Peter Jay Salzman, Michael Burian, Ori Pomerantz, Bob Mottram, Jim Huang


May 2, 2023

PIC

1 Introduction

The Linux Kernel Module Programming Guide is a free book; you may reproduce
and/or modify it under the terms of the Open Software License, version
3.0.

This book is distributed in the hope that it would be useful, but without any
warranty, without even the implied warranty of merchantability or fitness for a
particular purpose.

The author encourages wide distribution of this book for personal or commercial
use, provided the above copyright notice remains intact and the method adheres to
the provisions of the Open Software License. In summary, you may copy and
distribute this book free of charge or for a profit. No explicit permission is required
from the author for reproduction of this book in any medium, physical or
electronic.

Derivative works and translations of this document must be placed under the
Open Software License, and the original copyright notice must remain intact. If you
have contributed new material to this book, you must make the material and source
code available for your revisions. Please make revisions and updates available directly
to the document maintainer, Jim Huang . This will allow
for the merging of updates and provide consistent revisions to the Linux
community.

If you publish or distribute this book commercially, donations, royalties, and/or
printed copies are greatly appreciated by the author and the Linux Documentation
Project
(LDP). Contributing in this way shows your support for free software and
the LDP. If you have questions or comments, please contact the address
above.

1.1 Authorship

The Linux Kernel Module Programming Guide was originally written for the 2.2
kernels by Ori Pomerantz. Eventually, Ori no longer had time to maintain the
document. After all, the Linux kernel is a fast moving target. Peter Jay Salzman took
over maintenance and updated it for the 2.4 kernels. Eventually, Peter no longer had
time to follow developments with the 2.6 kernel, so Michael Burian became a
co-maintainer to update the document for the 2.6 kernels. Bob Mottram updated the
examples for 3.8+ kernels. Jim Huang upgraded to recent kernel versions (v5.x) and
revised the LaTeX document.

1.2 Acknowledgements

The following people have contributed corrections or good suggestions:

2011eric, 25077667, Arush Sharma, asas1asas200, Benno Bielmeier, Bob Lee,
Brad Baker, ccs100203, Chih-Yu Chen, Ching-Hua (Vivian) Lin,
ChinYikMing, Cyril Brulebois, Daniele Paolo Scarpazza, David Porter,
demonsome, Dimo Velev, Ekang Monyet, fennecJ, Francois Audeon,
gagachang, Gilad Reti, Horst Schirmeier, Hsin-Hsiang Peng, Ignacio Martin,
JianXing Wu, linD026, lyctw, manbing, Marconi Jiang, mengxinayan,
RinHizakura, Roman Lakeev, Stacy Prowell, Steven Lung, Tristan Lelong,
Tucker Polomik, VxTeemo, Wei-Lun Tsai, xatier, Ylowy.

1.3 What Is A Kernel Module?

So, you want to write a kernel module. You know C, you have written a few normal
programs to run as processes, and now you want to get to where the real action is, to
where a single wild pointer can wipe out your file system and a core dump means a
reboot.

What exactly is a kernel module? Modules are pieces of code that can be loaded
and unloaded into the kernel upon demand. They extend the functionality of the
kernel without the need to reboot the system. For example, one type of module is the
device driver, which allows the kernel to access hardware connected to the system.
Without modules, we would have to build monolithic kernels and add new
functionality directly into the kernel image. Besides having larger kernels, this has
the disadvantage of requiring us to rebuild and reboot the kernel every time we want
new functionality.

1.4 Kernel module package

Linux distributions provide the commands
modprobe
, insmod
and depmod
within a package.

On Ubuntu/Debian:

1sudo apt-get install build-essential kmod

On Arch Linux:

1sudo pacman -S gcc kmod

1.5 What Modules are in my Kernel?

To discover what modules are already loaded within your current kernel use the command
lsmod
.

1sudo lsmod

Modules are stored within the file /proc/modules, so you can also see them with:

1sudo cat /proc/modules

This can be a long list, and you might prefer to search for something particular.
To search for the fat module:

1sudo lsmod | grep fat

1.6 Do I need to download and compile the kernel?

For the purposes of following this guide you don’t necessarily need to do that.
However, it would be wise to run the examples within a test distribution running
on a virtual machine in order to avoid any possibility of messing up your
system.

1.7 Before We Begin

Before we delve into code, there are a few issues we need to cover. Everyone’s system
is different and everyone has their own groove. Getting your first “hello world”
program to compile and load correctly can sometimes be a trick. Rest assured, after
you get over the initial hurdle of doing it for the first time, it will be smooth sailing
thereafter.

  1. Modversioning. A module compiled for one kernel will not load if you boot
    a different kernel unless you enable CONFIG_MODVERSIONS
    in the kernel. We will not go into module versioning until later in this
    guide. Until we cover modversions, the examples in the guide may not

    work if you are running a kernel with modversioning turned on. However,
    most stock Linux distribution kernels come with it turned on. If you are
    having trouble loading the modules because of versioning errors, compile
    a kernel with modversioning turned off.

  2. Using X Window System. It is highly recommended that you extract,
    compile and load all the examples this guide discusses from a console. You
    should not be working on this stuff in X Window System.

    Modules can not print to the screen like printf()
    can, but they can log information and warnings, which ends up being
    printed on your screen, but only on a console. If you insmod
    a module from an xterm, the information and warnings will be logged, but
    only to your systemd journal. You will not see it unless you look through
    your journalctl
    . See 4 for details. To have immediate access to this information, do all
    your work from the console.

  3. SecureBoot. Many contemporary computers are pre-configured with UEFI
    SecureBoot enabled. It is a security standard that can make sure the
    device boots using only software that is trusted by original equipment
    manufacturer. The default Linux kernel from some distributions have also
    enabled the SecureBoot. For such distributions, the kernel module has to
    be signed with the security key or you would get the “ERROR: could not
    insert module” when you insert your first hello world module:

    1insmod ./hello-1.ko

    And then you can check further with
    dmesg
    and see the following text:

    Lockdown: insmod: unsigned module loading is restricted; see man kernel
    lockdown.7

    If you got this message, the simplest way is to disable the UEFI SecureBoot

    from the PC/laptop boot menu to have your “hello-1” to be inserted. Of course
    you can go through complicated steps to generate keys, install keys to your
    system, and finally sign your module to make it work. However, this is not
    suitable for beginners. You could read and follow the steps in SecureBoot if you
    are interested.

Before you can build anything you’ll need to install the header files for your
kernel.

On Ubuntu/Debian:

1sudo apt-get update 
2apt-cache search linux-headers-`uname -r`

This will tell you what kernel header files are available. Then for example:

1sudo apt-get install kmod linux-headers-5.4.0-80-generic

On Arch Linux:

1sudo pacman -S linux-headers

3 Examples

All the examples from this document are available within the examples
subdirectory.

If there are any compile errors then you might have a more recent kernel version
or need to install the corresponding kernel header files.

4 Hello World

4.1 The Simplest Module

Most people learning programming start out with some sort of “hello world
example. I don’t know what happens to people who break with this tradition, but I
think it is safer not to find out. We will start with a series of hello world
programs that demonstrate the different aspects of the basics of writing a kernel
module.

Here is the simplest module possible.

Make a test directory:

1mkdir -p ~/develop/kernel/hello-1 
2cd ~/develop/kernel/hello-1

Paste this into your favorite editor and save it as hello-1.c:

1/* 
2 * hello-1.c - The simplest kernel module. 
3 */ 
4#include  /* Needed by all modules */ 
5#include  /* Needed for pr_info() */ 
6 
7int init_module(void) 
8{ 
9    pr_info("Hello world 1.n"); 
10 
11    /* A non 0 return means init_module failed; module can't be loaded. */ 
12    return 0; 
13} 
14 
15void cleanup_module(void) 
16{ 
17    pr_info("Goodbye world 1.n"); 
18} 
19 
20MODULE_LICENSE("GPL");

Now you will need a Makefile. If you copy and paste this, change the indentation
to use tabs, not spaces.

1obj-m += hello-1.o 
2 
3PWD := $(CURDIR) 
4 
5all: 
6    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules 
7 
8clean: 
9    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

In Makefile, $(CURDIR) can set to the absolute pathname of the current working
directory(after all -C options are processed, if any). See more about CURDIR in GNU
make manual
.

And finally, just run make directly.

1make

If there is no PWD := $(CURDIR) statement in Makefile, then it may not compile
correctly with sudo make. Because some environment variables are specified by
the security policy, they can’t be inherited. The default security policy is
sudoers. In the sudoers security policy, env_reset is enabled by default,
which restricts environment variables. Specifically, path variables are not
retained from the user environment, they are set to default values (For more
information see: sudoers manual). You can see the environment variable settings
by:

$ sudo -s
# sudo -V

Here is a simple Makefile as an example to demonstrate the problem mentioned
above.

1all: 
2    echo $(PWD)

Then, we can use -p flag to print out the environment variable values from the
Makefile.

$ make -p | grep PWD
PWD = /home/ubuntu/temp
OLDPWD = /home/ubuntu
    echo $(PWD)

The PWD variable won’t be inherited with sudo.

$ sudo make -p | grep PWD
    echo $(PWD)

However, there are three ways to solve this problem.

  1. You can use the -E flag to temporarily preserve them.

    1    $ sudo -E make -p | grep PWD 
    2    PWD = /home/ubuntu/temp 
    3    OLDPWD = /home/ubuntu 
    4    echo $(PWD)
  2. You can set the env_reset disabled by editing the /etc/sudoers with
    root and visudo.

    1  ## sudoers file. 
    2  ## 
    3  ... 
    4  Defaults env_reset 
    5  ## Change env_reset to !env_reset in previous line to keep all environment variables

    Then execute env and sudo env individually.

    1    # disable the env_reset 
    2    echo "user:" > non-env_reset.log; env >> non-env_reset.log 
    3    echo "root:" >> non-env_reset.log; sudo env >> non-env_reset.log 
    4    # enable the env_reset 
    5    echo "user:" > env_reset.log; env >> env_reset.log 
    6    echo "root:" >> env_reset.log; sudo env >> env_reset.log

    You can view and compare these logs to find differences between
    env_reset and !env_reset.

  3. You can preserve environment variables by appending them to env_keep
    in /etc/sudoers.

    1  Defaults env_keep += "PWD"

    After applying the above change, you can check the environment variable
    settings by:

             $ sudo -s
             # sudo -V
           
    

If all goes smoothly you should then find that you have a compiled hello-1.ko
module. You can find info on it with the command:

1modinfo hello-1.ko

At this point the command:

1sudo lsmod | grep hello

should return nothing. You can try loading your shiny new module with:

1sudo insmod hello-1.ko

The dash character will get converted to an underscore, so when you again try:

1sudo lsmod | grep hello

you should now see your loaded module. It can be removed again with:

1sudo rmmod hello_1

Notice that the dash was replaced by an underscore. To see what just happened in
the logs:

1sudo journalctl --since "1 hour ago" | grep kernel

You now know the basics of creating, compiling, installing and removing modules.
Now for more of a description of how this module works.

Kernel modules must have at least two functions: a “start” (initialization) function
called init_module()
which is called when the module is insmod
ed into the kernel, and an “end” (cleanup) function called
cleanup_module()
which is called just before it is removed from the kernel. Actually, things have
changed starting with kernel 2.3.13. You can now use whatever name you like for the
start and end functions of a module, and you will learn how to do this in Section 4.2.
In fact, the new method is the preferred method. However, many people still use
init_module()
and cleanup_module()
for their start and end functions.

Typically, init_module()
either registers a handler for something with the kernel, or it replaces one of the kernel
functions with its own code (usually code to do something and then call the original function).
The cleanup_module()
function is supposed to undo whatever
init_module()
did, so the module can be unloaded safely.

Lastly, every kernel module needs to include . We
needed to include only for the macro expansion for the
pr_alert()
log level, which you’ll learn about in Section 2.

  1. A point about coding style. Another thing which may not be immediately
    obvious to anyone getting started with kernel programming is that
    indentation within your code should be using tabs and not spaces. It is
    one of the coding conventions of the kernel. You may not like it, but you’ll
    need to get used to it if you ever submit a patch upstream.
  2. Introducing print macros. In the beginning there was printk
    , usually followed by a priority such as KERN_INFO

    or KERN_DEBUG
    . More recently this can also be expressed in abbreviated form using a set of
    print macros, such as pr_info
    and pr_debug
    . This just saves some mindless keyboard bashing and looks a bit neater.
    They can be found within include/linux/printk.h. Take time to read through
    the available priority macros.

  3. About Compiling. Kernel modules need to be compiled a bit differently
    from regular userspace apps. Former kernel versions required us to
    care much about these settings, which are usually stored in Makefiles.
    Although hierarchically organized, many redundant settings accumulated
    in sublevel Makefiles and made them large and rather difficult to maintain.
    Fortunately, there is a new way of doing these things, called kbuild, and
    the build process for external loadable modules is now fully integrated into
    the standard kernel build mechanism. To learn more on how to compile
    modules which are not part of the official kernel (such as all the examples
    you will find in this guide), see file Documentation/kbuild/modules.rst.

    Additional details about Makefiles for kernel modules are available in
    Documentation/kbuild/makefiles.rst. Be sure to read this and the related
    files before starting to hack Makefiles. It will probably save you lots of
    work.

    Here is another exercise for the reader. See that comment above
    the return statement in init_module()
    ? Change the return value to something negative, recompile and
    load the module again. What happens?

4.2 Hello and Goodbye

In early kernel versions you had to use the

init_module
and cleanup_module
functions, as in the first hello world example, but these days you can name those anything you
want by using the module_init
and module_exit
macros. These macros are defined in include/linux/module.h. The only requirement
is that your init and cleanup functions must be defined before calling the those
macros, otherwise you’ll get compilation errors. Here is an example of this
technique:

1/* 
2 * hello-2.c - Demonstrating the module_init() and module_exit() macros. 
3 * This is preferred over using init_module() and cleanup_module(). 
4 */ 
5#include  /* Needed for the macros */ 
6#include  /* Needed by all modules */ 
7#include  /* Needed for pr_info() */ 
8 
9static int __init hello_2_init(void) 
10{ 
11    pr_info("Hello, world 2n"); 
12    return 0; 
13} 
14 
15static void __exit hello_2_exit(void) 
16{ 
17    pr_info("Goodbye, world 2n"); 
18} 
19 
20module_init(hello_2_init); 
21module_exit(hello_2_exit); 
22 
23MODULE_LICENSE("GPL");

So now we have two real kernel modules under our belt. Adding another module
is as simple as this:

1obj-m += hello-1.o 
2obj-m += hello-2.o 
3 
4PWD := $(CURDIR) 
5 
6all: 
7    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules 
8 
9clean: 
10    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Now have a look at drivers/char/Makefile for a real world example. As
you can see, some things got hardwired into the kernel (obj-y) but where
have all those obj-m gone? Those familiar with shell scripts will easily be
able to spot them. For those who are not, the obj-$(CONFIG_FOO) entries
you see everywhere expand into obj-y or obj-m, depending on whether the
CONFIG_FOO variable has been set to y or m. While we are at it, those were
exactly the kind of variables that you have set in the .config file in the
top-level directory of Linux kernel source tree, the last time when you said
make menuconfig
or something like that.

4.3 The __init and __exit Macros

The __init
macro causes the init function to be discarded and its memory freed once the init
function finishes for built-in drivers, but not loadable modules. If you think about
when the init function is invoked, this makes perfect sense.

There is also an __initdata
which works similarly to __init
but for init variables rather than functions.

The __exit
macro causes the omission of the function when the module is built into the kernel, and
like __init
, has no effect for loadable modules. Again, if you consider when the cleanup function
runs, this makes complete sense; built-in drivers do not need a cleanup function,

while loadable modules do.

These macros are defined in include/linux/init.h and serve to free up kernel
memory. When you boot your kernel and see something like Freeing unused kernel
memory: 236k freed, this is precisely what the kernel is freeing.

1/* 
2 * hello-3.c - Illustrating the __init, __initdata and __exit macros. 
3 */ 
4#include  /* Needed for the macros */ 
5#include  /* Needed by all modules */ 
6#include  /* Needed for pr_info() */ 
7 
8static int hello3_data __initdata = 3; 
9 
10static int __init hello_3_init(void) 
11{ 
12    pr_info("Hello, world %dn", hello3_data); 
13    return 0; 
14} 
15 
16static void __exit hello_3_exit(void) 
17{ 
18    pr_info("Goodbye, world 3n"); 
19} 
20 
21module_init(hello_3_init); 
22module_exit(hello_3_exit); 
23 
24MODULE_LICENSE("GPL");

4.4 Licensing and Module Documentation

Honestly, who loads or even cares about proprietary modules? If you do then you
might have seen something like this:

$ sudo insmod xxxxxx.ko
loading out-of-tree module taints kernel.
module license 'unspecified' taints kernel.

You can use a few macros to indicate the license for your module. Some examples
are “GPL”, “GPL v2”, “GPL and additional rights”, “Dual BSD/GPL”, “Dual
MIT/GPL”, “Dual MPL/GPL” and “Proprietary”. They are defined within
include/linux/module.h.

To reference what license you’re using a macro is available called
MODULE_LICENSE
. This and a few other macros describing the module are illustrated in the below
example.

1/* 
2 * hello-4.c - Demonstrates module documentation. 
3 */ 
4#include  /* Needed for the macros */ 
5#include  /* Needed by all modules */ 
6#include  /* Needed for pr_info() */ 
7 
8MODULE_LICENSE("GPL"); 
9MODULE_AUTHOR("LKMPG"); 
10MODULE_DESCRIPTION("A sample driver"); 
11 
12static int __init init_hello_4(void) 
13{ 
14    pr_info("Hello, world 4n"); 
15    return 0; 
16} 
17 
18static void __exit cleanup_hello_4(void) 
19{ 
20    pr_info("Goodbye, world 4n"); 
21} 
22 
23module_init(init_hello_4); 
24module_exit(cleanup_hello_4);

4.5 Passing Command Line Arguments to a Module

Modules can take command line arguments, but not with the argc/argv you might be
used to.

To allow arguments to be passed to your module, declare the variables that will
take the values of the command line arguments as global and then use the
module_param()
macro, (defined in include/linux/moduleparam.h) to set the mechanism up. At runtime,
insmod
will fill the variables with any command line arguments that are given, like
insmod mymodule.ko myvariable=5
. The variable declarations and macros should be placed at the beginning of the
module for clarity. The example code should clear up my admittedly lousy
explanation.

The module_param()
macro takes 3 arguments: the name of the variable, its type and
permissions for the corresponding file in sysfs. Integer types can be signed
as usual or unsigned. If you’d like to use arrays of integers or strings see
module_param_array()
and module_param_string()
.

1int myint = 3; 
2module_param(myint, int, 0);

Arrays are supported too, but things are a bit different now than they were in the
olden days. To keep track of the number of parameters you need to pass a pointer to
a count variable as third parameter. At your option, you could also ignore the count and
pass NULL
instead. We show both possibilities here:

1int myintarray[2]; 
2module_param_array(myintarray, int, NULL, 0); /* not interested in count */ 
3 
4short myshortarray[4]; 
5int count; 
6module_param_array(myshortarray, short, &count, 0); /* put count into "count" variable */

A good use for this is to have the module variable’s default values set, like a port
or IO address. If the variables contain the default values, then perform autodetection
(explained elsewhere). Otherwise, keep the current value. This will be made clear
later on.

Lastly, there is a macro function, MODULE_PARM_DESC()
, that is used to document arguments that the module can take. It takes two
parameters: a variable name and a free form string describing that variable.

1/* 
2 * hello-5.c - Demonstrates command line argument passing to a module. 
3 */ 
4#include  
5#include  /* for ARRAY_SIZE() */ 
6#include  
7#include  
8#include  
9#include  
10 
11MODULE_LICENSE("GPL"); 
12 
13static short int myshort = 1; 
14static int myint = 420; 
15static long int mylong = 9999; 
16static char *mystring = "blah"; 
17static int myintarray[2] = { 420, 420 }; 
18static int arr_argc = 0; 
19 
20/* module_param(foo, int, 0000) 
21 * The first param is the parameters name. 
22 * The second param is its data type. 
23 * The final argument is the permissions bits, 
24 * for exposing parameters in sysfs (if non-zero) at a later stage. 
25 */ 
26module_param(myshort, short, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP); 
27MODULE_PARM_DESC(myshort, "A short integer"); 
28module_param(myint, int, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH); 
29MODULE_PARM_DESC(myint, "An integer"); 
30module_param(mylong, long, S_IRUSR); 
31MODULE_PARM_DESC(mylong, "A long integer"); 
32module_param(mystring, charp, 0000); 
33MODULE_PARM_DESC(mystring, "A character string"); 
34 
35/* module_param_array(name, type, num, perm); 
36 * The first param is the parameter's (in this case the array's) name. 
37 * The second param is the data type of the elements of the array. 
38 * The third argument is a pointer to the variable that will store the number 
39 * of elements of the array initialized by the user at module loading time. 
40 * The fourth argument is the permission bits. 
41 */ 
42module_param_array(myintarray, int, &arr_argc, 0000); 
43MODULE_PARM_DESC(myintarray, "An array of integers"); 
44 
45static int __init hello_5_init(void) 
46{ 
47    int i; 
48 
49    pr_info("Hello, world 5n=============n"); 
50    pr_info("myshort is a short integer: %hdn", myshort); 
51    pr_info("myint is an integer: %dn", myint); 
52    pr_info("mylong is a long integer: %ldn", mylong); 
53    pr_info("mystring is a string: %sn", mystring); 
54 
55    for (i = 0; i < ARRAY_SIZE(myintarray); i++) 
56        pr_info("myintarray[%d] = %dn", i, myintarray[i]); 
57 
58    pr_info("got %d arguments for myintarray.n", arr_argc); 
59    return 0; 
60} 
61 
62static void __exit hello_5_exit(void) 
63{ 
64    pr_info("Goodbye, world 5n"); 
65} 
66 
67module_init(hello_5_init); 
68module_exit(hello_5_exit);

I would recommend playing around with this code:

$ sudo insmod hello-5.ko mystring="bebop" myintarray=-1
$ sudo dmesg -t | tail -7
myshort is a short integer: 1
myint is an integer: 420
mylong is a long integer: 9999
mystring is a string: bebop
myintarray[0] = -1
myintarray[1] = 420
got 1 arguments for myintarray.

$ sudo rmmod hello-5
$ sudo dmesg -t | tail -1
Goodbye, world 5

$ sudo insmod hello-5.ko mystring="supercalifragilisticexpialidocious" myintarray=-1,-1
$ sudo dmesg -t | tail -7
myshort is a short integer: 1
myint is an integer: 420
mylong is a long integer: 9999
mystring is a string: supercalifragilisticexpialidocious
myintarray[0] = -1
myintarray[1] = -1
got 2 arguments for myintarray.

$ sudo rmmod hello-5
$ sudo dmesg -t | tail -1
Goodbye, world 5

$ sudo insmod hello-5.ko mylong=hello
insmod: ERROR: could not insert module hello-5.ko: Invalid parameters

4.6 Modules Spanning Multiple Files

Sometimes it makes sense to divide a kernel module between several source
files.

Here is an example of such a kernel module.

1/* 
2 * start.c - Illustration of multi filed modules 
3 */ 
4 
5#include  /* We are doing kernel work */ 
6#include  /* Specifically, a module */ 
7 
8int init_module(void) 
9{ 
10    pr_info("Hello, world - this is the kernel speakingn"); 
11    return 0; 
12} 
13 
14MODULE_LICENSE("GPL");

The next file:

1/* 
2 * stop.c - Illustration of multi filed modules 
3 */ 
4 
5#include  /* We are doing kernel work */ 
6#include  /* Specifically, a module  */ 
7 
8void cleanup_module(void) 
9{ 
10    pr_info("Short is the life of a kernel modulen"); 
11} 
12 
13MODULE_LICENSE("GPL");

And finally, the makefile:

1obj-m += hello-1.o 
2obj-m += hello-2.o 
3obj-m += hello-3.o 
4obj-m += hello-4.o 
5obj-m += hello-5.o 
6obj-m += startstop.o 
7startstop-objs := start.o stop.o 
8 
9PWD := $(CURDIR) 
10 
11all: 
12    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules 
13 
14clean: 
15    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

This is the complete makefile for all the examples we have seen so far. The first
five lines are nothing special, but for the last example we will need two lines.
First we invent an object name for our combined module, second we tell
make
what object files are part of that module.

4.7 Building modules for a precompiled kernel

Obviously, we strongly suggest you to recompile your kernel, so that you can enable
a number of useful debugging features, such as forced module unloading
( MODULE_FORCE_UNLOAD
): when this option is enabled, you can force the kernel to unload a module even when it believes
it is unsafe, via a sudo rmmod -f module
command. This option can save you a lot of time and a number of reboots during
the development of a module. If you do not want to recompile your kernel then you
should consider running the examples within a test distribution on a virtual machine.
If you mess anything up then you can easily reboot or restore the virtual machine
(VM).

There are a number of cases in which you may want to load your module into a
precompiled running kernel, such as the ones shipped with common Linux
distributions, or a kernel you have compiled in the past. In certain circumstances you
could require to compile and insert a module into a running kernel which you are not
allowed to recompile, or on a machine that you prefer not to reboot. If you
can’t think of a case that will force you to use modules for a precompiled
kernel you might want to skip this and treat the rest of this chapter as a big
footnote.

Now, if you just install a kernel source tree, use it to compile your kernel module
and you try to insert your module into the kernel, in most cases you would obtain an
error as follows:

insmod: ERROR: could not insert module poet.ko: Invalid module format

Less cryptic information is logged to the systemd journal:

kernel: poet: disagrees about version of symbol module_layout

In other words, your kernel refuses to accept your module because version strings
(more precisely, version magic, see include/linux/vermagic.h) do not match. Incidentally,
version magic strings are stored in the module object in the form of a static string, starting
with vermagic:
. Version data are inserted in your module when it is linked against the kernel/module.o
file. To inspect version magics and other strings stored in a given module, issue the
command modinfo module.ko
:

$ modinfo hello-4.ko
description:    A sample driver
author:         LKMPG
license:        GPL
srcversion:     B2AA7FBFCC2C39AED665382
depends:
retpoline:      Y
name:           hello_4
vermagic:       5.4.0-70-generic SMP mod_unload modversions

To overcome this problem we could resort to the –force-vermagic option,
but this solution is potentially unsafe, and unquestionably unacceptable
in production modules. Consequently, we want to compile our module in
an environment which was identical to the one in which our precompiled
kernel was built. How to do this, is the subject of the remainder of this
chapter.

First of all, make sure that a kernel source tree is available, having exactly the same
version as your current kernel. Then, find the configuration file which was used to
compile your precompiled kernel. Usually, this is available in your current boot directory,
under a name like config-5.14.x. You may just want to copy it to your kernel source
tree: cp /boot/config-`uname -r` .config
.

Let’s focus again on the previous error message: a closer look at the version magic
strings suggests that, even with two configuration files which are exactly the same, a
slight difference in the version magic could be possible, and it is sufficient to prevent
insertion of the module into the kernel. That slight difference, namely the
custom string which appears in the module’s version magic and not in the
kernel’s one, is due to a modification with respect to the original, in the
makefile that some distributions include. Then, examine your Makefile,
and make sure that the specified version information matches exactly the
one used for your current kernel. For example, your makefile could start as
follows:

VERSION = 5
PATCHLEVEL = 14
SUBLEVEL = 0
EXTRAVERSION = -rc2

In this case, you need to restore the value of symbol EXTRAVERSION to
-rc2. We suggest to keep a backup copy of the makefile used to compile your kernel
available in /lib/modules/5.14.0-rc2/build. A simple command as following
should suffice.

1cp /lib/modules/`uname -r`/build/Makefile linux-`uname -r`

Here linux-`uname -r`
is the Linux kernel source you are attempting to build.

Now, please run make
to update configuration and version headers and objects:

$ make
  SYNC    include/config/auto.conf.cmd
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/confdata.o
  HOSTCC  scripts/kconfig/expr.o
  LEX     scripts/kconfig/lexer.lex.c
  YACC    scripts/kconfig/parser.tab.[ch]
  HOSTCC  scripts/kconfig/preprocess.o
  HOSTCC  scripts/kconfig/symbol.o
  HOSTCC  scripts/kconfig/util.o
  HOSTCC  scripts/kconfig/lexer.lex.o
  HOSTCC  scripts/kconfig/parser.tab.o
  HOSTLD  scripts/kconfig/conf

If you do not desire to actually compile the kernel, you can interrupt the build
process (CTRL-C) just after the SPLIT line, because at that time, the files you need
are ready. Now you can turn back to the directory of your module and compile it: It
will be built exactly according to your current kernel settings, and it will load into it
without any errors.

5 Preliminaries

5.1 How modules begin and end

A program usually begins with a main()
function, executes a bunch of instructions and terminates upon completion of those
instructions. Kernel modules work a bit differently. A module always begin with either
the init_module
or the function you specify with module_init
call. This is the entry function for modules; it tells the kernel what functionality the
module provides and sets up the kernel to run the module’s functions when they
are needed. Once it does this, entry function returns and the module does
nothing until the kernel wants to do something with the code that the module
provides.

All modules end by calling either cleanup_module
or the function you specify with the module_exit
call. This is the exit function for modules; it undoes whatever entry function did. It
unregisters the functionality that the entry function registered.

Every module must have an entry function and an exit function. Since there’s
more than one way to specify entry and exit functions, I will try my best to use the
terms “entry function” and “exit function”, but if I slip and simply refer to them as
init_module
and cleanup_module
, I think you will know what I mean.

5.2 Functions available to modules

Programmers use functions they do not define all the time. A prime example of this
is printf()
. You use these library functions which are provided by the standard C
library, libc. The definitions for these functions do not actually enter
your program until the linking stage, which insures that the code (for
printf()
for example) is available, and fixes the call instruction to point to that
code.

Kernel modules are different here, too. In the hello world
example, you might have noticed that we used a function,
pr_info()
but did not include a standard I/O library. That is because
modules are object files whose symbols get resolved upon running
insmod
or modprobe
. The definition for the symbols comes from the kernel itself; the only external
functions you can use are the ones provided by the kernel. If you’re curious about
what symbols have been exported by your kernel, take a look at /proc/kallsyms.

One point to keep in mind is the difference between library functions and system
calls. Library functions are higher level, run completely in user space and
provide a more convenient interface for the programmer to the functions
that do the real work — system calls. System calls run in kernel mode on
the user’s behalf and are provided by the kernel itself. The library function
printf()
may look like a very general printing function, but all it really does is format the
data into strings and write the string data using the low-level system call
write()
, which then sends the data to standard output.

Would you like to see what system calls are made by
printf()

? It is easy! Compile the following program:

1#include  
2 
3int main(void) 
4{ 
5    printf("hello"); 
6    return 0; 
7}

with gcc -Wall -o hello hello.c
. Run the executable with strace ./hello
. Are you impressed? Every line you see corresponds to a system call. strace is a
handy program that gives you details about what system calls a program is
making, including which call is made, what its arguments are and what it
returns. It is an invaluable tool for figuring out things like what files a program
is trying to access. Towards the end, you will see a line which looks like
write(1, "hello", 5hello)
. There it is. The face behind the printf()
mask. You may not be familiar with write, since most people use library functions for file
I/O (like fopen
, fputs
, fclose
). If that is the case, try looking at man 2 write. The 2nd man section is devoted to system
calls (like kill()
and read()
). The 3rd man section is devoted to library calls, which you would probably be more familiar
with (like cosh()
and random()
).

You can even write modules to replace the kernel’s system calls, which we will do
shortly. Crackers often make use of this sort of thing for backdoors or trojans, but
you can write your own modules to do more benign things, like have the kernel
write Tee hee, that tickles! every time someone tries to delete a file on your
system.

5.3 User Space vs Kernel Space

A kernel is all about access to resources, whether the resource in question happens to
be a video card, a hard drive or even memory. Programs often compete for the same
resource. As I just saved this document, updatedb started updating the locate
database. My vim session and updatedb are both using the hard drive concurrently.
The kernel needs to keep things orderly, and not give users access to resources
whenever they feel like it. To this end, a CPU can run in different modes. Each mode
gives a different level of freedom to do what you want on the system. The Intel 80386
architecture had 4 of these modes, which were called rings. Unix uses only
two rings; the highest ring (ring 0, also known as “supervisor mode” where
everything is allowed to happen) and the lowest ring, which is called “user
mode”.

Recall the discussion about library functions vs system calls. Typically, you use a
library function in user mode. The library function calls one or more system calls,
and these system calls execute on the library function’s behalf, but do so in
supervisor mode since they are part of the kernel itself. Once the system call
completes its task, it returns and execution gets transferred back to user
mode.

5.4 Name Space

When you write a small C program, you use variables which are convenient and make
sense to the reader. If, on the other hand, you are writing routines which will be part
of a bigger problem, any global variables you have are part of a community of other
peoples’ global variables; some of the variable names can clash. When a program has
lots of global variables which aren’t meaningful enough to be distinguished, you get
namespace pollution. In large projects, effort must be made to remember reserved
names, and to find ways to develop a scheme for naming unique variable names and
symbols.

When writing kernel code, even the smallest module will be linked against the
entire kernel, so this is definitely an issue. The best way to deal with this is to declare
all your variables as static and to use a well-defined prefix for your symbols. By
convention, all kernel prefixes are lowercase. If you do not want to declare everything
as static, another option is to declare a symbol table and register it with the kernel.
We will get to this later.

The file /proc/kallsyms holds all the symbols that the kernel knows about and
which are therefore accessible to your modules since they share the kernel’s
codespace.

5.5 Code space

Memory management is a very complicated subject and the majority of O’Reilly’s
Understanding The Linux Kernel exclusively covers memory management!
We are not setting out to be experts on memory managements, but we do
need to know a couple of facts to even begin worrying about writing real
modules.

If you have not thought about what a segfault really means, you may be surprised
to hear that pointers do not actually point to memory locations. Not real
ones, anyway. When a process is created, the kernel sets aside a portion of
real physical memory and hands it to the process to use for its executing
code, variables, stack, heap and other things which a computer scientist
would know about. This memory begins with 0x00000000 and extends up to
whatever it needs to be. Since the memory space for any two processes do not

overlap, every process that can access a memory address, say 0xbffff978, would
be accessing a different location in real physical memory! The processes
would be accessing an index named 0xbffff978 which points to some kind of
offset into the region of memory set aside for that particular process. For
the most part, a process like our Hello, World program can’t access the
space of another process, although there are ways which we will talk about
later.

The kernel has its own space of memory as well. Since a module is code which
can be dynamically inserted and removed in the kernel (as opposed to a
semi-autonomous object), it shares the kernel’s codespace rather than having its own.
Therefore, if your module segfaults, the kernel segfaults. And if you start writing
over data because of an off-by-one error, then you’re trampling on kernel
data (or code). This is even worse than it sounds, so try your best to be
careful.

By the way, I would like to point out that the above discussion is true for any
operating system which uses a monolithic kernel. This is not quite the same thing as
“building all your modules into the kernel”, although the idea is the same. There are
things called microkernels which have modules which get their own codespace. The
GNU Hurd and the Zircon kernel of Google Fuchsia are two examples of a
microkernel.

5.6 Device Drivers

One class of module is the device driver, which provides functionality for hardware
like a serial port. On Unix, each piece of hardware is represented by a file located in
/dev named a device file which provides the means to communicate with the
hardware. The device driver provides the communication on behalf of a
user program. So the es1370.ko sound card device driver might connect the
/dev/sound device file to the Ensoniq IS1370 sound card. A userspace program like
mp3blaster can use /dev/sound without ever knowing what kind of sound card is
installed.

Let’s look at some device files. Here are device files which represent the first three
partitions on the primary master IDE hard drive:

$ ls -l /dev/hda[1-3]
brw-rw----  1 root  disk  3, 1 Jul  5  2000 /dev/hda1
brw-rw----  1 root  disk  3, 2 Jul  5  2000 /dev/hda2
brw-rw----  1 root  disk  3, 3 Jul  5  2000 /dev/hda3

Notice the column of numbers separated by a comma. The first number is called
the device’s major number. The second number is the minor number. The major
number tells you which driver is used to access the hardware. Each driver is assigned
a unique major number; all device files with the same major number are controlled
by the same driver. All the above major numbers are 3, because they’re all controlled
by the same driver.

The minor number is used by the driver to distinguish between the various
hardware it controls. Returning to the example above, although all three devices are
handled by the same driver they have unique minor numbers because the driver sees
them as being different pieces of hardware.

Devices are divided into two types: character devices and block devices. The
difference is that block devices have a buffer for requests, so they can choose the best
order in which to respond to the requests. This is important in the case of storage
devices, where it is faster to read or write sectors which are close to each
other, rather than those which are further apart. Another difference is that
block devices can only accept input and return output in blocks (whose size
can vary according to the device), whereas character devices are allowed
to use as many or as few bytes as they like. Most devices in the world are
character, because they don’t need this type of buffering, and they don’t
operate with a fixed block size. You can tell whether a device file is for a block
device or a character device by looking at the first character in the output of
ls -l
. If it is ‘b’ then it is a block device, and if it is ‘c’ then it is a character device. The
devices you see above are block devices. Here are some character devices (the serial
ports):

crw-rw----  1 root  dial 4, 64 Feb 18 23:34 /dev/ttyS0
crw-r-----  1 root  dial 4, 65 Nov 17 10:26 /dev/ttyS1
crw-rw----  1 root  dial 4, 66 Jul  5  2000 /dev/ttyS2
crw-rw----  1 root  dial 4, 67 Jul  5  2000 /dev/ttyS3

If you want to see which major numbers have been assigned, you can look at
Documentation/admin-guide/devices.txt.

When the system was installed, all of those device files were created by the
mknod
command. To create a new char device named coffee with major/minor number 12 and 2,
simply do mknod /dev/coffee c 12 2
. You do not have to put your device files into /dev, but it is done by convention.
Linus put his device files in /dev, and so should you. However, when creating a
device file for testing purposes, it is probably OK to place it in your working
directory where you compile the kernel module. Just be sure to put it in the right
place when you’re done writing the device driver.

I would like to make a few last points which are implicit from the above
discussion, but I would like to make them explicit just in case. When a device file is
accessed, the kernel uses the major number of the file to determine which driver
should be used to handle the access. This means that the kernel doesn’t really need
to use or even know about the minor number. The driver itself is the only thing that
cares about the minor number. It uses the minor number to distinguish between
different pieces of hardware.

By the way, when I say “hardware”, I mean something a bit more abstract
than a PCI card that you can hold in your hand. Look at these two device
files:

$ ls -l /dev/sda /dev/sdb
brw-rw---- 1 root disk 8,  0 Jan  3 09:02 /dev/sda
brw-rw---- 1 root disk 8, 16 Jan  3 09:02 /dev/sdb

By now you can look at these two device files and know instantly that they are
block devices and are handled by same driver (block major 8). Sometimes two device
files with the same major but different minor number can actually represent the same
piece of physical hardware. So just be aware that the word “hardware” in our
discussion can mean something very abstract.

6 Character Device drivers

6.1 The file_operations Structure

The file_operations
structure is defined in include/linux/fs.h, and holds pointers to functions defined by
the driver that perform various operations on the device. Each field of the structure
corresponds to the address of some function defined by the driver to handle a
requested operation.

For example, every character driver needs to define a function that reads from the
device. The file_operations
structure holds the address of the module’s function that performs that operation.
Here is what the definition looks like for kernel 5.4:

1struct file_operations { 
2    struct module *owner; 
3    loff_t (*llseek) (struct file *, loff_t, int); 
4    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 
5    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 
6    ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 
7    ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 
8    int (*iopoll)(struct kiocb *kiocb, bool spin); 
9    int (*iterate) (struct file *, struct dir_context *); 
10    int (*iterate_shared) (struct file *, struct dir_context *); 
11    __poll_t (*poll) (struct file *, struct poll_table_struct *); 
12    long (*unlocked_ioctl) (struct file *, unsigned intunsigned long); 
13    long (*compat_ioctl) (struct file *, unsigned intunsigned long); 
14    int (*mmap) (struct file *, struct vm_area_struct *); 
15    unsigned long mmap_supported_flags; 
16    int (*open) (struct inode *, struct file *); 
17    int (*flush) (struct file *, fl_owner_t id); 
18    int (*release) (struct inode *, struct file *); 
19    int (*fsync) (struct file *, loff_t, loff_t, int datasync); 
20    int (*fasync) (intstruct file *, int); 
21    int (*lock) (struct file *, intstruct file_lock *); 
22    ssize_t (*sendpage) (struct file *, struct page *, intsize_t, loff_t *, int); 
23    unsigned long (*get_unmapped_area)(struct file *, unsigned longunsigned longunsigned longunsigned long); 
24    int (*check_flags)(int); 
25    int (*flock) (struct file *, intstruct file_lock *); 
26    ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_tunsigned int); 
27    ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_tunsigned int); 
28    int (*setlease)(struct file *, longstruct file_lock **, void **); 
29    long (*fallocate)(struct file *file, int mode, loff_t offset, 
30        loff_t len); 
31    void (*show_fdinfo)(struct seq_file *m, struct file *f); 
32    ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, 
33        loff_t, size_tunsigned int); 
34    loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, 
35             struct file *file_out, loff_t pos_out, 
36             loff_t len, unsigned int remap_flags); 
37    int (*fadvise)(struct file *, loff_t, loff_t, int); 
38} __randomize_layout;

Some operations are not implemented by a driver. For example, a driver that handles
a video card will not need to read from a directory structure. The corresponding entries
in the file_operations
structure should be set to NULL
.

There is a gcc extension that makes assigning to this structure more convenient.
You will see it in modern drivers, and may catch you by surprise. This is what the
new way of assigning to the structure looks like:

1struct file_operations fops = { 
2    read: device_read, 
3    write: device_write, 
4    open: device_open, 
5    release: device_release 
6};

However, there is also a C99 way of assigning to elements of a structure,
designated initializers, and this is definitely preferred over using the GNU extension.
You should use this syntax in case someone wants to port your driver. It will help
with compatibility:

1struct file_operations fops = { 
2    .read = device_read, 
3    .write = device_write, 
4    .open = device_open, 
5    .release = device_release 
6};

The meaning is clear, and you should be aware that any member of
the structure which you do not explicitly assign will be initialized to
NULL
by gcc.

An instance of struct file_operations
containing pointers to functions that are used to implement
read
, write
, open
, … system calls is commonly named fops
.

Since Linux v3.14, the read, write and seek operations are guaranteed for thread-safe by
using the f_pos
specific lock, which makes the file position update to become the mutual
exclusion. So, we can safely implement those operations without unnecessary
locking.

Additionally, since Linux v5.6, the proc_ops
structure was introduced to replace the use of the
file_operations
structure when registering proc handlers. See more information in the 7.1
section.

6.2 The file structure

Each device is represented in the kernel by a file structure, which is defined
in include/linux/fs.h. Be aware that a file is a kernel level structure and
never appears in a user space program. It is not the same thing as a
FILE
, which is defined by glibc and would never appear in a kernel space
function. Also, its name is a bit misleading; it represents an abstract open
‘file’, not a file on a disk, which is represented by a structure named
inode
.

An instance of struct file is commonly named
filp
. You’ll also see it referred to as a struct file object. Resist the temptation.

Go ahead and look at the definition of file. Most of the entries you see, like struct
dentry are not used by device drivers, and you can ignore them. This is because
drivers do not fill file directly; they only use structures contained in file which are
created elsewhere.

6.3 Registering A Device

As discussed earlier, char devices are accessed through device files, usually located in
/dev. This is by convention. When writing a driver, it is OK to put the
device file in your current directory. Just make sure you place it in /dev for a
production driver. The major number tells you which driver handles which
device file. The minor number is used only by the driver itself to differentiate
which device it is operating on, just in case the driver handles more than one
device.

Adding a driver to your system means registering it with the kernel. This is synonymous
with assigning it a major number during the module’s initialization. You do this by
using the register_chrdev
function, defined by include/linux/fs.h.

1int register_chrdev(unsigned int major, const char *name, struct file_operations *fops);

Where unsigned int major is the major number you want to request,
const char *name
is the name of the device as it will appear in /proc/devices and
struct file_operations *fops
is a pointer to the file_operations
table for your driver. A negative return value means the
registration failed. Note that we didn’t pass the minor number to
register_chrdev
. That is because the kernel doesn’t care about the minor number; only our driver
uses it.

Now the question is, how do you get a major number without hijacking
one that’s already in use? The easiest way would be to look through
Documentation/admin-guide/devices.txt and pick an unused one. That is a bad way
of doing things because you will never be sure if the number you picked will be
assigned later. The answer is that you can ask the kernel to assign you a dynamic
major number.

If you pass a major number of 0 to register_chrdev
, the return value will be the dynamically allocated major number. The
downside is that you can not make a device file in advance, since you do not

know what the major number will be. There are a couple of ways to do
this. First, the driver itself can print the newly assigned number and we
can make the device file by hand. Second, the newly registered device will
have an entry in /proc/devices, and we can either make the device file by
hand or write a shell script to read the file in and make the device file. The
third method is that we can have our driver make the device file using the
device_create
function after a successful registration and
device_destroy
during the call to cleanup_module
.

However, register_chrdev()
would occupy a range of minor numbers associated with the given major. The
recommended way to reduce waste for char device registration is using cdev
interface.

The newer interface completes the char device registration in two distinct steps.
First, we should register a range of device numbers, which can be completed with
register_chrdev_region
or alloc_chrdev_region
.

1int register_chrdev_region(dev_t from, unsigned count, const char *name); 
2int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name);

The choice between two different functions depends on
whether you know the major numbers for your device. Using
register_chrdev_region
if you know the device major number and
alloc_chrdev_region
if you would like to allocate a dynamicly-allocated major number.

Second, we should initialize the data structure
struct cdev
for our char device and associate it with the device numbers. To initialize the
struct cdev
, we can achieve by the similar sequence of the following codes.

1struct cdev *my_dev = cdev_alloc(); 
2my_cdev->ops = &my_fops;

However, the common usage pattern will embed the
struct cdev
within a device-specific structure of your own. In this case, we’ll need
cdev_init
for the initialization.

1void cdev_init(struct cdev *cdev, const struct file_operations *fops);

Once we finish the initialization, we can add the char device to the system by using
the cdev_add
.

1int cdev_add(struct cdev *p, dev_t dev, unsigned count);

To find a example using the interface, you can see ioctl.c described in section
9.

6.4 Unregistering A Device

We can not allow the kernel module to be
rmmod
’ed whenever root feels like it. If the device file is opened by a process and then we
remove the kernel module, using the file would cause a call to the memory location
where the appropriate function (read/write) used to be. If we are lucky, no
other code was loaded there, and we’ll get an ugly error message. If we are
unlucky, another kernel module was loaded into the same location, which
means a jump into the middle of another function within the kernel. The
results of this would be impossible to predict, but they can not be very
positive.

Normally, when you do not want to allow something, you return an error code
(a negative number) from the function which is supposed to do it. With
cleanup_module
that’s impossible because it is a void function. However, there is a counter
which keeps track of how many processes are using your module. You
can see what its value is by looking at the 3rd field with the command
cat /proc/modules
or sudo lsmod
. If this number isn’t zero, rmmod
will fail. Note that you do not have to check the counter within
cleanup_module
because the check will be performed for you by the system call
sys_delete_module
, defined in include/linux/syscalls.h. You should not use this counter directly, but
there are functions defined in include/linux/module.h which let you increase,
decrease and display this counter:

  • try_module_get(THIS_MODULE)
    : Increment the reference count of current module.
  • module_put(THIS_MODULE)
    : Decrement the reference count of current module.
  • module_refcount(THIS_MODULE)
    : Return the value of reference count of current module.

It is important to keep the counter accurate; if you ever do lose track of the
correct usage count, you will never be able to unload the module; it’s now reboot
time, boys and girls. This is bound to happen to you sooner or later during a
module’s development.

6.5 chardev.c

The next code sample creates a char driver named chardev. You can dump its device
file.

1cat /proc/devices

(or open the file with a program) and the driver will put the number of times the
device file has been read from into the file. We do not support writing to the file (like
echo "hi" > /dev/hello
), but catch these attempts and tell the user that the operation is not supported.
Don’t worry if you don’t see what we do with the data we read into the buffer; we
don’t do much with it. We simply read in the data and print a message
acknowledging that we received it.

In the multiple-threaded environment, without any protection, concurrent access
to the same memory may lead to the race condition, and will not preserve the
performance. In the kernel module, this problem may happen due to multiple
instances accessing the shared resources. Therefore, a solution is to enforce the
exclusive access. We use atomic Compare-And-Swap (CAS) to maintain the states,
CDEV_NOT_USED
and CDEV_EXCLUSIVE_OPEN

, to determine whether the file is currently opened by someone or not. CAS compares
the contents of a memory location with the expected value and, only if they are the
same, modifies the contents of that memory location to the desired value. See more
concurrency details in the 12 section.

1/* 
2 * chardev.c: Creates a read-only char device that says how many times 
3 * you have read from the dev file 
4 */ 
5 
6#include  
7#include  
8#include  
9#include  
10#include  
11#include  
12#include  /* for sprintf() */ 
13#include  
14#include  
15#include  
16#include  /* for get_user and put_user */ 
17 
18#include  
19 
20/*  Prototypes - this would normally go in a .h file */ 
21static int device_open(struct inode *, struct file *); 
22static int device_release(struct inode *, struct file *); 
23static ssize_t device_read(struct file *, char __user *, size_t, loff_t *); 
24static ssize_t device_write(struct file *, const char __user *, size_t, 
25                            loff_t *); 
26 
27#define SUCCESS 0 
28#define DEVICE_NAME "chardev" /* Dev name as it appears in /proc/devices   */ 
29#define BUF_LEN 80 /* Max length of the message from the device */ 
30 
31/* Global variables are declared as static, so are global within the file. */ 
32 
33static int major; /* major number assigned to our device driver */ 
34 
35enum { 
36    CDEV_NOT_USED = 0, 
37    CDEV_EXCLUSIVE_OPEN = 1, 
38}; 
39 
40/* Is device open? Used to prevent multiple access to device */ 
41static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
42 
43static char msg[BUF_LEN + 1]; /* The msg the device will give when asked */ 
44 
45static struct class *cls; 
46 
47static struct file_operations chardev_fops = { 
48    .read = device_read, 
49    .write = device_write, 
50    .open = device_open, 
51    .release = device_release, 
52}; 
53 
54static int __init chardev_init(void) 
55{ 
56    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
57 
58    if (major < 0) { 
59        pr_alert("Registering char device failed with %dn", major); 
60        return major; 
61    } 
62 
63    pr_info("I was assigned major number %d.n", major); 
64 
65    cls = class_create(THIS_MODULE, DEVICE_NAME); 
66    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
67 
68    pr_info("Device created on /dev/%sn", DEVICE_NAME); 
69 
70    return SUCCESS; 
71} 
72 
73static void __exit chardev_exit(void) 
74{ 
75    device_destroy(cls, MKDEV(major, 0)); 
76    class_destroy(cls); 
77 
78    /* Unregister the device */ 
79    unregister_chrdev(major, DEVICE_NAME); 
80} 
81 
82/* Methods */ 
83 
84/* Called when a process tries to open the device file, like 
85 * "sudo cat /dev/chardev" 
86 */ 
87static int device_open(struct inode *inode, struct file *file) 
88{ 
89    static int counter = 0; 
90 
91    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
92        return -EBUSY; 
93 
94    sprintf(msg, "I already told you %d times Hello world!n", counter++); 
95    try_module_get(THIS_MODULE); 
96 
97    return SUCCESS; 
98} 
99 
100/* Called when a process closes the device file. */ 
101static int device_release(struct inode *inode, struct file *file) 
102{ 
103    /* We're now ready for our next caller */ 
104    atomic_set(&already_open, CDEV_NOT_USED); 
105 
106    /* Decrement the usage count, or else once you opened the file, you will 
107     * never get rid of the module. 
108     */ 
109    module_put(THIS_MODULE); 
110 
111    return SUCCESS; 
112} 
113 
114/* Called when a process, which already opened the dev file, attempts to 
115 * read from it. 
116 */ 
117static ssize_t device_read(struct file *filp, /* see include/linux/fs.h   */ 
118                           char __user *buffer, /* buffer to fill with data */ 
119                           size_t length, /* length of the buffer     */ 
120                           loff_t *offset) 
121{ 
122    /* Number of bytes actually written to the buffer */ 
123    int bytes_read = 0; 
124    const char *msg_ptr = msg; 
125 
126    if (!*(msg_ptr + *offset)) { /* we are at the end of message */ 
127        *offset = 0; /* reset the offset */ 
128        return 0; /* signify end of file */ 
129    } 
130 
131    msg_ptr += *offset; 
132 
133    /* Actually put the data into the buffer */ 
134    while (length && *msg_ptr) { 
135        /* The buffer is in the user data segment, not the kernel 
136         * segment so "*" assignment won't work.  We have to use 
137         * put_user which copies data from the kernel data segment to 
138         * the user data segment. 
139         */ 
140        put_user(*(msg_ptr++), buffer++); 
141        length--; 
142        bytes_read++; 
143    } 
144 
145    *offset += bytes_read; 
146 
147    /* Most read functions return the number of bytes put into the buffer. */ 
148    return bytes_read; 
149} 
150 
151/* Called when a process writes to dev file: echo "hi" > /dev/hello */ 
152static ssize_t device_write(struct file *filp, const char __user *buff, 
153                            size_t len, loff_t *off) 
154{ 
155    pr_alert("Sorry, this operation is not supported.n"); 
156    return -EINVAL; 
157} 
158 
159module_init(chardev_init); 
160module_exit(chardev_exit); 
161 
162MODULE_LICENSE("GPL");

6.6 Writing Modules for Multiple Kernel Versions

The system calls, which are the major interface the kernel shows to the processes,
generally stay the same across versions. A new system call may be added, but
usually the old ones will behave exactly like they used to. This is necessary for
backward compatibility – a new kernel version is not supposed to break regular
processes. In most cases, the device files will also remain the same. On the other
hand, the internal interfaces within the kernel can and do change between
versions.

There are differences between different kernel versions, and if you want
to support multiple kernel versions, you will find yourself having to code
conditional compilation directives. The way to do this to compare the macro
LINUX_VERSION_CODE
to the macro KERNEL_VERSION
. In version a.b.c of the kernel, the value of this macro would be 216a+ 28b+ c  .

7 The /proc File System

In Linux, there is an additional mechanism for the kernel and kernel modules to send
information to processes — the /proc file system. Originally designed to allow easy
access to information about processes (hence the name), it is now used by every bit
of the kernel which has something interesting to report, such as /proc/modules
which provides the list of modules and /proc/meminfo which gathers memory usage
statistics.

The method to use the proc file system is very similar to the one used with device
drivers — a structure is created with all the information needed for the /proc file,
including pointers to any handler functions (in our case there is only one, the
one called when somebody attempts to read from the /proc file). Then,
init_module
registers the structure with the kernel and
cleanup_module
unregisters it.

Normal file systems are located on a disk, rather than just in memory (which is
where /proc is), and in that case the index-node (inode for short) number
is a pointer to a disk location where the file’s inode is located. The inode
contains information about the file, for example the file’s permissions, together
with a pointer to the disk location or locations where the file’s data can be
found.

Because we don’t get called when the file is opened or closed, there’s nowhere for
us to put try_module_get
and module_put
in this module, and if the file is opened and then the module is removed, there’s no
way to avoid the consequences.

Here a simple example showing how to use a /proc file. This is the HelloWorld for
the /proc filesystem. There are three parts: create the file /proc/helloworld in the
function init_module
, return a value (and a buffer) when the file /proc/helloworld is read in the callback
function procfile_read
, and delete the file /proc/helloworld in the function
cleanup_module
.

The /proc/helloworld is created when the module is loaded with the function
proc_create
. The return value is a struct proc_dir_entry
, and it will be used to configure the file /proc/helloworld (for example, the owner
of this file). A null return value means that the creation has failed.

Every time the file /proc/helloworld is read, the function
procfile_read
is called. Two parameters of this function are very important: the buffer
(the second parameter) and the offset (the fourth one). The content of the
buffer will be returned to the application which read it (for example the
cat
command). The offset is the current position in the file. If the return value of the
function is not null, then this function is called again. So be careful with this
function, if it never returns zero, the read function is called endlessly.

$ cat /proc/helloworld
HelloWorld!
1/* 
2 * procfs1.c 
3 */ 
4 
5#include  
6#include  
7#include  
8#include  
9#include  
10 
11#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0) 
12#define HAVE_PROC_OPS 
13#endif 
14 
15#define procfs_name "helloworld" 
16 
17static struct proc_dir_entry *our_proc_file; 
18 
19static ssize_t procfile_read(struct file *file_pointer, char __user *buffer, 
20                             size_t buffer_length, loff_t *offset) 
21{ 
22    char s[13] = "HelloWorld!n"; 
23    int len = sizeof(s); 
24    ssize_t ret = len; 
25 
26    if (*offset >= len || copy_to_user(buffer, s, len)) { 
27        pr_info("copy_to_user failedn"); 
28        ret = 0; 
29    } else { 
30        pr_info("procfile read %sn", file_pointer->f_path.dentry->d_name.name); 
31        *offset += len; 
32    } 
33 
34    return ret; 
35} 
36 
37#ifdef HAVE_PROC_OPS 
38static const struct proc_ops proc_file_fops = { 
39    .proc_read = procfile_read, 
40}; 
41#else 
42static const struct file_operations proc_file_fops = { 
43    .read = procfile_read, 
44}; 
45#endif 
46 
47static int __init procfs1_init(void) 
48{ 
49    our_proc_file = proc_create(procfs_name, 0644, NULL, &proc_file_fops); 
50    if (NULL == our_proc_file) { 
51        proc_remove(our_proc_file); 
52        pr_alert("Error:Could not initialize /proc/%sn", procfs_name); 
53        return -ENOMEM; 
54    } 
55 
56    pr_info("/proc/%s createdn", procfs_name); 
57    return 0; 
58} 
59 
60static void __exit procfs1_exit(void) 
61{ 
62    proc_remove(our_proc_file); 
63    pr_info("/proc/%s removedn", procfs_name); 
64} 
65 
66module_init(procfs1_init); 
67module_exit(procfs1_exit); 
68 
69MODULE_LICENSE("GPL");

7.1 The proc_ops Structure

The proc_ops
structure is defined in include/linux/proc_fs.h in Linux v5.6+. In older kernels, it
used file_operations
for custom hooks in /proc file system, but it contains some
members that are unnecessary in VFS, and every time VFS expands
file_operations
set, /proc code comes bloated. On the other hand, not only the space,
but also some operations were saved by this structure to improve its
performance. For example, the file which never disappears in /proc can set the
proc_flag
as PROC_ENTRY_PERMANENT
to save 2 atomic ops, 1 allocation, 1 free in per open/read/close sequence.

7.2 Read and Write a /proc File

We have seen a very simple example for a /proc file where we only read
the file /proc/helloworld. It is also possible to write in a /proc file. It
works the same way as read, a function is called when the /proc file
is written. But there is a little difference with read, data comes from
user, so you have to import data from user space to kernel space (with
copy_from_user
or get_user
)

The reason for copy_from_user
or get_user
is that Linux memory (on Intel architecture, it may be different under some

other processors) is segmented. This means that a pointer, by itself, does
not reference a unique location in memory, only a location in a memory
segment, and you need to know which memory segment it is to be able to use
it. There is one memory segment for the kernel, and one for each of the
processes.

The only memory segment accessible to a process is its own, so when
writing regular programs to run as processes, there is no need to worry about
segments. When you write a kernel module, normally you want to access
the kernel memory segment, which is handled automatically by the system.
However, when the content of a memory buffer needs to be passed between
the currently running process and the kernel, the kernel function receives
a pointer to the memory buffer which is in the process segment. The
put_user
and get_user
macros allow you to access that memory. These functions handle
only one character, you can handle several characters with
copy_to_user
and copy_from_user
. As the buffer (in read or write function) is in kernel space, for write function you
need to import data because it comes from user space, but not for the read function
because data is already in kernel space.

1/* 
2 * procfs2.c -  create a "file" in /proc 
3 */ 
4 
5#include  /* We're doing kernel work */ 
6#include  /* Specifically, a module */ 
7#include  /* Necessary because we use the proc fs */ 
8#include  /* for copy_from_user */ 
9#include  
10 
11#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0) 
12#define HAVE_PROC_OPS 
13#endif 
14 
15#define PROCFS_MAX_SIZE 1024 
16#define PROCFS_NAME "buffer1k" 
17 
18/* This structure hold information about the /proc file */ 
19static struct proc_dir_entry *our_proc_file; 
20 
21/* The buffer used to store character for this module */ 
22static char procfs_buffer[PROCFS_MAX_SIZE]; 
23 
24/* The size of the buffer */ 
25static unsigned long procfs_buffer_size = 0; 
26 
27/* This function is called then the /proc file is read */ 
28static ssize_t procfile_read(struct file *file_pointer, char __user *buffer, 
29                             size_t buffer_length, loff_t *offset) 
30{ 
31    char s[13] = "HelloWorld!n"; 
32    int len = sizeof(s); 
33    ssize_t ret = len; 
34 
35    if (*offset >= len || copy_to_user(buffer, s, len)) { 
36        pr_info("copy_to_user failedn"); 
37        ret = 0; 
38    } else { 
39        pr_info("procfile read %sn", file_pointer->f_path.dentry->d_name.name); 
40        *offset += len; 
41    } 
42 
43    return ret; 
44} 
45 
46/* This function is called with the /proc file is written. */ 
47static ssize_t procfile_write(struct file *file, const char __user *buff, 
48                              size_t len, loff_t *off) 
49{ 
50    procfs_buffer_size = len; 
51    if (procfs_buffer_size > PROCFS_MAX_SIZE) 
52        procfs_buffer_size = PROCFS_MAX_SIZE; 
53 
54    if (copy_from_user(procfs_buffer, buff, procfs_buffer_size)) 
55        return -EFAULT; 
56 
57    procfs_buffer[procfs_buffer_size & (PROCFS_MAX_SIZE - 1)] = ''; 
58    *off += procfs_buffer_size; 
59    pr_info("procfile write %sn", procfs_buffer); 
60 
61    return procfs_buffer_size; 
62} 
63 
64#ifdef HAVE_PROC_OPS 
65static const struct proc_ops proc_file_fops = { 
66    .proc_read = procfile_read, 
67    .proc_write = procfile_write, 
68}; 
69#else 
70static const struct file_operations proc_file_fops = { 
71    .read = procfile_read, 
72    .write = procfile_write, 
73}; 
74#endif 
75 
76static int __init procfs2_init(void) 
77{ 
78    our_proc_file = proc_create(PROCFS_NAME, 0644, NULL, &proc_file_fops); 
79    if (NULL == our_proc_file) { 
80        proc_remove(our_proc_file); 
81        pr_alert("Error:Could not initialize /proc/%sn", PROCFS_NAME); 
82        return -ENOMEM; 
83    } 
84 
85    pr_info("/proc/%s createdn", PROCFS_NAME); 
86    return 0; 
87} 
88 
89static void __exit procfs2_exit(void) 
90{ 
91    proc_remove(our_proc_file); 
92    pr_info("/proc/%s removedn", PROCFS_NAME); 
93} 
94 
95module_init(procfs2_init); 
96module_exit(procfs2_exit); 
97 
98MODULE_LICENSE("GPL");

7.3 Manage /proc file with standard filesystem

We have seen how to read and write a /proc file with the /proc interface. But it is
also possible to manage /proc file with inodes. The main concern is to use advanced
functions, like permissions.

In Linux, there is a standard mechanism for file system registration.
Since every file system has to have its own functions to handle inode and file
operations, there is a special structure to hold pointers to all those functions,
struct inode_operations
, which includes a pointer to struct proc_ops
.

The difference between file and inode operations is that file operations deal with
the file itself whereas inode operations deal with ways of referencing the file, such as
creating links to it.

In /proc, whenever we register a new file, we’re allowed to specify which
struct inode_operations
will be used to access to it. This is the mechanism we use, a
struct inode_operations

which includes a pointer to a struct proc_ops
which includes pointers to our procf_read
and procfs_write
functions.

Another interesting point here is the
module_permission
function. This function is called whenever a process tries to do something with the
/proc file, and it can decide whether to allow access or not. Right now it is only
based on the operation and the uid of the current user (as available in current, a
pointer to a structure which includes information on the currently running
process), but it could be based on anything we like, such as what other
processes are doing with the same file, the time of day, or the last input we
received.

It is important to note that the standard roles of read and write are reversed in
the kernel. Read functions are used for output, whereas write functions are used for
input. The reason for that is that read and write refer to the user’s point of view — if
a process reads something from the kernel, then the kernel needs to output it, and
if a process writes something to the kernel, then the kernel receives it as
input.

1/* 
2 * procfs3.c 
3 */ 
4 
5#include  
6#include  
7#include  
8#include  
9#include  
10#include  
11#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 10, 0) 
12#include  
13#endif 
14 
15#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0) 
16#define HAVE_PROC_OPS 
17#endif 
18 
19#define PROCFS_MAX_SIZE 2048UL 
20#define PROCFS_ENTRY_FILENAME "buffer2k" 
21 
22static struct proc_dir_entry *our_proc_file; 
23static char procfs_buffer[PROCFS_MAX_SIZE]; 
24static unsigned long procfs_buffer_size = 0; 
25 
26static ssize_t procfs_read(struct file *filp, char __user *buffer, 
27                           size_t length, loff_t *offset) 
28{ 
29    if (*offset || procfs_buffer_size == 0) { 
30        pr_debug("procfs_read: ENDn"); 
31        *offset = 0; 
32        return 0; 
33    } 
34    procfs_buffer_size = min(procfs_buffer_size, length); 
35    if (copy_to_user(buffer, procfs_buffer, procfs_buffer_size)) 
36        return -EFAULT; 
37    *offset += procfs_buffer_size; 
38 
39    pr_debug("procfs_read: read %lu bytesn", procfs_buffer_size); 
40    return procfs_buffer_size; 
41} 
42static ssize_t procfs_write(struct file *file, const char __user *buffer, 
43                            size_t len, loff_t *off) 
44{ 
45    procfs_buffer_size = min(PROCFS_MAX_SIZE, len); 
46    if (copy_from_user(procfs_buffer, buffer, procfs_buffer_size)) 
47        return -EFAULT; 
48    *off += procfs_buffer_size; 
49 
50    pr_debug("procfs_write: write %lu bytesn", procfs_buffer_size); 
51    return procfs_buffer_size; 
52} 
53static int procfs_open(struct inode *inode, struct file *file) 
54{ 
55    try_module_get(THIS_MODULE); 
56    return 0; 
57} 
58static int procfs_close(struct inode *inode, struct file *file) 
59{ 
60    module_put(THIS_MODULE); 
61    return 0; 
62} 
63 
64#ifdef HAVE_PROC_OPS 
65static struct proc_ops file_ops_4_our_proc_file = { 
66    .proc_read = procfs_read, 
67    .proc_write = procfs_write, 
68    .proc_open = procfs_open, 
69    .proc_release = procfs_close, 
70}; 
71#else 
72static const struct file_operations file_ops_4_our_proc_file = { 
73    .read = procfs_read, 
74    .write = procfs_write, 
75    .open = procfs_open, 
76    .release = procfs_close, 
77}; 
78#endif 
79 
80static int __init procfs3_init(void) 
81{ 
82    our_proc_file = proc_create(PROCFS_ENTRY_FILENAME, 0644, NULL, 
83                                &file_ops_4_our_proc_file); 
84    if (our_proc_file == NULL) { 
85        remove_proc_entry(PROCFS_ENTRY_FILENAME, NULL); 
86        pr_debug("Error: Could not initialize /proc/%sn", 
87                 PROCFS_ENTRY_FILENAME); 
88        return -ENOMEM; 
89    } 
90    proc_set_size(our_proc_file, 80); 
91    proc_set_user(our_proc_file, GLOBAL_ROOT_UID, GLOBAL_ROOT_GID); 
92 
93    pr_debug("/proc/%s createdn", PROCFS_ENTRY_FILENAME); 
94    return 0; 
95} 
96 
97static void __exit procfs3_exit(void) 
98{ 
99    remove_proc_entry(PROCFS_ENTRY_FILENAME, NULL); 
100    pr_debug("/proc/%s removedn", PROCFS_ENTRY_FILENAME); 
101} 
102 
103module_init(procfs3_init); 
104module_exit(procfs3_exit); 
105 
106MODULE_LICENSE("GPL");

Still hungry for procfs examples? Well, first of all keep in mind, there are rumors
around, claiming that procfs is on its way out, consider using sysfs instead. Consider
using this mechanism, in case you want to document something kernel related
yourself.

7.4 Manage /proc file with seq_file

As we have seen, writing a /proc file may be quite “complex”.
So to help people writting /proc file, there is an API named
seq_file
that helps formating a /proc file for output. It is based on sequence, which is composed of
3 functions: start()
, next()
, and stop()
. The seq_file
API starts a sequence when a user read the /proc file.

A sequence begins with the call of the function
start()
. If the return is a non NULL
value, the function next()
is called. This function is an iterator, the goal is to go through all the data. Each

time next()
is called, the function show()
is also called. It writes data values in the buffer read by the user. The function
next()
is called until it returns NULL
. The sequence ends when next()
returns NULL
, then the function stop()
is called.

BE CAREFUL: when a sequence is finished, another one starts. That means that at the end
of function stop()
, the function start()
is called again. This loop finishes when the function
start()
returns NULL
. You can see a scheme of this in the Figure 1.

srYrsNNYtaeenetoooertusetupstrxr((ntn))( tis)istrr teeaNreNatUaUtmLtLmeLmLen?e?ntntt

Figure 1:How seq_file works

The seq_file
provides basic functions for proc_ops
, such as seq_read
, seq_lseek
, and some others. But nothing to write in the /proc file. Of course, you can still use
the same way as in the previous example.

1/* 
2 * procfs4.c -  create a "file" in /proc 
3 * This program uses the seq_file library to manage the /proc file. 
4 */ 
5 
6#include  /* We are doing kernel work */ 
7#include  /* Specifically, a module */ 
8#include  /* Necessary because we use proc fs */ 
9#include  /* for seq_file */ 
10#include  
11 
12#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0) 
13#define HAVE_PROC_OPS 
14#endif 
15 
16#define PROC_NAME "iter" 
17 
18/* This function is called at the beginning of a sequence. 
19 * ie, when: 
20 *   - the /proc file is read (first time) 
21 *   - after the function stop (end of sequence) 
22 */ 
23static void *my_seq_start(struct seq_file *s, loff_t *pos) 
24{ 
25    static unsigned long counter = 0; 
26 
27    /* beginning a new sequence? */ 
28    if (*pos == 0) { 
29        /* yes => return a non null value to begin the sequence */ 
30        return &counter; 
31    } 
32 
33    /* no => it is the end of the sequence, return end to stop reading */ 
34    *pos = 0; 
35    return NULL; 
36} 
37 
38/* This function is called after the beginning of a sequence. 
39 * It is called untill the return is NULL (this ends the sequence). 
40 */ 
41static void *my_seq_next(struct seq_file *s, void *v, loff_t *pos) 
42{ 
43    unsigned long *tmp_v = (unsigned long *)v; 
44    (*tmp_v)++; 
45    (*pos)++; 
46    return NULL; 
47} 
48 
49/* This function is called at the end of a sequence. */ 
50static void my_seq_stop(struct seq_file *s, void *v) 
51{ 
52    /* nothing to do, we use a static value in start() */ 
53} 
54 
55/* This function is called for each "step" of a sequence. */ 
56static int my_seq_show(struct seq_file *s, void *v) 
57{ 
58    loff_t *spos = (loff_t *)v; 
59 
60    seq_printf(s, "%Ldn", *spos); 
61    return 0; 
62} 
63 
64/* This structure gather "function" to manage the sequence */ 
65static struct seq_operations my_seq_ops = { 
66    .start = my_seq_start, 
67    .next = my_seq_next, 
68    .stop = my_seq_stop, 
69    .show = my_seq_show, 
70}; 
71 
72/* This function is called when the /proc file is open. */ 
73static int my_open(struct inode *inode, struct file *file) 
74{ 
75    return seq_open(file, &my_seq_ops); 
76}; 
77 
78/* This structure gather "function" that manage the /proc file */ 
79#ifdef HAVE_PROC_OPS 
80static const struct proc_ops my_file_ops = { 
81    .proc_open = my_open, 
82    .proc_read = seq_read, 
83    .proc_lseek = seq_lseek, 
84    .proc_release = seq_release, 
85}; 
86#else 
87static const struct file_operations my_file_ops = { 
88    .open = my_open, 
89    .read = seq_read, 
90    .llseek = seq_lseek, 
91    .release = seq_release, 
92}; 
93#endif 
94 
95static int __init procfs4_init(void) 
96{ 
97    struct proc_dir_entry *entry; 
98 
99    entry = proc_create(PROC_NAME, 0, NULL, &my_file_ops); 
100    if (entry == NULL) { 
101        remove_proc_entry(PROC_NAME, NULL); 
102        pr_debug("Error: Could not initialize /proc/%sn", PROC_NAME); 
103        return -ENOMEM; 
104    } 
105 
106    return 0; 
107} 
108 
109static void __exit procfs4_exit(void) 
110{ 
111    remove_proc_entry(PROC_NAME, NULL); 
112    pr_debug("/proc/%s removedn", PROC_NAME); 
113} 
114 
115module_init(procfs4_init); 
116module_exit(procfs4_exit); 
117 
118MODULE_LICENSE("GPL");

If you want more information, you can read this web page:

You can also read the code of fs/seq_file.c in the linux kernel.

8 sysfs: Interacting with your module

sysfs allows you to interact with the running kernel from userspace by reading or
setting variables inside of modules. This can be useful for debugging purposes, or just
as an interface for applications or scripts. You can find sysfs directories and files
under the /sys directory on your system.

1ls -l /sys

Attributes can be exported for kobjects in the form of regular files in the
filesystem. Sysfs forwards file I/O operations to methods defined for the attributes,
providing a means to read and write kernel attributes.

An attribute definition in simply:

1struct attribute { 
2    char *name; 
3    struct module *owner; 
4    umode_t mode; 
5}; 
6 
7int sysfs_create_file(struct kobject * kobj, const struct attribute * attr); 
8void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr);

For example, the driver model defines
struct device_attribute
like:

1struct device_attribute { 
2    struct attribute attr; 
3    ssize_t (*show)(struct device *dev, struct device_attribute *attr, 
4                    char *buf); 
5    ssize_t (*store)(struct device *dev, struct device_attribute *attr, 
6                    const char *buf, size_t count); 
7}; 
8 
9int device_create_file(struct device *, const struct device_attribute *); 
10void device_remove_file(struct device *, const struct device_attribute *);

To read or write attributes, show()
or store()
method must be specified when declaring the attribute. For the
common cases include/linux/sysfs.h provides convenience macros
( __ATTR
, __ATTR_RO
, __ATTR_WO
, etc.) to make defining attributes easier as well as making code more concise and
readable.

An example of a hello world module which includes the creation of a variable
accessible via sysfs is given below.

1/* 
2 * hello-sysfs.c sysfs example 
3 */ 
4#include  
5#include  
6#include  
7#include  
8#include  
9#include  
10 
11static struct kobject *mymodule; 
12 
13/* the variable you want to be able to change */ 
14static int myvariable = 0; 
15 
16static ssize_t myvariable_show(struct kobject *kobj, 
17                               struct kobj_attribute *attr, char *buf) 
18{ 
19    return sprintf(buf, "%dn", myvariable); 
20} 
21 
22static ssize_t myvariable_store(struct kobject *kobj, 
23                                struct kobj_attribute *attr, char *buf, 
24                                size_t count) 
25{ 
26    sscanf(buf, "%du", &myvariable); 
27    return count; 
28} 
29 
30static struct kobj_attribute myvariable_attribute = 
31    __ATTR(myvariable, 0660, myvariable_show, (void *)myvariable_store); 
32 
33static int __init mymodule_init(void) 
34{ 
35    int error = 0; 
36 
37    pr_info("mymodule: initialisedn"); 
38 
39    mymodule = kobject_create_and_add("mymodule", kernel_kobj); 
40    if (!mymodule) 
41        return -ENOMEM; 
42 
43    error = sysfs_create_file(mymodule, &myvariable_attribute.attr); 
44    if (error) { 
45        pr_info("failed to create the myvariable file " 
46                "in /sys/kernel/mymodulen"); 
47    } 
48 
49    return error; 
50} 
51 
52static void __exit mymodule_exit(void) 
53{ 
54    pr_info("mymodule: Exit successn"); 
55    kobject_put(mymodule); 
56} 
57 
58module_init(mymodule_init); 
59module_exit(mymodule_exit); 
60 
61MODULE_LICENSE("GPL");

Make and install the module:

1make 
2sudo insmod hello-sysfs.ko

Check that it exists:

1sudo lsmod | grep hello_sysfs

What is the current value of myvariable
?

1cat /sys/kernel/mymodule/myvariable

Set the value of myvariable
and check that it changed.

1echo "32" > /sys/kernel/mymodule/myvariable 
2cat /sys/kernel/mymodule/myvariable

Finally, remove the test module:

1sudo rmmod hello_sysfs

In the above case, we use a simple kobject to create a directory under
sysfs, and communicate with its attributes. Since Linux v2.6.0, the
kobject
structure made its appearance. It was initially meant as a simple way of
unifying kernel code which manages reference counted objects. After a
bit of mission creep, it is now the glue that holds much of the device
model and its sysfs interface together. For more information about kobject
and sysfs, see Documentation/driver-api/driver-model/driver.rst and
https://lwn.net/Articles/51437/.

9 Talking To Device Files

Device files are supposed to represent physical devices. Most physical devices are
used for output as well as input, so there has to be some mechanism for
device drivers in the kernel to get the output to send to the device from
processes. This is done by opening the device file for output and writing to it,
just like writing to a file. In the following example, this is implemented by
device_write
.

This is not always enough. Imagine you had a serial port connected to a modem
(even if you have an internal modem, it is still implemented from the CPU’s
perspective as a serial port connected to a modem, so you don’t have to tax
your imagination too hard). The natural thing to do would be to use the
device file to write things to the modem (either modem commands or data to
be sent through the phone line) and read things from the modem (either
responses for commands or the data received through the phone line). However,
this leaves open the question of what to do when you need to talk to the
serial port itself, for example to configure the rate at which data is sent and
received.

The answer in Unix is to use a special function called
ioctl
(short for Input Output ConTroL). Every device can have its own
ioctl
commands, which can be read ioctl’s (to send information from a process to the
kernel), write ioctl’s (to return information to a process), both or neither. Notice
here the roles of read and write are reversed again, so in ioctl’s read is to
send information to the kernel and write is to receive information from the
kernel.

The ioctl function is called with three parameters: the file descriptor of the
appropriate device file, the ioctl number, and a parameter, which is of type long so
you can use a cast to use it to pass anything. You will not be able to pass a structure
this way, but you will be able to pass a pointer to the structure. Here is an
example:

1/* 
2 * ioctl.c 
3 */ 
4#include  
5#include  
6#include  
7#include  
8#include  
9#include  
10#include  
11 
12struct ioctl_arg { 
13    unsigned int val; 
14}; 
15 
16/* Documentation/ioctl/ioctl-number.txt */ 
17#define IOC_MAGIC 'x66' 
18 
19#define IOCTL_VALSET _IOW(IOC_MAGIC, 0, struct ioctl_arg) 
20#define IOCTL_VALGET _IOR(IOC_MAGIC, 1, struct ioctl_arg) 
21#define IOCTL_VALGET_NUM _IOR(IOC_MAGIC, 2, int) 
22#define IOCTL_VALSET_NUM _IOW(IOC_MAGIC, 3, int) 
23 
24#define IOCTL_VAL_MAXNR 3 
25#define DRIVER_NAME "ioctltest" 
26 
27static unsigned int test_ioctl_major = 0; 
28static unsigned int num_of_dev = 1; 
29static struct cdev test_ioctl_cdev; 
30static int ioctl_num = 0; 
31 
32struct test_ioctl_data { 
33    unsigned char val; 
34    rwlock_t lock; 
35}; 
36 
37static long test_ioctl_ioctl(struct file *filp, unsigned int cmd, 
38                             unsigned long arg) 
39{ 
40    struct test_ioctl_data *ioctl_data = filp->private_data; 
41    int retval = 0; 
42    unsigned char val; 
43    struct ioctl_arg data; 
44    memset(&data, 0, sizeof(data)); 
45 
46    switch (cmd) { 
47    case IOCTL_VALSET: 
48        if (copy_from_user(&data, (int __user *)arg, sizeof(data))) { 
49            retval = -EFAULT; 
50            goto done; 
51        } 
52 
53        pr_alert("IOCTL set val:%x .n", data.val); 
54        write_lock(&ioctl_data->lock); 
55        ioctl_data->val = data.val; 
56        write_unlock(&ioctl_data->lock); 
57        break; 
58 
59    case IOCTL_VALGET: 
60        read_lock(&ioctl_data->lock); 
61        val = ioctl_data->val; 
62        read_unlock(&ioctl_data->lock); 
63        data.val = val; 
64 
65        if (copy_to_user((int __user *)arg, &data, sizeof(data))) { 
66            retval = -EFAULT; 
67            goto done; 
68        } 
69 
70        break; 
71 
72    case IOCTL_VALGET_NUM: 
73        retval = __put_user(ioctl_num, (int __user *)arg); 
74        break; 
75 
76    case IOCTL_VALSET_NUM: 
77        ioctl_num = arg; 
78        break; 
79 
80    default: 
81        retval = -ENOTTY; 
82    } 
83 
84done: 
85    return retval; 
86} 
87 
88static ssize_t test_ioctl_read(struct file *filp, char __user *buf, 
89                               size_t count, loff_t *f_pos) 
90{ 
91    struct test_ioctl_data *ioctl_data = filp->private_data; 
92    unsigned char val; 
93    int retval; 
94    int i = 0; 
95 
96    read_lock(&ioctl_data->lock); 
97    val = ioctl_data->val; 
98    read_unlock(&ioctl_data->lock); 
99 
100    for (; i < count; i++) { 
101        if (copy_to_user(&buf[i], &val, 1)) { 
102            retval = -EFAULT; 
103            goto out; 
104        } 
105    } 
106 
107    retval = count; 
108out: 
109    return retval; 
110} 
111 
112static int test_ioctl_close(struct inode *inode, struct file *filp) 
113{ 
114    pr_alert("%s call.n", __func__); 
115 
116    if (filp->private_data) { 
117        kfree(filp->private_data); 
118        filp->private_data = NULL; 
119    } 
120 
121    return 0; 
122} 
123 
124static int test_ioctl_open(struct inode *inode, struct file *filp) 
125{ 
126    struct test_ioctl_data *ioctl_data; 
127 
128    pr_alert("%s call.n", __func__); 
129    ioctl_data = kmalloc(sizeof(struct test_ioctl_data), GFP_KERNEL); 
130 
131    if (ioctl_data == NULL) 
132        return -ENOMEM; 
133 
134    rwlock_init(&ioctl_data->lock); 
135    ioctl_data->val = 0xFF; 
136    filp->private_data = ioctl_data; 
137 
138    return 0; 
139} 
140 
141static struct file_operations fops = { 
142    .owner = THIS_MODULE, 
143    .open = test_ioctl_open, 
144    .release = test_ioctl_close, 
145    .read = test_ioctl_read, 
146    .unlocked_ioctl = test_ioctl_ioctl, 
147}; 
148 
149static int ioctl_init(void) 
150{ 
151    dev_t dev; 
152    int alloc_ret = -1; 
153    int cdev_ret = -1; 
154    alloc_ret = alloc_chrdev_region(&dev, 0, num_of_dev, DRIVER_NAME); 
155 
156    if (alloc_ret) 
157        goto error; 
158 
159    test_ioctl_major = MAJOR(dev); 
160    cdev_init(&test_ioctl_cdev, &fops); 
161    cdev_ret = cdev_add(&test_ioctl_cdev, dev, num_of_dev); 
162 
163    if (cdev_ret) 
164        goto error; 
165 
166    pr_alert("%s driver(major: %d) installed.n", DRIVER_NAME, 
167             test_ioctl_major); 
168    return 0; 
169error: 
170    if (cdev_ret == 0) 
171        cdev_del(&test_ioctl_cdev); 
172    if (alloc_ret == 0) 
173        unregister_chrdev_region(dev, num_of_dev); 
174    return -1; 
175} 
176 
177static void ioctl_exit(void) 
178{ 
179    dev_t dev = MKDEV(test_ioctl_major, 0); 
180 
181    cdev_del(&test_ioctl_cdev); 
182    unregister_chrdev_region(dev, num_of_dev); 
183    pr_alert("%s driver removed.n", DRIVER_NAME); 
184} 
185 
186module_init(ioctl_init); 
187module_exit(ioctl_exit); 
188 
189MODULE_LICENSE("GPL"); 
190MODULE_DESCRIPTION("This is test_ioctl module");

You can see there is an argument called
cmd
in test_ioctl_ioctl()
function. It is the ioctl number. The ioctl number encodes the major
device number, the type of the ioctl, the command, and the type of
the parameter. This ioctl number is usually created by a macro call
( _IO
, _IOR
, _IOW
or _IOWR
— depending on the type) in a header file. This header file should then be
included both by the programs which will use ioctl (so they can generate the
appropriate ioctl’s) and by the kernel module (so it can understand it). In the
example below, the header file is chardev.h and the program which uses it is
userspace_ioctl.c.

If you want to use ioctls in your own kernel modules, it is best to receive an
official ioctl assignment, so if you accidentally get somebody else’s ioctls, or if they
get yours, you’ll know something is wrong. For more information, consult the kernel
source tree at Documentation/userspace-api/ioctl/ioctl-number.rst.

Also, we need to be careful that concurrent access to the shared resources will
lead to the race condition. The solution is using atomic Compare-And-Swap (CAS),
which we mentioned at 6.5 section, to enforce the exclusive access.

1/* 
2 * chardev2.c - Create an input/output character device 
3 */ 
4 
5#include  
6#include  
7#include  
8#include  
9#include  
10#include  
11#include  /* Specifically, a module */ 
12#include  
13#include  
14#include  /* for get_user and put_user */ 
15 
16#include  
17 
18#include "chardev.h" 
19#define SUCCESS 0 
20#define DEVICE_NAME "char_dev" 
21#define BUF_LEN 80 
22 
23enum { 
24    CDEV_NOT_USED = 0, 
25    CDEV_EXCLUSIVE_OPEN = 1, 
26}; 
27 
28/* Is the device open right now? Used to prevent concurrent access into 
29 * the same device 
30 */ 
31static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
32 
33/* The message the device will give when asked */ 
34static char message[BUF_LEN + 1]; 
35 
36static struct class *cls; 
37 
38/* This is called whenever a process attempts to open the device file */ 
39static int device_open(struct inode *inode, struct file *file) 
40{ 
41    pr_info("device_open(%p)n", file); 
42 
43    try_module_get(THIS_MODULE); 
44    return SUCCESS; 
45} 
46 
47static int device_release(struct inode *inode, struct file *file) 
48{ 
49    pr_info("device_release(%p,%p)n", inode, file); 
50 
51    module_put(THIS_MODULE); 
52    return SUCCESS; 
53} 
54 
55/* This function is called whenever a process which has already opened the 
56 * device file attempts to read from it. 
57 */ 
58static ssize_t device_read(struct file *file, /* see include/linux/fs.h   */ 
59                           char __user *buffer, /* buffer to be filled  */ 
60                           size_t length, /* length of the buffer     */ 
61                           loff_t *offset) 
62{ 
63    /* Number of bytes actually written to the buffer */ 
64    int bytes_read = 0; 
65    /* How far did the process reading the message get? Useful if the message 
66     * is larger than the size of the buffer we get to fill in device_read. 
67     */ 
68    const char *message_ptr = message; 
69 
70    if (!*(message_ptr + *offset)) { /* we are at the end of message */ 
71        *offset = 0; /* reset the offset */ 
72        return 0; /* signify end of file */ 
73    } 
74 
75    message_ptr += *offset; 
76 
77    /* Actually put the data into the buffer */ 
78    while (length && *message_ptr) { 
79        /* Because the buffer is in the user data segment, not the kernel 
80         * data segment, assignment would not work. Instead, we have to 
81         * use put_user which copies data from the kernel data segment to 
82         * the user data segment. 
83         */ 
84        put_user(*(message_ptr++), buffer++); 
85        length--; 
86        bytes_read++; 
87    } 
88 
89    pr_info("Read %d bytes, %ld leftn", bytes_read, length); 
90 
91    *offset += bytes_read; 
92 
93    /* Read functions are supposed to return the number of bytes actually 
94     * inserted into the buffer. 
95     */ 
96    return bytes_read; 
97} 
98 
99/* called when somebody tries to write into our device file. */ 
100static ssize_t device_write(struct file *file, const char __user *buffer, 
101                            size_t length, loff_t *offset) 
102{ 
103    int i; 
104 
105    pr_info("device_write(%p,%p,%ld)", file, buffer, length); 
106 
107    for (i = 0; i < length && i < BUF_LEN; i++) 
108        get_user(message[i], buffer + i); 
109 
110    /* Again, return the number of input characters used. */ 
111    return i; 
112} 
113 
114/* This function is called whenever a process tries to do an ioctl on our 
115 * device file. We get two extra parameters (additional to the inode and file 
116 * structures, which all device functions get): the number of the ioctl called 
117 * and the parameter given to the ioctl function. 
118 * 
119 * If the ioctl is write or read/write (meaning output is returned to the 
120 * calling process), the ioctl call returns the output of this function. 
121 */ 
122static long 
123device_ioctl(struct file *file, /* ditto */ 
124             unsigned int ioctl_num, /* number and param for ioctl */ 
125             unsigned long ioctl_param) 
126{ 
127    int i; 
128    long ret = SUCCESS; 
129 
130    /* We don't want to talk to two processes at the same time. */ 
131    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
132        return -EBUSY; 
133 
134    /* Switch according to the ioctl called */ 
135    switch (ioctl_num) { 
136    case IOCTL_SET_MSG: { 
137        /* Receive a pointer to a message (in user space) and set that to 
138         * be the device's message. Get the parameter given to ioctl by 
139         * the process. 
140         */ 
141        char __user *tmp = (char __user *)ioctl_param; 
142        char ch; 
143 
144        /* Find the length of the message */ 
145        get_user(ch, tmp); 
146        for (i = 0; ch && i < BUF_LEN; i++, tmp++) 
147            get_user(ch, tmp); 
148 
149        device_write(file, (char __user *)ioctl_param, i, NULL); 
150        break; 
151    } 
152    case IOCTL_GET_MSG: { 
153        loff_t offset = 0; 
154 
155        /* Give the current message to the calling process - the parameter 
156         * we got is a pointer, fill it. 
157         */ 
158        i = device_read(file, (char __user *)ioctl_param, 99, &offset); 
159 
160        /* Put a zero at the end of the buffer, so it will be properly 
161         * terminated. 
162         */ 
163        put_user('', (char __user *)ioctl_param + i); 
164        break; 
165    } 
166    case IOCTL_GET_NTH_BYTE: 
167        /* This ioctl is both input (ioctl_param) and output (the return 
168         * value of this function). 
169         */ 
170        ret = (long)message[ioctl_param]; 
171        break; 
172    } 
173 
174    /* We're now ready for our next caller */ 
175    atomic_set(&already_open, CDEV_NOT_USED); 
176 
177    return ret; 
178} 
179 
180/* Module Declarations */ 
181 
182/* This structure will hold the functions to be called when a process does 
183 * something to the device we created. Since a pointer to this structure 
184 * is kept in the devices table, it can't be local to init_module. NULL is 
185 * for unimplemented functions. 
186 */ 
187static struct file_operations fops = { 
188    .read = device_read, 
189    .write = device_write, 
190    .unlocked_ioctl = device_ioctl, 
191    .open = device_open, 
192    .release = device_release, /* a.k.a. close */ 
193}; 
194 
195/* Initialize the module - Register the character device */ 
196static int __init chardev2_init(void) 
197{ 
198    /* Register the character device (atleast try) */ 
199    int ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &fops); 
200 
201    /* Negative values signify an error */ 
202    if (ret_val < 0) { 
203        pr_alert("%s failed with %dn", 
204                 "Sorry, registering the character device ", ret_val); 
205        return ret_val; 
206    } 
207 
208    cls = class_create(THIS_MODULE, DEVICE_FILE_NAME); 
209    device_create(cls, NULL, MKDEV(MAJOR_NUM, 0), NULL, DEVICE_FILE_NAME); 
210 
211    pr_info("Device created on /dev/%sn", DEVICE_FILE_NAME); 
212 
213    return 0; 
214} 
215 
216/* Cleanup - unregister the appropriate file from /proc */ 
217static void __exit chardev2_exit(void) 
218{ 
219    device_destroy(cls, MKDEV(MAJOR_NUM, 0)); 
220    class_destroy(cls); 
221 
222    /* Unregister the device */ 
223    unregister_chrdev(MAJOR_NUM, DEVICE_NAME); 
224} 
225 
226module_init(chardev2_init); 
227module_exit(chardev2_exit); 
228 
229MODULE_LICENSE("GPL");
1/* 
2 * chardev.h - the header file with the ioctl definitions. 
3 * 
4 * The declarations here have to be in a header file, because they need 
5 * to be known both to the kernel module (in chardev2.c) and the process 
6 * calling ioctl() (in userspace_ioctl.c). 
7 */ 
8 
9#ifndef CHARDEV_H 
10#define CHARDEV_H 
11 
12#include  
13 
14/* The major device number. We can not rely on dynamic registration 
15 * any more, because ioctls need to know it. 
16 */ 
17#define MAJOR_NUM 100 
18 
19/* Set the message of the device driver */ 
20#define IOCTL_SET_MSG _IOW(MAJOR_NUM, 0, char *) 
21/* _IOW means that we are creating an ioctl command number for passing 
22 * information from a user process to the kernel module. 
23 * 
24 * The first arguments, MAJOR_NUM, is the major device number we are using. 
25 * 
26 * The second argument is the number of the command (there could be several 
27 * with different meanings). 
28 * 
29 * The third argument is the type we want to get from the process to the 
30 * kernel. 
31 */ 
32 
33/* Get the message of the device driver */ 
34#define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *) 
35/* This IOCTL is used for output, to get the message of the device driver. 
36 * However, we still need the buffer to place the message in to be input, 
37 * as it is allocated by the process. 
38 */ 
39 
40/* Get the n'th byte of the message */ 
41#define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int) 
42/* The IOCTL is used for both input and output. It receives from the user 
43 * a number, n, and returns message[n]. 
44 */ 
45 
46/* The name of the device file */ 
47#define DEVICE_FILE_NAME "char_dev" 
48#define DEVICE_PATH "/dev/char_dev" 
49 
50#endif
1/*  userspace_ioctl.c - the process to use ioctl's to control the kernel module 
2 * 
3 *  Until now we could have used cat for input and output.  But now 
4 *  we need to do ioctl's, which require writing our own process.  
5 */ 
6 
7/* device specifics, such as ioctl numbers and the  
8 * major device file. */ 
9#include "../chardev.h" 
10 
11#include  /* standard I/O */ 
12#include  /* open */ 
13#include  /* close */ 
14#include  /* exit */ 
15#include  /* ioctl */ 
16 
17/* Functions for the ioctl calls */ 
18 
19int ioctl_set_msg(int file_desc, char *message) 
20{ 
21    int ret_val; 
22 
23    ret_val = ioctl(file_desc, IOCTL_SET_MSG, message); 
24 
25    if (ret_val < 0) { 
26        printf("ioctl_set_msg failed:%dn", ret_val); 
27    } 
28 
29    return ret_val; 
30} 
31 
32int ioctl_get_msg(int file_desc) 
33{ 
34    int ret_val; 
35    char message[100] = { 0 }; 
36 
37    /* Warning - this is dangerous because we don't tell  
38   * the kernel how far it's allowed to write, so it  
39   * might overflow the buffer. In a real production  
40   * program, we would have used two ioctls - one to tell 
41   * the kernel the buffer length and another to give  
42   * it the buffer to fill 
43   */ 
44    ret_val = ioctl(file_desc, IOCTL_GET_MSG, message); 
45 
46    if (ret_val < 0) { 
47        printf("ioctl_get_msg failed:%dn", ret_val); 
48    } 
49    printf("get_msg message:%s", message); 
50 
51    return ret_val; 
52} 
53 
54int ioctl_get_nth_byte(int file_desc) 
55{ 
56    int i, c; 
57 
58    printf("get_nth_byte message:"); 
59 
60    i = 0; 
61    do { 
62        c = ioctl(file_desc, IOCTL_GET_NTH_BYTE, i++); 
63 
64        if (c < 0) { 
65            printf("nioctl_get_nth_byte failed at the %d'th byte:n", i); 
66            return c; 
67        } 
68 
69        putchar(c); 
70    } while (c != 0); 
71 
72    return 0; 
73} 
74 
75/* Main - Call the ioctl functions */ 
76int main(void) 
77{ 
78    int file_desc, ret_val; 
79    char *msg = "Message passed by ioctln"; 
80 
81    file_desc = open(DEVICE_PATH, O_RDWR); 
82    if (file_desc < 0) { 
83        printf("Can't open device file: %s, error:%dn", DEVICE_PATH, 
84               file_desc); 
85        exit(EXIT_FAILURE); 
86    } 
87 
88    ret_val = ioctl_set_msg(file_desc, msg); 
89    if (ret_val) 
90        goto error; 
91    ret_val = ioctl_get_nth_byte(file_desc); 
92    if (ret_val) 
93        goto error; 
94    ret_val = ioctl_get_msg(file_desc); 
95    if (ret_val) 
96        goto error; 
97 
98    close(file_desc); 
99    return 0; 
100error: 
101    close(file_desc); 
102    exit(EXIT_FAILURE); 
103}

10 System Calls

So far, the only thing we’ve done was to use well defined kernel mechanisms to
register /proc files and device handlers. This is fine if you want to do something the
kernel programmers thought you’d want, such as write a device driver. But what if

you want to do something unusual, to change the behavior of the system in some
way? Then, you are mostly on your own.

If you are not being sensible and using a virtual machine then this is where kernel
programming can become hazardous. While writing the example below, I killed the
open()
system call. This meant I could not open any files, I could not run any
programs, and I could not shutdown the system. I had to restart the virtual
machine. No important files got annihilated, but if I was doing this on some live
mission critical system then that could have been a possible outcome. To
ensure you do not lose any files, even within a test environment, please run
sync
right before you do the insmod
and the rmmod
.

Forget about /proc files, forget about device files. They are just minor details.
Minutiae in the vast expanse of the universe. The real process to kernel
communication mechanism, the one used by all processes, is system calls. When a
process requests a service from the kernel (such as opening a file, forking to a new
process, or requesting more memory), this is the mechanism used. If you want to
change the behaviour of the kernel in interesting ways, this is the place to do
it. By the way, if you want to see which system calls a program uses, run
strace 
.

In general, a process is not supposed to be able to access the kernel. It can not
access kernel memory and it can’t call kernel functions. The hardware of the CPU
enforces this (that is the reason why it is called “protected mode” or “page
protection”).

System calls are an exception to this general rule. What happens is that the
process fills the registers with the appropriate values and then calls a special
instruction which jumps to a previously defined location in the kernel (of course, that
location is readable by user processes, it is not writable by them). Under Intel CPUs,
this is done by means of interrupt 0x80. The hardware knows that once you jump to
this location, you are no longer running in restricted user mode, but as the
operating system kernel — and therefore you’re allowed to do whatever you
want.

The location in the kernel a process can jump to is called system_call. The
procedure at that location checks the system call number, which tells the kernel what
service the process requested. Then, it looks at the table of system calls
( sys_call_table
) to see the address of the kernel function to call. Then it calls the function, and after
it returns, does a few system checks and then return back to the process (or to a
different process, if the process time ran out). If you want to read this code, it is
at the source file arch/$(architecture)/kernel/entry.S, after the line
ENTRY(system_call)
.

So, if we want to change the way a certain system call works, what we need to do

is to write our own function to implement it (usually by adding a bit of our own
code, and then calling the original function) and then change the pointer at
sys_call_table
to point to our function. Because we might be removed later and we
don’t want to leave the system in an unstable state, it’s important for
cleanup_module
to restore the table to its original state.

To modify the content of sys_call_table
, we need to consider the control register. A control register is a processor
register that changes or controls the general behavior of the CPU. For x86
architecture, the cr0 register has various control flags that modify the basic
operation of the processor. The WP flag in cr0 stands for write protection.
Once the WP flag is set, the processor disallows further write attempts to the
read-only sections Therefore, we must disable the WP flag before modifying
sys_call_table
. Since Linux v5.3, the write_cr0
function cannot be used because of the sensitive cr0 bits pinned by the security
issue, the attacker may write into CPU control registers to disable CPU protections
like write protection. As a result, we have to provide the custom assembly routine to
bypass it.

However, sys_call_table
symbol is unexported to prevent misuse. But there have few ways to get the symbol, manual
symbol lookup and kallsyms_lookup_name
. Here we use both depend on the kernel version.

Because of the control-flow integrity, which is a technique to prevent the redirect
execution code from the attacker, for making sure that the indirect calls go to the
expected addresses and the return addresses are not changed. Since Linux v5.7, the
kernel patched the series of control-flow enforcement (CET) for x86, and some
configurations of GCC, like GCC versions 9 and 10 in Ubuntu, will add with CET
(the -fcf-protection option) in the kernel by default. Using that GCC to compile
the kernel with retpoline off may result in CET being enabled in the kernel. You can
use the following command to check out the -fcf-protection option is enabled or
not:

$ gcc -v -Q -O2 --help=target | grep protection
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
...
gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)
COLLECT_GCC_OPTIONS='-v' '-Q' '-O2' '--help=target' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/9/cc1 -v ... -fcf-protection ...
 GNU C17 (Ubuntu 9.3.0-17ubuntu1~20.04) version 9.3.0 (x86_64-linux-gnu)
...

But CET should not be enabled in the kernel, it may break the Kprobes and bpf.
Consequently, CET is disabled since v5.11. To guarantee the manual symbol lookup
worked, we only use up to v5.4.

Unfortunately, since Linux v5.7 kallsyms_lookup_name
is also unexported, it needs certain trick to get the address of
kallsyms_lookup_name
. If CONFIG_KPROBES
is enabled, we can facilitate the retrieval of function addresses by means of Kprobes
to dynamically break into the specific kernel routine. Kprobes inserts a breakpoint at
the entry of function by replacing the first bytes of the probed instruction. When a
CPU hits the breakpoint, registers are stored, and the control will pass to Kprobes. It
passes the addresses of the saved registers and the Kprobe struct to the handler
you defined, then executes it. Kprobes can be registered by symbol name
or address. Within the symbol name, the address will be handled by the
kernel.

Otherwise, specify the address of sys_call_table
from /proc/kallsyms and /boot/System.map into
sym
parameter. Following is the sample usage for /proc/kallsyms:

$ sudo grep sys_call_table /proc/kallsyms
ffffffff82000280 R x32_sys_call_table
ffffffff820013a0 R sys_call_table
ffffffff820023e0 R ia32_sys_call_table
$ sudo insmod syscall.ko sym=0xffffffff820013a0

Using the address from /boot/System.map, be careful about KASLR (Kernel
Address Space Layout Randomization). KASLR may randomize the address of
kernel code and data at every boot time, such as the static address listed in
/boot/System.map will offset by some entropy. The purpose of KASLR is to protect
the kernel space from the attacker. Without KASLR, the attacker may find the target
address in the fixed address easily. Then the attacker can use return-oriented
programming to insert some malicious codes to execute or receive the target data by
a tampered pointer. KASLR mitigates these kinds of attacks because the attacker
cannot immediately know the target address, but a brute-force attack can still work.
If the address of a symbol in /proc/kallsyms is different from the address in
/boot/System.map, KASLR is enabled with the kernel, which your system running
on.

$ grep GRUB_CMDLINE_LINUX_DEFAULT /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
$ sudo grep sys_call_table /boot/System.map-$(uname -r)
ffffffff82000300 R sys_call_table
$ sudo grep sys_call_table /proc/kallsyms
ffffffff820013a0 R sys_call_table
# Reboot
$ sudo grep sys_call_table /boot/System.map-$(uname -r)
ffffffff82000300 R sys_call_table
$ sudo grep sys_call_table /proc/kallsyms
ffffffff86400300 R sys_call_table

If KASLR is enabled, we have to take care of the address from /proc/kallsyms each
time we reboot the machine. In order to use the address from /boot/System.map,
make sure that KASLR is disabled. You can add the nokaslr for disabling KASLR in
next booting time:

$ grep GRUB_CMDLINE_LINUX_DEFAULT /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
$ sudo perl -i -pe 'm/quiet/ and s//quiet nokaslr/' /etc/default/grub
$ grep quiet /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet nokaslr splash"
$ sudo update-grub

For more information, check out the following:

The source code here is an example of such a kernel module. We want to “spy” on a certain
user, and to pr_info()
a message whenever that user opens a file. Towards this end, we
replace the system call to open a file with our own function, called
our_sys_openat
. This function checks the uid (user’s id) of the current process, and if it is equal to the uid we

spy on, it calls pr_info()
to display the name of the file to be opened. Then, either way, it calls the original
openat()
function with the same parameters, to actually open the file.

The init_module
function replaces the appropriate location in
sys_call_table
and keeps the original pointer in a variable. The
cleanup_module
function uses that variable to restore everything back to normal. This approach is
dangerous, because of the possibility of two kernel modules changing the same system
call. Imagine we have two kernel modules, A and B. A’s openat system call will be
A_openat
and B’s will be B_openat
. Now, when A is inserted into the kernel, the system call is replaced with
A_openat
, which will call the original sys_openat
when it is done. Next, B is inserted into the kernel, which replaces the system call
with B_openat
, which will call what it thinks is the original system call,
A_openat
, when it’s done.

Now, if B is removed first, everything will be well — it will simply restore the system
call to A_openat
, which calls the original. However, if A is removed and then B is removed, the
system will crash. A’s removal will restore the system call to the original,
sys_openat
, cutting B out of the loop. Then, when B is removed, it will restore the system call to what it thinks
is the original, A_openat
, which is no longer in memory. At first glance, it appears we could solve
this particular problem by checking if the system call is equal to our
open function and if so not changing it at all (so that B won’t change
the system call when it is removed), but that will cause an even worse
problem. When A is removed, it sees that the system call was changed to
B_openat
so that it is no longer pointing to A_openat
, so it will not restore it to sys_openat
before it is removed from memory. Unfortunately,
B_openat
will still try to call A_openat
which is no longer there, so that even without removing B the system would
crash.

Note that all the related problems make syscall stealing unfeasible for
production use. In order to keep people from doing potential harmful things
sys_call_table
is no longer exported. This means, if you want to do something more than a mere

dry run of this example, you will have to patch your current kernel in order to have
sys_call_table
exported.

1/* 
2 * syscall.c 
3 * 
4 * System call "stealing" sample. 
5 * 
6 * Disables page protection at a processor level by changing the 16th bit 
7 * in the cr0 register (could be Intel specific). 
8 * 
9 * Based on example by Peter Jay Salzman and 
10 * https://bbs.archlinux.org/viewtopic.php?id=139406 
11 */ 
12 
13#include  
14#include  
15#include  
16#include  /* which will have params */ 
17#include  /* The list of system calls */ 
18#include  /* For current_uid() */ 
19#include  /* For __kuid_val() */ 
20#include  
21 
22/* For the current (process) structure, we need this to know who the 
23 * current user is. 
24 */ 
25#include  
26#include  
27 
28/* The way we access "sys_call_table" varies as kernel internal changes. 
29 * - Prior to v5.4 : manual symbol lookup 
30 * - v5.5 to v5.6  : use kallsyms_lookup_name() 
31 * - v5.7+         : Kprobes or specific kernel module parameter 
32 */ 
33 
34/* The in-kernel calls to the ksys_close() syscall were removed in Linux v5.11+. 
35 */ 
36#if (LINUX_VERSION_CODE < KERNEL_VERSION(5, 7, 0)) 
37 
38#if LINUX_VERSION_CODE <= KERNEL_VERSION(5, 4, 0) 
39#define HAVE_KSYS_CLOSE 1 
40#include  /* For ksys_close() */ 
41#else 
42#include  /* For kallsyms_lookup_name */ 
43#endif 
44 
45#else 
46 
47#if defined(CONFIG_KPROBES) 
48#define HAVE_KPROBES 1 
49#include  
50#else 
51#define HAVE_PARAM 1 
52#include  /* For sprint_symbol */ 
53/* The address of the sys_call_table, which can be obtained with looking up 
54 * "/boot/System.map" or "/proc/kallsyms". When the kernel version is v5.7+, 
55 * without CONFIG_KPROBES, you can input the parameter or the module will look 
56 * up all the memory. 
57 */ 
58static unsigned long sym = 0; 
59module_param(sym, ulong, 0644); 
60#endif /* CONFIG_KPROBES */ 
61 
62#endif /* Version < v5.7 */ 
63 
64static unsigned long **sys_call_table; 
65 
66/* UID we want to spy on - will be filled from the command line. */ 
67static uid_t uid = -1; 
68module_param(uid, int, 0644); 
69 
70/* A pointer to the original system call. The reason we keep this, rather 
71 * than call the original function (sys_openat), is because somebody else 
72 * might have replaced the system call before us. Note that this is not 
73 * 100% safe, because if another module replaced sys_openat before us, 
74 * then when we are inserted, we will call the function in that module - 
75 * and it might be removed before we are. 
76 * 
77 * Another reason for this is that we can not get sys_openat. 
78 * It is a static variable, so it is not exported. 
79 */ 
80#ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER 
81static asmlinkage long (*original_call)(const struct pt_regs *); 
82#else 
83static asmlinkage long (*original_call)(intconst char __user *, int, umode_t); 
84#endif 
85 
86/* The function we will replace sys_openat (the function called when you 
87 * call the open system call) with. To find the exact prototype, with 
88 * the number and type of arguments, we find the original function first 
89 * (it is at fs/open.c). 
90 * 
91 * In theory, this means that we are tied to the current version of the 
92 * kernel. In practice, the system calls almost never change (it would 
93 * wreck havoc and require programs to be recompiled, since the system 
94 * calls are the interface between the kernel and the processes). 
95 */ 
96#ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER 
97static asmlinkage long our_sys_openat(const struct pt_regs *regs) 
98#else 
99static asmlinkage long our_sys_openat(int dfd, const char __user *filename, 
100                                      int flags, umode_t mode) 
101#endif 
102{ 
103    int i = 0; 
104    char ch; 
105 
106    if (__kuid_val(current_uid()) != uid) 
107        goto orig_call; 
108 
109    /* Report the file, if relevant */ 
110    pr_info("Opened file by %d: ", uid); 
111    do { 
112#ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER 
113        get_user(ch, (char __user *)regs->si + i); 
114#else 
115        get_user(ch, (char __user *)filename + i); 
116#endif 
117        i++; 
118        pr_info("%c", ch); 
119    } while (ch != 0); 
120    pr_info("n"); 
121 
122orig_call: 
123    /* Call the original sys_openat - otherwise, we lose the ability to 
124     * open files. 
125     */ 
126#ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER 
127    return original_call(regs); 
128#else 
129    return original_call(dfd, filename, flags, mode); 
130#endif 
131} 
132 
133static unsigned long **acquire_sys_call_table(void) 
134{ 
135#ifdef HAVE_KSYS_CLOSE 
136    unsigned long int offset = PAGE_OFFSET; 
137    unsigned long **sct; 
138 
139    while (offset < ULLONG_MAX) { 
140        sct = (unsigned long **)offset; 
141 
142        if (sct[__NR_close] == (unsigned long *)ksys_close) 
143            return sct; 
144 
145        offset += sizeof(void *); 
146    } 
147 
148    return NULL; 
149#endif 
150 
151#ifdef HAVE_PARAM 
152    const char sct_name[15] = "sys_call_table"; 
153    char symbol[40] = { 0 }; 
154 
155    if (sym == 0) { 
156        pr_alert("For Linux v5.7+, Kprobes is the preferable way to get " 
157                 "symbol.n"); 
158        pr_info("If Kprobes is absent, you have to specify the address of " 
159                "sys_call_table symboln"); 
160        pr_info("by /boot/System.map or /proc/kallsyms, which contains all the " 
161                "symbol addresses, into sym parameter.n"); 
162        return NULL; 
163    } 
164    sprint_symbol(symbol, sym); 
165    if (!strncmp(sct_name, symbol, sizeof(sct_name) - 1)) 
166        return (unsigned long **)sym; 
167 
168    return NULL; 
169#endif 
170 
171#ifdef HAVE_KPROBES 
172    unsigned long (*kallsyms_lookup_name)(const char *name); 
173    struct kprobe kp = { 
174        .symbol_name = "kallsyms_lookup_name", 
175    }; 
176 
177    if (register_kprobe(&kp) < 0) 
178        return NULL; 
179    kallsyms_lookup_name = (unsigned long (*)(const char *name))kp.addr; 
180    unregister_kprobe(&kp); 
181#endif 
182 
183    return (unsigned long **)kallsyms_lookup_name("sys_call_table"); 
184} 
185 
186#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 3, 0) 
187static inline void __write_cr0(unsigned long cr0) 
188{ 
189    asm volatile("mov %0,%%cr0" : "+r"(cr0) : : "memory"); 
190} 
191#else 
192#define __write_cr0 write_cr0 
193#endif 
194 
195static void enable_write_protection(void) 
196{ 
197    unsigned long cr0 = read_cr0(); 
198    set_bit(16, &cr0); 
199    __write_cr0(cr0); 
200} 
201 
202static void disable_write_protection(void) 
203{ 
204    unsigned long cr0 = read_cr0(); 
205    clear_bit(16, &cr0); 
206    __write_cr0(cr0); 
207} 
208 
209static int __init syscall_start(void) 
210{ 
211    if (!(sys_call_table = acquire_sys_call_table())) 
212        return -1; 
213 
214    disable_write_protection(); 
215 
216    /* keep track of the original open function */ 
217    original_call = (void *)sys_call_table[__NR_openat]; 
218 
219    /* use our openat function instead */ 
220    sys_call_table[__NR_openat] = (unsigned long *)our_sys_openat; 
221 
222    enable_write_protection(); 
223 
224    pr_info("Spying on UID:%dn", uid); 
225 
226    return 0; 
227} 
228 
229static void __exit syscall_end(void) 
230{ 
231    if (!sys_call_table) 
232        return; 
233 
234    /* Return the system call back to normal */ 
235    if (sys_call_table[__NR_openat] != (unsigned long *)our_sys_openat) { 
236        pr_alert("Somebody else also played with the "); 
237        pr_alert("open system calln"); 
238        pr_alert("The system may be left in "); 
239        pr_alert("an unstable state.n"); 
240    } 
241 
242    disable_write_protection(); 
243    sys_call_table[__NR_openat] = (unsigned long *)original_call; 
244    enable_write_protection(); 
245 
246    msleep(2000); 
247} 
248 
249module_init(syscall_start); 
250module_exit(syscall_end); 
251 
252MODULE_LICENSE("GPL");

11 Blocking Processes and threads

11.1 Sleep

What do you do when somebody asks you for something you can not do right
away? If you are a human being and you are bothered by a human being, the
only thing you can say is: “Not right now, I’m busy. Go away!“. But if you
are a kernel module and you are bothered by a process, you have another
possibility. You can put the process to sleep until you can service it. After all,
processes are being put to sleep by the kernel and woken up all the time (that
is the way multiple processes appear to run on the same time on a single
CPU).

This kernel module is an example of this. The file (called /proc/sleep) can only
be opened by a single process at a time. If the file is already open, the kernel module
calls wait_event_interruptible
. The easiest way to keep a file open is to open it with:

1tail -f

This function changes the status of the task (a task is the kernel data structure
which holds information about a process and the system call it is in, if any) to
TASK_INTERRUPTIBLE
, which means that the task will not run until it is woken up somehow, and adds it to
WaitQ, the queue of tasks waiting to access the file. Then, the function calls the
scheduler to context switch to a different process, one which has some use for the
CPU.

When a process is done with the file, it closes it, and
module_close
is called. That function wakes up all the processes in the queue (there’s no
mechanism to only wake up one of them). It then returns and the process which just

closed the file can continue to run. In time, the scheduler decides that that
process has had enough and gives control of the CPU to another process.
Eventually, one of the processes which was in the queue will be given control
of the CPU by the scheduler. It starts at the point right after the call to
wait_event_interruptible
.

This means that the process is still in kernel mode – as far as the process
is concerned, it issued the open system call and the system call has not
returned yet. The process does not know somebody else used the CPU for
most of the time between the moment it issued the call and the moment it
returned.

It can then proceed to set a global variable to tell all the other processes that the
file is still open and go on with its life. When the other processes get a piece of the
CPU, they’ll see that global variable and go back to sleep.

So we will use tail -f
to keep the file open in the background, while trying to access it with another
process (again in the background, so that we need not switch to a different vt). As
soon as the first background process is killed with kill %1 , the second is woken up, is
able to access the file and finally terminates.

To make our life more interesting, module_close
does not have a monopoly on waking up the processes which wait to access the file.
A signal, such as Ctrl +c (SIGINT) can also wake up a process. This is because we
used wait_event_interruptible
. We could have used wait_event
instead, but that would have resulted in extremely angry users whose Ctrl+c’s are
ignored.

In that case, we want to return with
-EINTR
immediately. This is important so users can, for example, kill the process before it
receives the file.

There is one more point to remember. Some times processes don’t want to sleep, they want
either to get what they want immediately, or to be told it cannot be done. Such processes
use the O_NONBLOCK
flag when opening the file. The kernel is supposed to respond by returning with the error
code -EAGAIN
from operations which would otherwise block, such as opening the file in this example. The
program cat_nonblock
, available in the examples/other directory, can be used to open a file with
O_NONBLOCK
.

$ sudo insmod sleep.ko
$ cat_nonblock /proc/sleep
Last input:
$ tail -f /proc/sleep &
Last input:
Last input:
Last input:
Last input:
Last input:
Last input:
Last input:
tail: /proc/sleep: file truncated
[1] 6540
$ cat_nonblock /proc/sleep
Open would block
$ kill %1
[1]+  Terminated              tail -f /proc/sleep
$ cat_nonblock /proc/sleep
Last input:
$
1/* 
2 * sleep.c - create a /proc file, and if several processes try to open it 
3 * at the same time, put all but one to sleep. 
4 */ 
5 
6#include  
7#include  
8#include  /* for sprintf() */ 
9#include  /* Specifically, a module */ 
10#include  
11#include  /* Necessary because we use proc fs */ 
12#include  
13#include  /* for get_user and put_user */ 
14#include  
15#include  /* For putting processes to sleep and 
16                                   waking them up */  
17  
18#include  
19#include  
20 
21#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0) 
22#define HAVE_PROC_OPS 
23#endif 
24 
25/* Here we keep the last message received, to prove that we can process our 
26 * input. 
27 */ 
28#define MESSAGE_LENGTH 80 
29static char message[MESSAGE_LENGTH]; 
30 
31static struct proc_dir_entry *our_proc_file; 
32#define PROC_ENTRY_FILENAME "sleep" 
33 
34/* Since we use the file operations struct, we can't use the special proc 
35 * output provisions - we have to use a standard read function, which is this 
36 * function. 
37 */ 
38static ssize_t module_output(struct file *file, /* see include/linux/fs.h   */ 
39                             char __user *buf, /* The buffer to put data to 
40                                                   (in the user segment)    */  
41                             size_t len, /* The length of the buffer */ 
42                             loff_t *offset) 
43{ 
44    static int finished = 0; 
45    int i; 
46    char output_msg[MESSAGE_LENGTH + 30]; 
47 
48    /* Return 0 to signify end of file - that we have nothing more to say 
49     * at this point. 
50     */ 
51    if (finished) { 
52        finished = 0; 
53        return 0; 
54    } 
55 
56    sprintf(output_msg, "Last input:%sn", message); 
57    for (i = 0; i < len && output_msg[i]; i++) 
58        put_user(output_msg[i], buf + i); 
59 
60    finished = 1; 
61    return i; /* Return the number of bytes "read" */ 
62} 
63 
64/* This function receives input from the user when the user writes to the 
65 * /proc file. 
66 */ 
67static ssize_t module_input(struct file *file, /* The file itself */ 
68                            const char __user *buf, /* The buffer with input */ 
69                            size_t length, /* The buffer's length */ 
70                            loff_t *offset) /* offset to file - ignore */ 
71{ 
72    int i; 
73 
74    /* Put the input into Message, where module_output will later be able 
75     * to use it. 
76     */ 
77    for (i = 0; i < MESSAGE_LENGTH - 1 && i < length; i++) 
78        get_user(message[i], buf + i); 
79    /* we want a standard, zero terminated string */ 
80    message[i] = ''; 
81 
82    /* We need to return the number of input characters used */ 
83    return i; 
84} 
85 
86/* 1 if the file is currently open by somebody */ 
87static atomic_t already_open = ATOMIC_INIT(0); 
88 
89/* Queue of processes who want our file */ 
90static DECLARE_WAIT_QUEUE_HEAD(waitq); 
91 
92/* Called when the /proc file is opened */ 
93static int module_open(struct inode *inode, struct file *file) 
94{ 
95    /* If the file's flags include O_NONBLOCK, it means the process does not 
96     * want to wait for the file. In this case, if the file is already open, 
97     * we should fail with -EAGAIN, meaning "you will have to try again", 
98     * instead of blocking a process which would rather stay awake. 
99     */ 
100    if ((file->f_flags & O_NONBLOCK) && atomic_read(&already_open)) 
101        return -EAGAIN; 
102 
103    /* This is the correct place for try_module_get(THIS_MODULE) because if 
104     * a process is in the loop, which is within the kernel module, 
105     * the kernel module must not be removed. 
106     */ 
107    try_module_get(THIS_MODULE); 
108 
109    while (atomic_cmpxchg(&already_open, 0, 1)) { 
110        int i, is_sig = 0; 
111 
112        /* This function puts the current process, including any system 
113         * calls, such as us, to sleep.  Execution will be resumed right 
114         * after the function call, either because somebody called 
115         * wake_up(&waitq) (only module_close does that, when the file 
116         * is closed) or when a signal, such as Ctrl-C, is sent 
117         * to the process 
118         */ 
119        wait_event_interruptible(waitq, !atomic_read(&already_open)); 
120 
121        /* If we woke up because we got a signal we're not blocking, 
122         * return -EINTR (fail the system call).  This allows processes 
123         * to be killed or stopped. 
124         */ 
125        for (i = 0; i < _NSIG_WORDS && !is_sig; i++) 
126            is_sig = current->pending.signal.sig[i] & ~current->blocked.sig[i]; 
127 
128        if (is_sig) { 
129            /* It is important to put module_put(THIS_MODULE) here, because 
130             * for processes where the open is interrupted there will never 
131             * be a corresponding close. If we do not decrement the usage 
132             * count here, we will be left with a positive usage count 
133             * which we will have no way to bring down to zero, giving us 
134             * an immortal module, which can only be killed by rebooting 
135             * the machine. 
136             */ 
137            module_put(THIS_MODULE); 
138            return -EINTR; 
139        } 
140    } 
141 
142    return 0; /* Allow the access */ 
143} 
144 
145/* Called when the /proc file is closed */ 
146static int module_close(struct inode *inode, struct file *file) 
147{ 
148    /* Set already_open to zero, so one of the processes in the waitq will 
149     * be able to set already_open back to one and to open the file. All 
150     * the other processes will be called when already_open is back to one, 
151     * so they'll go back to sleep. 
152     */ 
153    atomic_set(&already_open, 0); 
154 
155    /* Wake up all the processes in waitq, so if anybody is waiting for the 
156     * file, they can have it. 
157     */ 
158    wake_up(&waitq); 
159 
160    module_put(THIS_MODULE); 
161 
162    return 0; /* success */ 
163} 
164 
165/* Structures to register as the /proc file, with pointers to all the relevant 
166 * functions. 
167 */ 
168 
169/* File operations for our proc file. This is where we place pointers to all 
170 * the functions called when somebody tries to do something to our file. NULL 
171 * means we don't want to deal with something. 
172 */ 
173#ifdef HAVE_PROC_OPS 
174static const struct proc_ops file_ops_4_our_proc_file = { 
175    .proc_read = module_output, /* "read" from the file */ 
176    .proc_write = module_input, /* "write" to the file */ 
177    .proc_open = module_open, /* called when the /proc file is opened */ 
178    .proc_release = module_close, /* called when it's closed */ 
179    .proc_lseek = noop_llseek, /* return file->f_pos */ 
180}; 
181#else 
182static const struct file_operations file_ops_4_our_proc_file = { 
183    .read = module_output, 
184    .write = module_input, 
185    .open = module_open, 
186    .release = module_close, 
187    .llseek = noop_llseek, 
188}; 
189#endif 
190 
191/* Initialize the module - register the proc file */ 
192static int __init sleep_init(void) 
193{ 
194    our_proc_file = 
195        proc_create(PROC_ENTRY_FILENAME, 0644, NULL, &file_ops_4_our_proc_file); 
196    if (our_proc_file == NULL) { 
197        remove_proc_entry(PROC_ENTRY_FILENAME, NULL); 
198        pr_debug("Error: Could not initialize /proc/%sn", PROC_ENTRY_FILENAME); 
199        return -ENOMEM; 
200    } 
201    proc_set_size(our_proc_file, 80); 
202    proc_set_user(our_proc_file, GLOBAL_ROOT_UID, GLOBAL_ROOT_GID); 
203 
204    pr_info("/proc/%s createdn", PROC_ENTRY_FILENAME); 
205 
206    return 0; 
207} 
208 
209/* Cleanup - unregister our file from /proc.  This could get dangerous if 
210 * there are still processes waiting in waitq, because they are inside our 
211 * open function, which will get unloaded. I'll explain how to avoid removal 
212 * of a kernel module in such a case in chapter 10. 
213 */ 
214static void __exit sleep_exit(void) 
215{ 
216    remove_proc_entry(PROC_ENTRY_FILENAME, NULL); 
217    pr_debug("/proc/%s removedn", PROC_ENTRY_FILENAME); 
218} 
219 
220module_init(sleep_init); 
221module_exit(sleep_exit); 
222 
223MODULE_LICENSE("GPL");
1/* 
2 *  cat_nonblock.c - open a file and display its contents, but exit rather than 
3 *  wait for input. 
4 */ 
5#include  /* for errno */ 
6#include  /* for open */ 
7#include  /* standard I/O */ 
8#include  /* for exit */ 
9#include  /* for read */ 
10 
11#define MAX_BYTES 1024 * 4 
12 
13int main(int argc, char *argv[]) 
14{ 
15    int fd; /* The file descriptor for the file to read */ 
16    size_t bytes; /* The number of bytes read */ 
17    char buffer[MAX_BYTES]; /* The buffer for the bytes */ 
18 
19    /* Usage */ 
20    if (argc != 2) { 
21        printf("Usage: %s n", argv[0]); 
22        puts("Reads the content of a file, but doesn't wait for input"); 
23        exit(-1); 
24    } 
25 
26    /* Open the file for reading in non blocking mode */ 
27    fd = open(argv[1], O_RDONLY | O_NONBLOCK); 
28 
29    /* If open failed */ 
30    if (fd == -1) { 
31        puts(errno == EAGAIN ? "Open would block" : "Open failed"); 
32        exit(-1); 
33    } 
34 
35    /* Read the file and output its contents */ 
36    do { 
37        /* Read characters from the file */ 
38        bytes = read(fd, buffer, MAX_BYTES); 
39 
40        /* If there's an error, report it and die */ 
41        if (bytes == -1) { 
42            if (errno == EAGAIN) 
43                puts("Normally I'd block, but you told me not to"); 
44            else 
45                puts("Another read error"); 
46            exit(-1); 
47        } 
48 
49        /* Print the characters */ 
50        if (bytes > 0) { 
51            for (int i = 0; i < bytes; i++) 
52                putchar(buffer[i]); 
53        } 
54 
55        /* While there are no errors and the file isn't over */ 
56    } while (bytes > 0); 
57 
58    return 0; 
59}

11.2 Completions

Sometimes one thing should happen before another within a module having multiple threads.
Rather than using /bin/sleep
commands, the kernel has another way to do this which allows timeouts or
interrupts to also happen.

In the following example two threads are started, but one needs to start before
another.

1/* 
2 * completions.c 
3 */ 
4#include  
5#include  /* for IS_ERR() */ 
6#include  
7#include  
8#include  
9#include  
10#include  
11 
12static struct { 
13    struct completion crank_comp; 
14    struct completion flywheel_comp; 
15} machine; 
16 
17static int machine_crank_thread(void *arg) 
18{ 
19    pr_info("Turn the crankn"); 
20 
21    complete_all(&machine.crank_comp); 
22#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 17, 0) 
23    kthread_complete_and_exit(&machine.crank_comp, 0); 
24#else 
25    complete_and_exit(&machine.crank_comp, 0); 
26#endif 
27} 
28 
29static int machine_flywheel_spinup_thread(void *arg) 
30{ 
31    wait_for_completion(&machine.crank_comp); 
32 
33    pr_info("Flywheel spins upn"); 
34 
35    complete_all(&machine.flywheel_comp); 
36#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 17, 0) 
37    kthread_complete_and_exit(&machine.flywheel_comp, 0); 
38#else 
39    complete_and_exit(&machine.flywheel_comp, 0); 
40#endif 
41} 
42 
43static int completions_init(void) 
44{ 
45    struct task_struct *crank_thread; 
46    struct task_struct *flywheel_thread; 
47 
48    pr_info("completions examplen"); 
49 
50    init_completion(&machine.crank_comp); 
51    init_completion(&machine.flywheel_comp); 
52 
53    crank_thread = kthread_create(machine_crank_thread, NULL, "KThread Crank"); 
54    if (IS_ERR(crank_thread)) 
55        goto ERROR_THREAD_1; 
56 
57    flywheel_thread = kthread_create(machine_flywheel_spinup_thread, NULL, 
58                                     "KThread Flywheel"); 
59    if (IS_ERR(flywheel_thread)) 
60        goto ERROR_THREAD_2; 
61 
62    wake_up_process(flywheel_thread); 
63    wake_up_process(crank_thread); 
64 
65    return 0; 
66 
67ERROR_THREAD_2: 
68    kthread_stop(crank_thread); 
69ERROR_THREAD_1: 
70 
71    return -1; 
72} 
73 
74static void completions_exit(void) 
75{ 
76    wait_for_completion(&machine.crank_comp); 
77    wait_for_completion(&machine.flywheel_comp); 
78 
79    pr_info("completions exitn"); 
80} 
81 
82module_init(completions_init); 
83module_exit(completions_exit); 
84 
85MODULE_DESCRIPTION("Completions example"); 
86MODULE_LICENSE("GPL");

The machine
structure stores the completion states for the two threads. At the exit
point of each thread the respective completion state is updated, and
wait_for_completion
is used by the flywheel thread to ensure that it does not begin prematurely.

So even though flywheel_thread
is started first you should notice if you load this module and run
dmesg
that turning the crank always happens first because the flywheel thread waits for it
to complete.

There are other variations upon the
wait_for_completion
function, which include timeouts or being interrupted, but this basic mechanism is
enough for many common situations without adding a lot of complexity.

12 Avoiding Collisions and Deadlocks

If processes running on different CPUs or in different threads try to access the same
memory, then it is possible that strange things can happen or your system can lock
up. To avoid this, various types of mutual exclusion kernel functions are available.
These indicate if a section of code is “locked” or “unlocked” so that simultaneous
attempts to run it can not happen.

12.1 Mutex

You can use kernel mutexes (mutual exclusions) in much the same manner that you
might deploy them in userland. This may be all that is needed to avoid collisions in
most cases.

1/* 
2 * example_mutex.c 
3 */ 
4#include  
5#include  
6#include  
7 
8static DEFINE_MUTEX(mymutex); 
9 
10static int example_mutex_init(void) 
11{ 
12    int ret; 
13 
14    pr_info("example_mutex initn"); 
15 
16    ret = mutex_trylock(&mymutex); 
17    if (ret != 0) { 
18        pr_info("mutex is lockedn"); 
19 
20        if (mutex_is_locked(&mymutex) == 0) 
21            pr_info("The mutex failed to lock!n"); 
22 
23        mutex_unlock(&mymutex); 
24        pr_info("mutex is unlockedn"); 
25    } else 
26        pr_info("Failed to lockn"); 
27 
28    return 0; 
29} 
30 
31static void example_mutex_exit(void) 
32{ 
33    pr_info("example_mutex exitn"); 
34} 
35 
36module_init(example_mutex_init); 
37module_exit(example_mutex_exit); 
38 
39MODULE_DESCRIPTION("Mutex example"); 
40MODULE_LICENSE("GPL");

12.2 Spinlocks

As the name suggests, spinlocks lock up the CPU that the code is running on,
taking 100% of its resources. Because of this you should only use the spinlock

mechanism around code which is likely to take no more than a few milliseconds to
run and so will not noticeably slow anything down from the user’s point of
view.

The example here is “irq safe” in that if interrupts happen during the lock then
they will not be forgotten and will activate when the unlock happens, using the
flags
variable to retain their state.

1/* 
2 * example_spinlock.c 
3 */ 
4#include  
5#include  
6#include  
7#include  
8 
9static DEFINE_SPINLOCK(sl_static); 
10static spinlock_t sl_dynamic; 
11 
12static void example_spinlock_static(void) 
13{ 
14    unsigned long flags; 
15 
16    spin_lock_irqsave(&sl_static, flags); 
17    pr_info("Locked static spinlockn"); 
18 
19    /* Do something or other safely. Because this uses 100% CPU time, this 
20     * code should take no more than a few milliseconds to run. 
21     */ 
22 
23    spin_unlock_irqrestore(&sl_static, flags); 
24    pr_info("Unlocked static spinlockn"); 
25} 
26 
27static void example_spinlock_dynamic(void) 
28{ 
29    unsigned long flags; 
30 
31    spin_lock_init(&sl_dynamic); 
32    spin_lock_irqsave(&sl_dynamic, flags); 
33    pr_info("Locked dynamic spinlockn"); 
34 
35    /* Do something or other safely. Because this uses 100% CPU time, this 
36     * code should take no more than a few milliseconds to run. 
37     */ 
38 
39    spin_unlock_irqrestore(&sl_dynamic, flags); 
40    pr_info("Unlocked dynamic spinlockn"); 
41} 
42 
43static int example_spinlock_init(void) 
44{ 
45    pr_info("example spinlock startedn"); 
46 
47    example_spinlock_static(); 
48    example_spinlock_dynamic(); 
49 
50    return 0; 
51} 
52 
53static void example_spinlock_exit(void) 
54{ 
55    pr_info("example spinlock exitn"); 
56} 
57 
58module_init(example_spinlock_init); 
59module_exit(example_spinlock_exit); 
60 
61MODULE_DESCRIPTION("Spinlock example"); 
62MODULE_LICENSE("GPL");

12.3 Read and write locks

Read and write locks are specialised kinds of spinlocks so that you can exclusively
read from something or write to something. Like the earlier spinlocks example, the
one below shows an “irq safe” situation in which if other functions were triggered
from irqs which might also read and write to whatever you are concerned with
then they would not disrupt the logic. As before it is a good idea to keep
anything done within the lock as short as possible so that it does not hang up
the system and cause users to start revolting against the tyranny of your
module.

1/* 
2 * example_rwlock.c 
3 */ 
4#include  
5#include  
6#include  
7 
8static DEFINE_RWLOCK(myrwlock); 
9 
10static void example_read_lock(void) 
11{ 
12    unsigned long flags; 
13 
14    read_lock_irqsave(&myrwlock, flags); 
15    pr_info("Read Lockedn"); 
16 
17    /* Read from something */ 
18 
19    read_unlock_irqrestore(&myrwlock, flags); 
20    pr_info("Read Unlockedn"); 
21} 
22 
23static void example_write_lock(void) 
24{ 
25    unsigned long flags; 
26 
27    write_lock_irqsave(&myrwlock, flags); 
28    pr_info("Write Lockedn"); 
29 
30    /* Write to something */ 
31 
32    write_unlock_irqrestore(&myrwlock, flags); 
33    pr_info("Write Unlockedn"); 
34} 
35 
36static int example_rwlock_init(void) 
37{ 
38    pr_info("example_rwlock startedn"); 
39 
40    example_read_lock(); 
41    example_write_lock(); 
42 
43    return 0; 
44} 
45 
46static void example_rwlock_exit(void) 
47{ 
48    pr_info("example_rwlock exitn"); 
49} 
50 
51module_init(example_rwlock_init); 
52module_exit(example_rwlock_exit); 
53 
54MODULE_DESCRIPTION("Read/Write locks example"); 
55MODULE_LICENSE("GPL");

Of course, if you know for sure that there are no functions triggered by irqs
which could possibly interfere with your logic then you can use the simpler
read_lock(&myrwlock)
and read_unlock(&myrwlock)
or the corresponding write functions.

12.4 Atomic operations

If you are doing simple arithmetic: adding, subtracting or bitwise operations, then
there is another way in the multi-CPU and multi-hyperthreaded world to stop other
parts of the system from messing with your mojo. By using atomic operations you
can be confident that your addition, subtraction or bit flip did actually happen
and was not overwritten by some other shenanigans. An example is shown
below.

1/* 
2 * example_atomic.c 
3 */ 
4#include  
5#include  
6#include  
7#include  
8 
9#define BYTE_TO_BINARY_PATTERN "%c%c%c%c%c%c%c%c" 
10#define BYTE_TO_BINARY(byte)                                                    
11    ((byte & 0x80) ? '1' : '0'), ((byte & 0x40) ? '1' : '0'),                   
12        ((byte & 0x20) ? '1' : '0'), ((byte & 0x10) ? '1' : '0'),               
13        ((byte & 0x08) ? '1' : '0'), ((byte & 0x04) ? '1' : '0'),               
14        ((byte & 0x02) ? '1' : '0'), ((byte & 0x01) ? '1' : '0') 
15 
16static void atomic_add_subtract(void) 
17{ 
18    atomic_t debbie; 
19    atomic_t chris = ATOMIC_INIT(50); 
20 
21    atomic_set(&debbie, 45); 
22 
23    /* subtract one */ 
24    atomic_dec(&debbie); 
25 
26    atomic_add(7, &debbie); 
27 
28    /* add one */ 
29    atomic_inc(&debbie); 
30 
31    pr_info("chris: %d, debbie: %dn", atomic_read(&chris), 
32            atomic_read(&debbie)); 
33} 
34 
35static void atomic_bitwise(void) 
36{ 
37    unsigned long word = 0; 
38 
39    pr_info("Bits 0: " BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(word)); 
40    set_bit(3, &word); 
41    set_bit(5, &word); 
42    pr_info("Bits 1: " BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(word)); 
43    clear_bit(5, &word); 
44    pr_info("Bits 2: " BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(word)); 
45    change_bit(3, &word); 
46 
47    pr_info("Bits 3: " BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(word)); 
48    if (test_and_set_bit(3, &word)) 
49        pr_info("wrongn"); 
50    pr_info("Bits 4: " BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(word)); 
51 
52    word = 255; 
53    pr_info("Bits 5: " BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(word)); 
54} 
55 
56static int example_atomic_init(void) 
57{ 
58    pr_info("example_atomic startedn"); 
59 
60    atomic_add_subtract(); 
61    atomic_bitwise(); 
62 
63    return 0; 
64} 
65 
66static void example_atomic_exit(void) 
67{ 
68    pr_info("example_atomic exitn"); 
69} 
70 
71module_init(example_atomic_init); 
72module_exit(example_atomic_exit); 
73 
74MODULE_DESCRIPTION("Atomic operations example"); 
75MODULE_LICENSE("GPL");

Before the C11 standard adopts the built-in atomic types, the kernel already
provided a small set of atomic types by using a bunch of tricky architecture-specific
codes. Implementing the atomic types by C11 atomics may allow the kernel to throw
away the architecture-specific codes and letting the kernel code be more friendly to
the people who understand the standard. But there are some problems, such as the
memory model of the kernel doesn’t match the model formed by the C11 atomics.
For further details, see:

13 Replacing Print Macros

13.1 Replacement

In Section 2, I said that X Window System and kernel module programming do not
mix. That is true for developing kernel modules. But in actual use, you want to be
able to send messages to whichever tty the command to load the module came
from.

“tty” is an abbreviation of teletype: originally a combination keyboard-printer
used to communicate with a Unix system, and today an abstraction for the text
stream used for a Unix program, whether it is a physical terminal, an xterm on an X
display, a network connection used with ssh, etc.

The way this is done is by using current, a pointer to the currently running task,
to get the current task’s tty structure. Then, we look inside that tty structure to find
a pointer to a string write function, which we use to write a string to the
tty.

1/* 
2 * print_string.c - Send output to the tty we're running on, regardless if 
3 * it is through X11, telnet, etc.  We do this by printing the string to the 
4 * tty associated with the current task. 
5 */ 
6#include  
7#include  
8#include  
9#include  /* For current */ 
10#include  /* For the tty declarations */ 
11 
12static void print_string(char *str) 
13{ 
14    /* The tty for the current task */ 
15    struct tty_struct *my_tty = get_current_tty(); 
16 
17    /* If my_tty is NULL, the current task has no tty you can print to (i.e., 
18     * if it is a daemon). If so, there is nothing we can do. 
19     */ 
20    if (my_tty) { 
21        const struct tty_operations *ttyops = my_tty->driver->ops; 
22        /* my_tty->driver is a struct which holds the tty's functions, 
23         * one of which (write) is used to write strings to the tty. 
24         * It can be used to take a string either from the user's or 
25         * kernel's memory segment. 
26         * 
27         * The function's 1st parameter is the tty to write to, because the 
28         * same function would normally be used for all tty's of a certain 
29         * type. 
30         * The 2nd parameter is a pointer to a string. 
31         * The 3rd parameter is the length of the string. 
32         * 
33         * As you will see below, sometimes it's necessary to use 
34         * preprocessor stuff to create code that works for different 
35         * kernel versions. The (naive) approach we've taken here does not 
36         * scale well. The right way to deal with this is described in 
37         * section 2 of 
38         * linux/Documentation/SubmittingPatches 
39         */ 
40        (ttyops->write)(my_tty, /* The tty itself */ 
41                        str, /* String */ 
42                        strlen(str)); /* Length */ 
43 
44        /* ttys were originally hardware devices, which (usually) strictly 
45         * followed the ASCII standard. In ASCII, to move to a new line you 
46         * need two characters, a carriage return and a line feed. On Unix, 
47         * the ASCII line feed is used for both purposes - so we can not 
48         * just use n, because it would not have a carriage return and the 
49         * next line will start at the column right after the line feed. 
50         * 
51         * This is why text files are different between Unix and MS Windows. 
52         * In CP/M and derivatives, like MS-DOS and MS Windows, the ASCII 
53         * standard was strictly adhered to, and therefore a newline requires 
54         * both a LF and a CR. 
55         */ 
56        (ttyops->write)(my_tty, "1512", 2); 
57    } 
58} 
59 
60static int __init print_string_init(void) 
61{ 
62    print_string("The module has been inserted.  Hello world!"); 
63    return 0; 
64} 
65 
66static void __exit print_string_exit(void) 
67{ 
68    print_string("The module has been removed.  Farewell world!"); 
69} 
70 
71module_init(print_string_init); 
72module_exit(print_string_exit); 
73 
74MODULE_LICENSE("GPL");

13.2 Flashing keyboard LEDs

In certain conditions, you may desire a simpler and more direct way to communicate
to the external world. Flashing keyboard LEDs can be such a solution: It is an
immediate way to attract attention or to display a status condition. Keyboard LEDs
are present on every hardware, they are always visible, they do not need any setup,
and their use is rather simple and non-intrusive, compared to writing to a tty or a
file.

From v4.14 to v4.15, the timer API made a series of changes
to improve memory safety. A buffer overflow in the area of a
timer_list
structure may be able to overwrite the
function
and data
fields, providing the attacker with a way to use return-object programming (ROP)
to call arbitrary functions within the kernel. Also, the function prototype of the callback,
containing a unsigned long
argument, will prevent work from any type checking. Furthermore, the function prototype
with unsigned long
argument may be an obstacle to the forward-edge protection of control-flow integrity.
Thus, it is better to use a unique prototype to separate from the cluster that takes an
unsigned long
argument. The timer callback should be passed a pointer to the
timer_list
structure rather than an unsigned long
argument. Then, it wraps all the information the callback needs, including the
timer_list
structure, into a larger structure, and it can use the
container_of
macro instead of the unsigned long
value. For more information see: Improving the kernel timers API.

Before Linux v4.14, setup_timer
was used to initialize the timer and the
timer_list
structure looked like:

1struct timer_list { 
2    unsigned long expires; 
3    void (*function)(unsigned long); 
4    unsigned long data; 
5    u32 flags; 
6    /* ... */ 
7}; 
8 
9void setup_timer(struct timer_list *timer, void (*callback)(unsigned long), 
10                 unsigned long data);

Since Linux v4.14, timer_setup
is adopted and the kernel step by step converting to
timer_setup
from setup_timer
. One of the reasons why API was changed is it need to coexist with the old version interface.
Moreover, the timer_setup

was implemented by setup_timer
at first.

1void timer_setup(struct timer_list *timer, 
2                 void (*callback)(struct timer_list *), unsigned int flags);

The setup_timer
was then removed since v4.15. As a result, the
timer_list
structure had changed to the following.

1struct timer_list { 
2    unsigned long expires; 
3    void (*function)(struct timer_list *); 
4    u32 flags; 
5    /* ... */ 
6};

The following source code illustrates a minimal kernel module which, when
loaded, starts blinking the keyboard LEDs until it is unloaded.

1/* 
2 * kbleds.c - Blink keyboard leds until the module is unloaded. 
3 */ 
4 
5#include  
6#include  /* For KDSETLED */ 
7#include  
8#include  /* For tty_struct */ 
9#include  /* For MAX_NR_CONSOLES */ 
10#include  /* for fg_console */ 
11#include  /* For vc_cons */ 
12 
13MODULE_DESCRIPTION("Example module illustrating the use of Keyboard LEDs."); 
14 
15static struct timer_list my_timer; 
16static struct tty_driver *my_driver; 
17static unsigned long kbledstatus = 0; 
18 
19#define BLINK_DELAY HZ / 5 
20#define ALL_LEDS_ON 0x07 
21#define RESTORE_LEDS 0xFF 
22 
23/* Function my_timer_func blinks the keyboard LEDs periodically by invoking 
24 * command KDSETLED of ioctl() on the keyboard driver. To learn more on virtual 
25 * terminal ioctl operations, please see file: 
26 *   drivers/tty/vt/vt_ioctl.c, function vt_ioctl(). 
27 * 
28 * The argument to KDSETLED is alternatively set to 7 (thus causing the led 
29 * mode to be set to LED_SHOW_IOCTL, and all the leds are lit) and to 0xFF 
30 * (any value above 7 switches back the led mode to LED_SHOW_FLAGS, thus 
31 * the LEDs reflect the actual keyboard status).  To learn more on this, 
32 * please see file: drivers/tty/vt/keyboard.c, function setledstate(). 
33 */ 
34static void my_timer_func(struct timer_list *unused) 
35{ 
36    struct tty_struct *t = vc_cons[fg_console].d->port.tty; 
37 
38    if (kbledstatus == ALL_LEDS_ON) 
39        kbledstatus = RESTORE_LEDS; 
40    else 
41        kbledstatus = ALL_LEDS_ON; 
42 
43    (my_driver->ops->ioctl)(t, KDSETLED, kbledstatus); 
44 
45    my_timer.expires = jiffies + BLINK_DELAY; 
46    add_timer(&my_timer); 
47} 
48 
49static int __init kbleds_init(void) 
50{ 
51    int i; 
52 
53    pr_info("kbleds: loadingn"); 
54    pr_info("kbleds: fgconsole is %xn", fg_console); 
55    for (i = 0; i < MAX_NR_CONSOLES; i++) { 
56        if (!vc_cons[i].d) 
57            break; 
58        pr_info("poet_atkm: console[%i/%i] #%i, tty %pn", i, MAX_NR_CONSOLES, 
59                vc_cons[i].d->vc_num, (void *)vc_cons[i].d->port.tty); 
60    } 
61    pr_info("kbleds: finished scanning consolesn"); 
62 
63    my_driver = vc_cons[fg_console].d->port.tty->driver; 
64    pr_info("kbleds: tty driver magic %xn", my_driver->magic); 
65 
66    /* Set up the LED blink timer the first time. */ 
67    timer_setup(&my_timer, my_timer_func, 0); 
68    my_timer.expires = jiffies + BLINK_DELAY; 
69    add_timer(&my_timer); 
70 
71    return 0; 
72} 
73 
74static void __exit kbleds_cleanup(void) 
75{ 
76    pr_info("kbleds: unloading...n"); 
77    del_timer(&my_timer); 
78    (my_driver->ops->ioctl)(vc_cons[fg_console].d->port.tty, KDSETLED, 
79                            RESTORE_LEDS); 
80} 
81 
82module_init(kbleds_init); 
83module_exit(kbleds_cleanup); 
84 
85MODULE_LICENSE("GPL");

If none of the examples in this chapter fit your debugging needs,
there might yet be some other tricks to try. Ever wondered what
CONFIG_LL_DEBUG
in make menuconfig
is good for? If you activate that you get low level access to the serial port. While this
might not sound very powerful by itself, you can patch kernel/printk.c or any other
essential syscall to print ASCII characters, thus making it possible to trace virtually
everything what your code does over a serial line. If you find yourself porting the
kernel to some new and former unsupported architecture, this is usually amongst the
first things that should be implemented. Logging over a netconsole might also be
worth a try.

While you have seen lots of stuff that can be used to aid debugging here, there are
some things to be aware of. Debugging is almost always intrusive. Adding debug code
can change the situation enough to make the bug seem to disappear. Thus, you
should keep debug code to a minimum and make sure it does not show up in
production code.

14 Scheduling Tasks

There are two main ways of running tasks: tasklets and work queues. Tasklets are a
quick and easy way of scheduling a single function to be run. For example, when
triggered from an interrupt, whereas work queues are more complicated but also
better suited to running multiple things in a sequence.

14.1 Tasklets

Here is an example tasklet module. The
tasklet_fn
function runs for a few seconds. In the meantime, execution of the
example_tasklet_init
function may continue to the exit point, depending on whether it is interrupted by
softirq.

1/* 
2 * example_tasklet.c 
3 */ 
4#include  
5#include  
6#include  
7#include  
8 
9/* Macro DECLARE_TASKLET_OLD exists for compatibility. 
10 * See https://lwn.net/Articles/830964/ 
11 */ 
12#ifndef DECLARE_TASKLET_OLD 
13#define DECLARE_TASKLET_OLD(arg1, arg2) DECLARE_TASKLET(arg1, arg2, 0L) 
14#endif 
15 
16static void tasklet_fn(unsigned long data) 
17{ 
18    pr_info("Example tasklet startsn"); 
19    mdelay(5000); 
20    pr_info("Example tasklet endsn"); 
21} 
22 
23static DECLARE_TASKLET_OLD(mytask, tasklet_fn); 
24 
25static int example_tasklet_init(void) 
26{ 
27    pr_info("tasklet example initn"); 
28    tasklet_schedule(&mytask); 
29    mdelay(200); 
30    pr_info("Example tasklet init continues...n"); 
31    return 0; 
32} 
33 
34static void example_tasklet_exit(void) 
35{ 
36    pr_info("tasklet example exitn"); 
37    tasklet_kill(&mytask); 
38} 
39 
40module_init(example_tasklet_init); 
41module_exit(example_tasklet_exit); 
42 
43MODULE_DESCRIPTION("Tasklet example"); 
44MODULE_LICENSE("GPL");

So with this example loaded dmesg
should show:

tasklet example init
Example tasklet starts
Example tasklet init continues...
Example tasklet ends

Although tasklet is easy to use, it comes with several defators, and developers are
discussing about getting rid of tasklet in linux kernel. The tasklet callback
runs in atomic context, inside a software interrupt, meaning that it cannot
sleep or access user-space data, so not all work can be done in a tasklet
handler. Also, the kernel only allows one instance of any given tasklet to be
running at any given time; multiple different tasklet callbacks can run in
parallel.

In recent kernels, tasklets can be replaced by workqueues, timers, or threaded
interrupts.
While the removal of tasklets remains a longer-term goal, the current kernel contains more
than a hundred uses of tasklets. Now developers are proceeding with the API changes and
the macro DECLARE_TASKLET_OLD
exists for compatibility. For further information, see https://lwn.net/Articles/830964/.

14.2 Work queues

To add a task to the scheduler we can use a workqueue. The kernel then uses the
Completely Fair Scheduler (CFS) to execute work within the queue.

1/* 
2 * sched.c 
3 */ 
4#include  
5#include  
6#include  
7 
8static struct workqueue_struct *queue = NULL; 
9static struct work_struct work; 
10 
11static void work_handler(struct work_struct *data) 
12{ 
13    pr_info("work handler function.n"); 
14} 
15 
16static int __init sched_init(void) 
17{ 
18    queue = alloc_workqueue("HELLOWORLD", WQ_UNBOUND, 1); 
19    INIT_WORK(&work, work_handler); 
20    schedule_work(&work); 
21    return 0; 
22} 
23 
24static void __exit sched_exit(void) 
25{ 
26    destroy_workqueue(queue); 
27} 
28 
29module_init(sched_init); 
30module_exit(sched_exit); 
31 
32MODULE_LICENSE("GPL"); 
33MODULE_DESCRIPTION("Workqueue example");

15 Interrupt Handlers

15.1 Interrupt Handlers

Except for the last chapter, everything we did in the kernel so far we have done as a
response to a process asking for it, either by dealing with a special file, sending an
ioctl()
, or issuing a system call. But the job of the kernel is not just to respond to process
requests. Another job, which is every bit as important, is to speak to the hardware
connected to the machine.

There are two types of interaction between the CPU and the rest of the
computer’s hardware. The first type is when the CPU gives orders to the hardware,
the other is when the hardware needs to tell the CPU something. The second, called
interrupts, is much harder to implement because it has to be dealt with when
convenient for the hardware, not the CPU. Hardware devices typically have a very
small amount of RAM, and if you do not read their information when available, it is
lost.

Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
are two types of IRQ’s, short and long. A short IRQ is one which is expected to take
a very short period of time, during which the rest of the machine will be blocked and
no other interrupts will be handled. A long IRQ is one which can take longer, and
during which other interrupts may occur (but not interrupts from the same
device). If at all possible, it is better to declare an interrupt handler to be
long.

When the CPU receives an interrupt, it stops whatever it is doing (unless it is
processing a more important interrupt, in which case it will deal with this one only
when the more important one is done), saves certain parameters on the stack and
calls the interrupt handler. This means that certain things are not allowed in the
interrupt handler itself, because the system is in an unknown state. Linux kernel
solves the problem by splitting interrupt handling into two parts. The first part
executes right away and masks the interrupt line. Hardware interrupts must be
handled quickly, and that is why we need the second part to handle the
heavy work deferred from an interrupt handler. Historically, BH (Linux
naming for Bottom Halves) statistically book-keeps the deferred functions.
Softirq and its higher level abstraction, Tasklet, replace BH since Linux
2.3.

The way to implement this is to call
request_irq()
to get your interrupt handler called when the relevant IRQ is received.

In practice IRQ handling can be a bit more complex. Hardware is often designed
in a way that chains two interrupt controllers, so that all the IRQs from
interrupt controller B are cascaded to a certain IRQ from interrupt controller A.
Of course, that requires that the kernel finds out which IRQ it really was
afterwards and that adds overhead. Other architectures offer some special,
very low overhead, so called “fast IRQ” or FIQs. To take advantage of them
requires handlers to be written in assembly language, so they do not really
fit into the kernel. They can be made to work similar to the others, but
after that procedure, they are no longer any faster than “common” IRQs.
SMP enabled kernels running on systems with more than one processor

need to solve another truckload of problems. It is not enough to know if a
certain IRQs has happened, it’s also important to know what CPU(s) it was
for. People still interested in more details, might want to refer to “APIC”
now.

This function receives the IRQ number, the name of the function,
flags, a name for /proc/interrupts and a parameter to be passed to the
interrupt handler. Usually there is a certain number of IRQs available.
How many IRQs there are is hardware-dependent. The flags can include
SA_SHIRQ
to indicate you are willing to share the IRQ with other interrupt handlers
(usually because a number of hardware devices sit on the same IRQ) and
SA_INTERRUPT
to indicate this is a fast interrupt. This function will only succeed if there is not
already a handler on this IRQ, or if you are both willing to share.

15.2 Detecting button presses

Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
bunch of GPIO pins. Attaching buttons to those and then having a button press do
something is a classic case in which you might need to use interrupts, so that instead
of having the CPU waste time and battery power polling for a change in input state,
it is better for the input to trigger the CPU to then run a particular handling
function.

Here is an example where buttons are connected to GPIO numbers 17 and 18 and
an LED is connected to GPIO 4. You can change those numbers to whatever is
appropriate for your board.

1/* 
2 * intrpt.c - Handling GPIO with interrupts 
3 * 
4 * Based upon the RPi example by Stefan Wendler (devnull@kaltpost.de) 
5 * from: 
6 *   https://github.com/wendlers/rpi-kmod-samples 
7 * 
8 * Press one button to turn on a LED and another to turn it off. 
9 */ 
10 
11#include  
12#include  
13#include  /* for ARRAY_SIZE() */ 
14#include  
15#include  
16 
17static int button_irqs[] = { -1, -1 }; 
18 
19/* Define GPIOs for LEDs. 
20 * TODO: Change the numbers for the GPIO on your board. 
21 */ 
22static struct gpio leds[] = { { 4, GPIOF_OUT_INIT_LOW, "LED 1" } }; 
23 
24/* Define GPIOs for BUTTONS 
25 * TODO: Change the numbers for the GPIO on your board. 
26 */ 
27static struct gpio buttons[] = { { 17, GPIOF_IN, "LED 1 ON BUTTON" }, 
28                                 { 18, GPIOF_IN, "LED 1 OFF BUTTON" } }; 
29 
30/* interrupt function triggered when a button is pressed. */ 
31static irqreturn_t button_isr(int irq, void *data) 
32{ 
33    /* first button */ 
34    if (irq == button_irqs[0] && !gpio_get_value(leds[0].gpio)) 
35        gpio_set_value(leds[0].gpio, 1); 
36    /* second button */ 
37    else if (irq == button_irqs[1] && gpio_get_value(leds[0].gpio)) 
38        gpio_set_value(leds[0].gpio, 0); 
39 
40    return IRQ_HANDLED; 
41} 
42 
43static int __init intrpt_init(void) 
44{ 
45    int ret = 0; 
46 
47    pr_info("%sn", __func__); 
48 
49    /* register LED gpios */ 
50    ret = gpio_request_array(leds, ARRAY_SIZE(leds)); 
51 
52    if (ret) { 
53        pr_err("Unable to request GPIOs for LEDs: %dn", ret); 
54        return ret; 
55    } 
56 
57    /* register BUTTON gpios */ 
58    ret = gpio_request_array(buttons, ARRAY_SIZE(buttons)); 
59 
60    if (ret) { 
61        pr_err("Unable to request GPIOs for BUTTONs: %dn", ret); 
62        goto fail1; 
63    } 
64 
65    pr_info("Current button1 value: %dn", gpio_get_value(buttons[0].gpio)); 
66 
67    ret = gpio_to_irq(buttons[0].gpio); 
68 
69    if (ret < 0) { 
70        pr_err("Unable to request IRQ: %dn", ret); 
71        goto fail2; 
72    } 
73 
74    button_irqs[0] = ret; 
75 
76    pr_info("Successfully requested BUTTON1 IRQ # %dn", button_irqs[0]); 
77 
78    ret = request_irq(button_irqs[0], button_isr, 
79                      IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING, 
80                      "gpiomod#button1", NULL); 
81 
82    if (ret) { 
83        pr_err("Unable to request IRQ: %dn", ret); 
84        goto fail2; 
85    } 
86 
87    ret = gpio_to_irq(buttons[1].gpio); 
88 
89    if (ret < 0) { 
90        pr_err("Unable to request IRQ: %dn", ret); 
91        goto fail2; 
92    } 
93 
94    button_irqs[1] = ret; 
95 
96    pr_info("Successfully requested BUTTON2 IRQ # %dn", button_irqs[1]); 
97 
98    ret = request_irq(button_irqs[1], button_isr, 
99                      IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING, 
100                      "gpiomod#button2", NULL); 
101 
102    if (ret) { 
103        pr_err("Unable to request IRQ: %dn", ret); 
104        goto fail3; 
105    } 
106 
107    return 0; 
108 
109/* cleanup what has been setup so far */ 
110fail3: 
111    free_irq(button_irqs[0], NULL); 
112 
113fail2: 
114    gpio_free_array(buttons, ARRAY_SIZE(leds)); 
115 
116fail1: 
117    gpio_free_array(leds, ARRAY_SIZE(leds)); 
118 
119    return ret; 
120} 
121 
122static void __exit intrpt_exit(void) 
123{ 
124    int i; 
125 
126    pr_info("%sn", __func__); 
127 
128    /* free irqs */ 
129    free_irq(button_irqs[0], NULL); 
130    free_irq(button_irqs[1], NULL); 
131 
132    /* turn all LEDs off */ 
133    for (i = 0; i < ARRAY_SIZE(leds); i++) 
134        gpio_set_value(leds[i].gpio, 0); 
135 
136    /* unregister */ 
137    gpio_free_array(leds, ARRAY_SIZE(leds)); 
138    gpio_free_array(buttons, ARRAY_SIZE(buttons)); 
139} 
140 
141module_init(intrpt_init); 
142module_exit(intrpt_exit); 
143 
144MODULE_LICENSE("GPL"); 
145MODULE_DESCRIPTION("Handle some GPIO interrupts");

15.3 Bottom Half

Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
way to do that without rendering the interrupt unavailable for a significant duration
is to combine it with a tasklet. This pushes the bulk of the work off into the
scheduler.

The example below modifies the previous example to also run an additional task
when an interrupt is triggered.

1/* 
2 * bottomhalf.c - Top and bottom half interrupt handling 
3 * 
4 * Based upon the RPi example by Stefan Wendler (devnull@kaltpost.de) 
5 * from: 
6 *    https://github.com/wendlers/rpi-kmod-samples 
7 * 
8 * Press one button to turn on an LED and another to turn it off 
9 */ 
10 
11#include  
12#include  
13#include  
14#include  
15#include  
16 
17/* Macro DECLARE_TASKLET_OLD exists for compatibiity. 
18 * See https://lwn.net/Articles/830964/ 
19 */ 
20#ifndef DECLARE_TASKLET_OLD 
21#define DECLARE_TASKLET_OLD(arg1, arg2) DECLARE_TASKLET(arg1, arg2, 0L) 
22#endif 
23 
24static int button_irqs[] = { -1, -1 }; 
25 
26/* Define GPIOs for LEDs. 
27 * TODO: Change the numbers for the GPIO on your board. 
28 */ 
29static struct gpio leds[] = { { 4, GPIOF_OUT_INIT_LOW, "LED 1" } }; 
30 
31/* Define GPIOs for BUTTONS 
32 * TODO: Change the numbers for the GPIO on your board. 
33 */ 
34static struct gpio buttons[] = { 
35    { 17, GPIOF_IN, "LED 1 ON BUTTON" }, 
36    { 18, GPIOF_IN, "LED 1 OFF BUTTON" }, 
37}; 
38 
39/* Tasklet containing some non-trivial amount of processing */ 
40static void bottomhalf_tasklet_fn(unsigned long data) 
41{ 
42    pr_info("Bottom half tasklet startsn"); 
43    /* do something which takes a while */ 
44    mdelay(500); 
45    pr_info("Bottom half tasklet endsn"); 
46} 
47 
48static DECLARE_TASKLET_OLD(buttontask, bottomhalf_tasklet_fn); 
49 
50/* interrupt function triggered when a button is pressed */ 
51static irqreturn_t button_isr(int irq, void *data) 
52{ 
53    /* Do something quickly right now */ 
54    if (irq == button_irqs[0] && !gpio_get_value(leds[0].gpio)) 
55        gpio_set_value(leds[0].gpio, 1); 
56    else if (irq == button_irqs[1] && gpio_get_value(leds[0].gpio)) 
57        gpio_set_value(leds[0].gpio, 0); 
58 
59    /* Do the rest at leisure via the scheduler */ 
60    tasklet_schedule(&buttontask); 
61 
62    return IRQ_HANDLED; 
63} 
64 
65static int __init bottomhalf_init(void) 
66{ 
67    int ret = 0; 
68 
69    pr_info("%sn", __func__); 
70 
71    /* register LED gpios */ 
72    ret = gpio_request_array(leds, ARRAY_SIZE(leds)); 
73 
74    if (ret) { 
75        pr_err("Unable to request GPIOs for LEDs: %dn", ret); 
76        return ret; 
77    } 
78 
79    /* register BUTTON gpios */ 
80    ret = gpio_request_array(buttons, ARRAY_SIZE(buttons)); 
81 
82    if (ret) { 
83        pr_err("Unable to request GPIOs for BUTTONs: %dn", ret); 
84        goto fail1; 
85    } 
86 
87    pr_info("Current button1 value: %dn", gpio_get_value(buttons[0].gpio)); 
88 
89    ret = gpio_to_irq(buttons[0].gpio); 
90 
91    if (ret < 0) { 
92        pr_err("Unable to request IRQ: %dn", ret); 
93        goto fail2; 
94    } 
95 
96    button_irqs[0] = ret; 
97 
98    pr_info("Successfully requested BUTTON1 IRQ # %dn", button_irqs[0]); 
99 
100    ret = request_irq(button_irqs[0], button_isr, 
101                      IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING, 
102                      "gpiomod#button1", NULL); 
103 
104    if (ret) { 
105        pr_err("Unable to request IRQ: %dn", ret); 
106        goto fail2; 
107    } 
108 
109    ret = gpio_to_irq(buttons[1].gpio); 
110 
111    if (ret < 0) { 
112        pr_err("Unable to request IRQ: %dn", ret); 
113        goto fail2; 
114    } 
115 
116    button_irqs[1] = ret; 
117 
118    pr_info("Successfully requested BUTTON2 IRQ # %dn", button_irqs[1]); 
119 
120    ret = request_irq(button_irqs[1], button_isr, 
121                      IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING, 
122                      "gpiomod#button2", NULL); 
123 
124    if (ret) { 
125        pr_err("Unable to request IRQ: %dn", ret); 
126        goto fail3; 
127    } 
128 
129    return 0; 
130 
131/* cleanup what has been setup so far */ 
132fail3: 
133    free_irq(button_irqs[0], NULL); 
134 
135fail2: 
136    gpio_free_array(buttons, ARRAY_SIZE(leds)); 
137 
138fail1: 
139    gpio_free_array(leds, ARRAY_SIZE(leds)); 
140 
141    return ret; 
142} 
143 
144static void __exit bottomhalf_exit(void) 
145{ 
146    int i; 
147 
148    pr_info("%sn", __func__); 
149 
150    /* free irqs */ 
151    free_irq(button_irqs[0], NULL); 
152    free_irq(button_irqs[1], NULL); 
153 
154    /* turn all LEDs off */ 
155    for (i = 0; i < ARRAY_SIZE(leds); i++) 
156        gpio_set_value(leds[i].gpio, 0); 
157 
158    /* unregister */ 
159    gpio_free_array(leds, ARRAY_SIZE(leds)); 
160    gpio_free_array(buttons, ARRAY_SIZE(buttons)); 
161} 
162 
163module_init(bottomhalf_init); 
164module_exit(bottomhalf_exit); 
165 
166MODULE_LICENSE("GPL"); 
167MODULE_DESCRIPTION("Interrupt with top and bottom half");

16 Crypto

At the dawn of the internet, everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written, it was a more innocent
era in which almost nobody actually gave a damn about crypto – least of all kernel
developers. That is certainly no longer the case now. To handle crypto stuff, the
kernel has its own API enabling common methods of encryption, decryption and your
favourite hash functions.

16.1 Hash functions

Calculating and checking the hashes of things is a common operation.
Here is a demonstration of how to calculate a sha256 hash within a
kernel module. To provide the sha256 algorithm support, make sure
CONFIG_CRYPTO_SHA256
is enabled in kernel.

1/* 
2 * cryptosha256.c 
3 */ 
4#include  
5#include  
6 
7#define SHA256_LENGTH 32 
8 
9static void show_hash_result(char *plaintext, char *hash_sha256) 
10{ 
11    int i; 
12    char str[SHA256_LENGTH * 2 + 1]; 
13 
14    pr_info("sha256 test for string: "%s"n", plaintext); 
15    for (i = 0; i < SHA256_LENGTH; i++) 
16        sprintf(&str[i * 2], "%02x", (unsigned char)hash_sha256[i]); 
17    str[i * 2] = 0; 
18    pr_info("%sn", str); 
19} 
20 
21static int cryptosha256_init(void) 
22{ 
23    char *plaintext = "This is a test"; 
24    char hash_sha256[SHA256_LENGTH]; 
25    struct crypto_shash *sha256; 
26    struct shash_desc *shash; 
27 
28    sha256 = crypto_alloc_shash("sha256", 0, 0); 
29    if (IS_ERR(sha256)) { 
30        pr_err( 
31            "%s(): Failed to allocate sha256 algorithm, enable CONFIG_CRYPTO_SHA256 and try again.n", 
32            __func__); 
33        return -1; 
34    } 
35 
36    shash = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(sha256), 
37                    GFP_KERNEL); 
38    if (!shash) 
39        return -ENOMEM; 
40 
41    shash->tfm = sha256; 
42 
43    if (crypto_shash_init(shash)) 
44        return -1; 
45 
46    if (crypto_shash_update(shash, plaintext, strlen(plaintext))) 
47        return -1; 
48 
49    if (crypto_shash_final(shash, hash_sha256)) 
50        return -1; 
51 
52    kfree(shash); 
53    crypto_free_shash(sha256); 
54 
55    show_hash_result(plaintext, hash_sha256); 
56 
57    return 0; 
58} 
59 
60static void cryptosha256_exit(void) 
61{ 
62} 
63 
64module_init(cryptosha256_init); 
65module_exit(cryptosha256_exit); 
66 
67MODULE_DESCRIPTION("sha256 hash test"); 
68MODULE_LICENSE("GPL");

Install the module:

1sudo insmod cryptosha256.ko 
2sudo dmesg

And you should see that the hash was calculated for the test string.

Finally, remove the test module:

1sudo rmmod cryptosha256

16.2 Symmetric key encryption

Here is an example of symmetrically encrypting a string using the AES algorithm
and a password.

1/* 
2 * cryptosk.c 
3 */ 
4#include  
5#include  
6#include  
7#include  
8#include  
9 
10#define SYMMETRIC_KEY_LENGTH 32 
11#define CIPHER_BLOCK_SIZE 16 
12 
13struct tcrypt_result { 
14    struct completion completion; 
15    int err; 
16}; 
17 
18struct skcipher_def { 
19    struct scatterlist sg; 
20    struct crypto_skcipher *tfm; 
21    struct skcipher_request *req; 
22    struct tcrypt_result result; 
23    char *scratchpad; 
24    char *ciphertext; 
25    char *ivdata; 
26}; 
27 
28static struct skcipher_def sk; 
29 
30static void test_skcipher_finish(struct skcipher_def *sk) 
31{ 
32    if (sk->tfm) 
33        crypto_free_skcipher(sk->tfm); 
34    if (sk->req) 
35        skcipher_request_free(sk->req); 
36    if (sk->ivdata) 
37        kfree(sk->ivdata); 
38    if (sk->scratchpad) 
39        kfree(sk->scratchpad); 
40    if (sk->ciphertext) 
41        kfree(sk->ciphertext); 
42} 
43 
44static int test_skcipher_result(struct skcipher_def *sk, int rc) 
45{ 
46    switch (rc) { 
47    case 0: 
48        break; 
49    case -EINPROGRESS || -EBUSY: 
50        rc = wait_for_completion_interruptible(&sk->result.completion); 
51        if (!rc && !sk->result.err) { 
52            reinit_completion(&sk->result.completion); 
53            break; 
54        } 
55        pr_info("skcipher encrypt returned with %d result %dn", rc, 
56                sk->result.err); 
57        break; 
58    default: 
59        pr_info("skcipher encrypt returned with %d result %dn", rc, 
60                sk->result.err); 
61        break; 
62    } 
63 
64    init_completion(&sk->result.completion); 
65 
66    return rc; 
67} 
68 
69static void test_skcipher_callback(struct crypto_async_request *req, int error) 
70{ 
71    struct tcrypt_result *result = req->data; 
72 
73    if (error == -EINPROGRESS) 
74        return; 
75 
76    result->err = error; 
77    complete(&result->completion); 
78    pr_info("Encryption finished successfullyn"); 
79 
80    /* decrypt data */ 
81#if 0 
82    memset((void*)sk.scratchpad, '-', CIPHER_BLOCK_SIZE); 
83    ret = crypto_skcipher_decrypt(sk.req); 
84    ret = test_skcipher_result(&sk, ret); 
85    if (ret) 
86        return; 
87 
88    sg_copy_from_buffer(&sk.sg, 1, sk.scratchpad, CIPHER_BLOCK_SIZE); 
89    sk.scratchpad[CIPHER_BLOCK_SIZE-1] = 0; 
90 
91    pr_info("Decryption request successfuln"); 
92    pr_info("Decrypted: %sn", sk.scratchpad); 
93#endif 
94} 
95 
96static int test_skcipher_encrypt(char *plaintext, char *password, 
97                                 struct skcipher_def *sk) 
98{ 
99    int ret = -EFAULT; 
100    unsigned char key[SYMMETRIC_KEY_LENGTH]; 
101 
102    if (!sk->tfm) { 
103        sk->tfm = crypto_alloc_skcipher("cbc-aes-aesni", 0, 0); 
104        if (IS_ERR(sk->tfm)) { 
105            pr_info("could not allocate skcipher handlen"); 
106            return PTR_ERR(sk->tfm); 
107        } 
108    } 
109 
110    if (!sk->req) { 
111        sk->req = skcipher_request_alloc(sk->tfm, GFP_KERNEL); 
112        if (!sk->req) { 
113            pr_info("could not allocate skcipher requestn"); 
114            ret = -ENOMEM; 
115            goto out; 
116        } 
117    } 
118 
119    skcipher_request_set_callback(sk->req, CRYPTO_TFM_REQ_MAY_BACKLOG, 
120                                  test_skcipher_callback, &sk->result); 
121 
122    /* clear the key */ 
123    memset((void *)key, '', SYMMETRIC_KEY_LENGTH); 
124 
125    /* Use the world's favourite password */ 
126    sprintf((char *)key, "%s", password); 
127 
128    /* AES 256 with given symmetric key */ 
129    if (crypto_skcipher_setkey(sk->tfm, key, SYMMETRIC_KEY_LENGTH)) { 
130        pr_info("key could not be setn"); 
131        ret = -EAGAIN; 
132        goto out; 
133    } 
134    pr_info("Symmetric key: %sn", key); 
135    pr_info("Plaintext: %sn", plaintext); 
136 
137    if (!sk->ivdata) { 
138        /* see https://en.wikipedia.org/wiki/Initialization_vector */ 
139        sk->ivdata = kmalloc(CIPHER_BLOCK_SIZE, GFP_KERNEL); 
140        if (!sk->ivdata) { 
141            pr_info("could not allocate ivdatan"); 
142            goto out; 
143        } 
144        get_random_bytes(sk->ivdata, CIPHER_BLOCK_SIZE); 
145    } 
146 
147    if (!sk->scratchpad) { 
148        /* The text to be encrypted */ 
149        sk->scratchpad = kmalloc(CIPHER_BLOCK_SIZE, GFP_KERNEL); 
150        if (!sk->scratchpad) { 
151            pr_info("could not allocate scratchpadn"); 
152            goto out; 
153        } 
154    } 
155    sprintf((char *)sk->scratchpad, "%s", plaintext); 
156 
157    sg_init_one(&sk->sg, sk->scratchpad, CIPHER_BLOCK_SIZE); 
158    skcipher_request_set_crypt(sk->req, &sk->sg, &sk->sg, CIPHER_BLOCK_SIZE, 
159                               sk->ivdata); 
160    init_completion(&sk->result.completion); 
161 
162    /* encrypt data */ 
163    ret = crypto_skcipher_encrypt(sk->req); 
164    ret = test_skcipher_result(sk, ret); 
165    if (ret) 
166        goto out; 
167 
168    pr_info("Encryption request successfuln"); 
169 
170out: 
171    return ret; 
172} 
173 
174static int cryptoapi_init(void) 
175{ 
176    /* The world's favorite password */ 
177    char *password = "password123"; 
178 
179    sk.tfm = NULL; 
180    sk.req = NULL; 
181    sk.scratchpad = NULL; 
182    sk.ciphertext = NULL; 
183    sk.ivdata = NULL; 
184 
185    test_skcipher_encrypt("Testing", password, &sk); 
186    return 0; 
187} 
188 
189static void cryptoapi_exit(void) 
190{ 
191    test_skcipher_finish(&sk); 
192} 
193 
194module_init(cryptoapi_init); 
195module_exit(cryptoapi_exit); 
196 
197MODULE_DESCRIPTION("Symmetric key encryption example"); 
198MODULE_LICENSE("GPL");

17 Virtual Input Device Driver

The input device driver is a module that provides a way to communicate
with the interaction device via the event. For example, the keyboard
can send the press or release event to tell the kernel what we want to
do. The input device driver will allocate a new input structure with
input_allocate_device()
and sets up input bitfields, device id, version, etc. After that, registers it by calling
input_register_device()
.

Here is an example, vinput, It is an API to allow easy
development of virtual input drivers. The drivers needs to export a
vinput_device()
that contains the virtual device name and
vinput_ops
structure that describes:

  • the init function: init()
  • the input event injection function: send()
  • the readback function: read()

Then using vinput_register_device()
and vinput_unregister_device()
will add a new device to the list of support virtual input devices.

1int init(struct vinput *);

This function is passed a struct vinput
already initialized with an allocated struct input_dev
. The init()
function is responsible for initializing the capabilities of the input device and register
it.

1int send(struct vinput *, char *, int);

This function will receive a user string to interpret and inject the event using the
input_report_XXXX
or input_event
call. The string is already copied from user.

1int read(struct vinput *, char *, int);

This function is used for debugging and should fill the buffer parameter with the
last event sent in the virtual input device format. The buffer will then be copied to
user.

vinput devices are created and destroyed using sysfs. And, event injection is done
through a /dev node. The device name will be used by the userland to export a new
virtual input device.

The class_attribute
structure is similar to other attribute types we talked about in section 8:

1struct class_attribute { 
2    struct attribute attr; 
3    ssize_t (*show)(struct class *class, struct class_attribute *attr, 
4                    char *buf); 
5    ssize_t (*store)(struct class *class, struct class_attribute *attr, 
6                    const char *buf, size_t count); 
7};

In vinput.c, the macro CLASS_ATTR_WO(export/unexport)
defined in include/linux/device.h (in this case, device.h is included in include/linux/input.h)
will generate the class_attribute
structures which are named class_attr_export/unexport. Then, put them into
vinput_class_attrs
array and the macro ATTRIBUTE_GROUPS(vinput_class)
will generate the struct attribute_group vinput_class_group

that should be assigned in vinput_class
. Finally, call class_register(&vinput_class)
to create attributes in sysfs.

To create a vinputX sysfs entry and /dev node.

1echo "vkbd" | sudo tee /sys/class/vinput/export

To unexport the device, just echo its id in unexport:

1echo "0" | sudo tee /sys/class/vinput/unexport
1/* 
2 * vinput.h 
3 */ 
4 
5#ifndef VINPUT_H 
6#define VINPUT_H 
7 
8#include  
9#include  
10 
11#define VINPUT_MAX_LEN 128 
12#define MAX_VINPUT 32 
13#define VINPUT_MINORS MAX_VINPUT 
14 
15#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
16 
17struct vinput_device; 
18 
19struct vinput { 
20    long id; 
21    long devno; 
22    long last_entry; 
23    spinlock_t lock; 
24 
25    void *priv_data; 
26 
27    struct device dev; 
28    struct list_head list; 
29    struct input_dev *input; 
30    struct vinput_device *type; 
31}; 
32 
33struct vinput_ops { 
34    int (*init)(struct vinput *); 
35    int (*kill)(struct vinput *); 
36    int (*send)(struct vinput *, char *, int); 
37    int (*read)(struct vinput *, char *, int); 
38}; 
39 
40struct vinput_device { 
41    char name[16]; 
42    struct list_head list; 
43    struct vinput_ops *ops; 
44}; 
45 
46int vinput_register(struct vinput_device *dev); 
47void vinput_unregister(struct vinput_device *dev); 
48 
49#endif
1/* 
2 * vinput.c 
3 */ 
4 
5#include  
6#include  
7#include  
8#include  
9#include  
10 
11#include  
12 
13#include "vinput.h" 
14 
15#define DRIVER_NAME "vinput" 
16 
17#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
18 
19static DECLARE_BITMAP(vinput_ids, VINPUT_MINORS); 
20 
21static LIST_HEAD(vinput_devices); 
22static LIST_HEAD(vinput_vdevices); 
23 
24static int vinput_dev; 
25static struct spinlock vinput_lock; 
26static struct class vinput_class; 
27 
28/* Search the name of vinput device in the vinput_devices linked list, 
29 * which added at vinput_register(). 
30 */ 
31static struct vinput_device *vinput_get_device_by_type(const char *type) 
32{ 
33    int found = 0; 
34    struct vinput_device *vinput; 
35    struct list_head *curr; 
36 
37    spin_lock(&vinput_lock); 
38    list_for_each (curr, &vinput_devices) { 
39        vinput = list_entry(curr, struct vinput_device, list); 
40        if (vinput && strncmp(type, vinput->name, strlen(vinput->name)) == 0) { 
41            found = 1; 
42            break; 
43        } 
44    } 
45    spin_unlock(&vinput_lock); 
46 
47    if (found) 
48        return vinput; 
49    return ERR_PTR(-ENODEV); 
50} 
51 
52/* Search the id of virtual device in the vinput_vdevices linked list, 
53 * which added at vinput_alloc_vdevice(). 
54 */ 
55static struct vinput *vinput_get_vdevice_by_id(long id) 
56{ 
57    struct vinput *vinput = NULL; 
58    struct list_head *curr; 
59 
60    spin_lock(&vinput_lock); 
61    list_for_each (curr, &vinput_vdevices) { 
62        vinput = list_entry(curr, struct vinput, list); 
63        if (vinput && vinput->id == id) 
64            break; 
65    } 
66    spin_unlock(&vinput_lock); 
67 
68    if (vinput && vinput->id == id) 
69        return vinput; 
70    return ERR_PTR(-ENODEV); 
71} 
72 
73static int vinput_open(struct inode *inode, struct file *file) 
74{ 
75    int err = 0; 
76    struct vinput *vinput = NULL; 
77 
78    vinput = vinput_get_vdevice_by_id(iminor(inode)); 
79 
80    if (IS_ERR(vinput)) 
81        err = PTR_ERR(vinput); 
82    else 
83        file->private_data = vinput; 
84 
85    return err; 
86} 
87 
88static int vinput_release(struct inode *inode, struct file *file) 
89{ 
90    return 0; 
91} 
92 
93static ssize_t vinput_read(struct file *file, char __user *buffer, size_t count, 
94                           loff_t *offset) 
95{ 
96    int len; 
97    char buff[VINPUT_MAX_LEN + 1]; 
98    struct vinput *vinput = file->private_data; 
99 
100    len = vinput->type->ops->read(vinput, buff, count); 
101 
102    if (*offset > len) 
103        count = 0; 
104    else if (count + *offset > VINPUT_MAX_LEN) 
105        count = len - *offset; 
106 
107    if (raw_copy_to_user(buffer, buff + *offset, count)) 
108        count = -EFAULT; 
109 
110    *offset += count; 
111 
112    return count; 
113} 
114 
115static ssize_t vinput_write(struct file *file, const char __user *buffer, 
116                            size_t count, loff_t *offset) 
117{ 
118    char buff[VINPUT_MAX_LEN + 1]; 
119    struct vinput *vinput = file->private_data; 
120 
121    memset(buff, 0, sizeof(char) * (VINPUT_MAX_LEN + 1)); 
122 
123    if (count > VINPUT_MAX_LEN) { 
124        dev_warn(&vinput->dev, "Too long. %d bytes allowedn", VINPUT_MAX_LEN); 
125        return -EINVAL; 
126    } 
127 
128    if (raw_copy_from_user(buff, buffer, count)) 
129        return -EFAULT; 
130 
131    return vinput->type->ops->send(vinput, buff, count); 
132} 
133 
134static const struct file_operations vinput_fops = { 
135    .owner = THIS_MODULE, 
136    .open = vinput_open, 
137    .release = vinput_release, 
138    .read = vinput_read, 
139    .write = vinput_write, 
140}; 
141 
142static void vinput_unregister_vdevice(struct vinput *vinput) 
143{ 
144    input_unregister_device(vinput->input); 
145    if (vinput->type->ops->kill) 
146        vinput->type->ops->kill(vinput); 
147} 
148 
149static void vinput_destroy_vdevice(struct vinput *vinput) 
150{ 
151    /* Remove from the list first */ 
152    spin_lock(&vinput_lock); 
153    list_del(&vinput->list); 
154    clear_bit(vinput->id, vinput_ids); 
155    spin_unlock(&vinput_lock); 
156 
157    module_put(THIS_MODULE); 
158 
159    kfree(vinput); 
160} 
161 
162static void vinput_release_dev(struct device *dev) 
163{ 
164    struct vinput *vinput = dev_to_vinput(dev); 
165    int id = vinput->id; 
166 
167    vinput_destroy_vdevice(vinput); 
168 
169    pr_debug("released vinput%d.n", id); 
170} 
171 
172static struct vinput *vinput_alloc_vdevice(void) 
173{ 
174    int err; 
175    struct vinput *vinput = kzalloc(sizeof(struct vinput), GFP_KERNEL); 
176 
177    try_module_get(THIS_MODULE); 
178 
179    memset(vinput, 0, sizeof(struct vinput)); 
180 
181    spin_lock_init(&vinput->lock); 
182 
183    spin_lock(&vinput_lock); 
184    vinput->id = find_first_zero_bit(vinput_ids, VINPUT_MINORS); 
185    if (vinput->id >= VINPUT_MINORS) { 
186        err = -ENOBUFS; 
187        goto fail_id; 
188    } 
189    set_bit(vinput->id, vinput_ids); 
190    list_add(&vinput->list, &vinput_vdevices); 
191    spin_unlock(&vinput_lock); 
192 
193    /* allocate the input device */ 
194    vinput->input = input_allocate_device(); 
195    if (vinput->input == NULL) { 
196        pr_err("vinput: Cannot allocate vinput input devicen"); 
197        err = -ENOMEM; 
198        goto fail_input_dev; 
199    } 
200 
201    /* initialize device */ 
202    vinput->dev.class = &vinput_class; 
203    vinput->dev.release = vinput_release_dev; 
204    vinput->dev.devt = MKDEV(vinput_dev, vinput->id); 
205    dev_set_name(&vinput->dev, DRIVER_NAME "%lu", vinput->id); 
206 
207    return vinput; 
208 
209fail_input_dev: 
210    spin_lock(&vinput_lock); 
211    list_del(&vinput->list); 
212fail_id: 
213    spin_unlock(&vinput_lock); 
214    module_put(THIS_MODULE); 
215    kfree(vinput); 
216 
217    return ERR_PTR(err); 
218} 
219 
220static int vinput_register_vdevice(struct vinput *vinput) 
221{ 
222    int err = 0; 
223 
224    /* register the input device */ 
225    vinput->input->name = vinput->type->name; 
226    vinput->input->phys = "vinput"; 
227    vinput->input->dev.parent = &vinput->dev; 
228 
229    vinput->input->id.bustype = BUS_VIRTUAL; 
230    vinput->input->id.product = 0x0000; 
231    vinput->input->id.vendor = 0x0000; 
232    vinput->input->id.version = 0x0000; 
233 
234    err = vinput->type->ops->init(vinput); 
235 
236    if (err == 0) 
237        dev_info(&vinput->dev, "Registered virtual input %s %ldn", 
238                 vinput->type->name, vinput->id); 
239 
240    return err; 
241} 
242 
243static ssize_t export_store(struct class *class, struct class_attribute *attr, 
244                            const char *buf, size_t len) 
245{ 
246    int err; 
247    struct vinput *vinput; 
248    struct vinput_device *device; 
249 
250    device = vinput_get_device_by_type(buf); 
251    if (IS_ERR(device)) { 
252        pr_info("vinput: This virtual device isn't registeredn"); 
253        err = PTR_ERR(device); 
254        goto fail; 
255    } 
256 
257    vinput = vinput_alloc_vdevice(); 
258    if (IS_ERR(vinput)) { 
259        err = PTR_ERR(vinput); 
260        goto fail; 
261    } 
262 
263    vinput->type = device; 
264    err = device_register(&vinput->dev); 
265    if (err < 0) 
266        goto fail_register; 
267 
268    err = vinput_register_vdevice(vinput); 
269    if (err < 0) 
270        goto fail_register_vinput; 
271 
272    return len; 
273 
274fail_register_vinput: 
275    device_unregister(&vinput->dev); 
276fail_register: 
277    vinput_destroy_vdevice(vinput); 
278fail: 
279    return err; 
280} 
281/* This macro generates class_attr_export structure and export_store() */ 
282static CLASS_ATTR_WO(export); 
283 
284static ssize_t unexport_store(struct class *class, struct class_attribute *attr, 
285                              const char *buf, size_t len) 
286{ 
287    int err; 
288    unsigned long id; 
289    struct vinput *vinput; 
290 
291    err = kstrtol(buf, 10, &id); 
292    if (err) { 
293        err = -EINVAL; 
294        goto failed; 
295    } 
296 
297    vinput = vinput_get_vdevice_by_id(id); 
298    if (IS_ERR(vinput)) { 
299        pr_err("vinput: No such vinput device %ldn", id); 
300        err = PTR_ERR(vinput); 
301        goto failed; 
302    } 
303 
304    vinput_unregister_vdevice(vinput); 
305    device_unregister(&vinput->dev); 
306 
307    return len; 
308failed: 
309    return err; 
310} 
311/* This macro generates class_attr_unexport structure and unexport_store() */ 
312static CLASS_ATTR_WO(unexport); 
313 
314static struct attribute *vinput_class_attrs[] = { 
315    &class_attr_export.attr, 
316    &class_attr_unexport.attr, 
317    NULL, 
318}; 
319 
320/* This macro generates vinput_class_groups structure */ 
321ATTRIBUTE_GROUPS(vinput_class); 
322 
323static struct class vinput_class = { 
324    .name = "vinput", 
325    .owner = THIS_MODULE, 
326    .class_groups = vinput_class_groups, 
327}; 
328 
329int vinput_register(struct vinput_device *dev) 
330{ 
331    spin_lock(&vinput_lock); 
332    list_add(&dev->list, &vinput_devices); 
333    spin_unlock(&vinput_lock); 
334 
335    pr_info("vinput: registered new virtual input device '%s'n", dev->name); 
336 
337    return 0; 
338} 
339EXPORT_SYMBOL(vinput_register); 
340 
341void vinput_unregister(struct vinput_device *dev) 
342{ 
343    struct list_head *curr, *next; 
344 
345    /* Remove from the list first */ 
346    spin_lock(&vinput_lock); 
347    list_del(&dev->list); 
348    spin_unlock(&vinput_lock); 
349 
350    /* unregister all devices of this type */ 
351    list_for_each_safe (curr, next, &vinput_vdevices) { 
352        struct vinput *vinput = list_entry(curr, struct vinput, list); 
353        if (vinput && vinput->type == dev) { 
354            vinput_unregister_vdevice(vinput); 
355            device_unregister(&vinput->dev); 
356        } 
357    } 
358 
359    pr_info("vinput: unregistered virtual input device '%s'n", dev->name); 
360} 
361EXPORT_SYMBOL(vinput_unregister); 
362 
363static int __init vinput_init(void) 
364{ 
365    int err = 0; 
366 
367    pr_info("vinput: Loading virtual input drivern"); 
368 
369    vinput_dev = register_chrdev(0, DRIVER_NAME, &vinput_fops); 
370    if (vinput_dev < 0) { 
371        pr_err("vinput: Unable to allocate char dev regionn"); 
372        err = vinput_dev; 
373        goto failed_alloc; 
374    } 
375 
376    spin_lock_init(&vinput_lock); 
377 
378    err = class_register(&vinput_class); 
379    if (err < 0) { 
380        pr_err("vinput: Unable to register vinput classn"); 
381        goto failed_class; 
382    } 
383 
384    return 0; 
385failed_class: 
386    class_unregister(&vinput_class); 
387failed_alloc: 
388    return err; 
389} 
390 
391static void __exit vinput_end(void) 
392{ 
393    pr_info("vinput: Unloading virtual input drivern"); 
394 
395    unregister_chrdev(vinput_dev, DRIVER_NAME); 
396    class_unregister(&vinput_class); 
397} 
398 
399module_init(vinput_init); 
400module_exit(vinput_end); 
401 
402MODULE_LICENSE("GPL"); 
403MODULE_DESCRIPTION("Emulate input events");

Here the virtual keyboard is one of example to use vinput. It supports all
KEY_MAX
keycodes. The injection format is the KEY_CODE
such as defined in include/linux/input.h. A positive value means
KEY_PRESS
while a negative value is a KEY_RELEASE
. The keyboard supports repetition when the key stays pressed for too long. The
following demonstrates how simulation work.

Simulate a key press on “g” ( KEY_G
= 34):

1echo "+34" | sudo tee /dev/vinput0

Simulate a key release on “g” ( KEY_G
= 34):

1echo "-34" | sudo tee /dev/vinput0
1/* 
2 * vkbd.c 
3 */ 
4 
5#include  
6#include  
7#include  
8#include  
9 
10#include "vinput.h" 
11 
12#define VINPUT_KBD "vkbd" 
13#define VINPUT_RELEASE 0 
14#define VINPUT_PRESS 1 
15 
16static unsigned short vkeymap[KEY_MAX]; 
17 
18static int vinput_vkbd_init(struct vinput *vinput) 
19{ 
20    int i; 
21 
22    /* Set up the input bitfield */ 
23    vinput->input->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REP); 
24    vinput->input->keycodesize = sizeof(unsigned short); 
25    vinput->input->keycodemax = KEY_MAX; 
26    vinput->input->keycode = vkeymap; 
27 
28    for (i = 0; i < KEY_MAX; i++) 
29        set_bit(vkeymap[i], vinput->input->keybit); 
30 
31    /* vinput will help us allocate new input device structure via 
32     * input_allocate_device(). So, we can register it straightforwardly. 
33     */ 
34    return input_register_device(vinput->input); 
35} 
36 
37static int vinput_vkbd_read(struct vinput *vinput, char *buff, int len) 
38{ 
39    spin_lock(&vinput->lock); 
40    len = snprintf(buff, len, "%+ldn", vinput->last_entry); 
41    spin_unlock(&vinput->lock); 
42 
43    return len; 
44} 
45 
46static int vinput_vkbd_send(struct vinput *vinput, char *buff, int len) 
47{ 
48    int ret; 
49    long key = 0; 
50    short type = VINPUT_PRESS; 
51 
52    /* Determine which event was received (press or release) 
53     * and store the state. 
54     */ 
55    if (buff[0] == '+') 
56        ret = kstrtol(buff + 1, 10, &key); 
57    else 
58        ret = kstrtol(buff, 10, &key); 
59    if (ret) 
60        dev_err(&vinput->dev, "error during kstrtol: -%dn", ret); 
61    spin_lock(&vinput->lock); 
62    vinput->last_entry = key; 
63    spin_unlock(&vinput->lock); 
64 
65    if (key < 0) { 
66        type = VINPUT_RELEASE; 
67        key = -key; 
68    } 
69 
70    dev_info(&vinput->dev, "Event %s code %ldn", 
71             (type == VINPUT_RELEASE) ? "VINPUT_RELEASE" : "VINPUT_PRESS", key); 
72 
73    /* Report the state received to input subsystem. */ 
74    input_report_key(vinput->input, key, type); 
75    /* Tell input subsystem that it finished the report. */ 
76    input_sync(vinput->input); 
77 
78    return len; 
79} 
80 
81static struct vinput_ops vkbd_ops = { 
82    .init = vinput_vkbd_init, 
83    .send = vinput_vkbd_send, 
84    .read = vinput_vkbd_read, 
85}; 
86 
87static struct vinput_device vkbd_dev = { 
88    .name = VINPUT_KBD, 
89    .ops = &vkbd_ops, 
90}; 
91 
92static int __init vkbd_init(void) 
93{ 
94    int i; 
95 
96    for (i = 0; i < KEY_MAX; i++) 
97        vkeymap[i] = i; 
98    return vinput_register(&vkbd_dev); 
99} 
100 
101static void __exit vkbd_end(void) 
102{ 
103    vinput_unregister(&vkbd_dev); 
104} 
105 
106module_init(vkbd_init); 
107module_exit(vkbd_end); 
108 
109MODULE_LICENSE("GPL"); 
110MODULE_DESCRIPTION("Emulate keyboard input events through /dev/vinput");

18 Standardizing the interfaces: The Device Model

Up to this point we have seen all kinds of modules doing all kinds of things, but there
was no consistency in their interfaces with the rest of the kernel. To impose some
consistency such that there is at minimum a standardized way to start, suspend and
resume a device a device model was added. An example is shown below, and you can
use this as a template to add your own suspend, resume or other interface
functions.

1/* 
2 * devicemodel.c 
3 */ 
4#include  
5#include  
6#include  
7 
8struct devicemodel_data { 
9    char *greeting; 
10    int number; 
11}; 
12 
13static int devicemodel_probe(struct platform_device *dev) 
14{ 
15    struct devicemodel_data *pd = 
16        (struct devicemodel_data *)(dev->dev.platform_data); 
17 
18    pr_info("devicemodel proben"); 
19    pr_info("devicemodel greeting: %s; %dn", pd->greeting, pd->number); 
20 
21    /* Your device initialization code */ 
22 
23    return 0; 
24} 
25 
26static int devicemodel_remove(struct platform_device *dev) 
27{ 
28    pr_info("devicemodel example removedn"); 
29 
30    /* Your device removal code */ 
31 
32    return 0; 
33} 
34 
35static int devicemodel_suspend(struct device *dev) 
36{ 
37    pr_info("devicemodel example suspendn"); 
38 
39    /* Your device suspend code */ 
40 
41    return 0; 
42} 
43 
44static int devicemodel_resume(struct device *dev) 
45{ 
46    pr_info("devicemodel example resumen"); 
47 
48    /* Your device resume code */ 
49 
50    return 0; 
51} 
52 
53static const struct dev_pm_ops devicemodel_pm_ops = { 
54    .suspend = devicemodel_suspend, 
55    .resume = devicemodel_resume, 
56    .poweroff = devicemodel_suspend, 
57    .freeze = devicemodel_suspend, 
58    .thaw = devicemodel_resume, 
59    .restore = devicemodel_resume, 
60}; 
61 
62static struct platform_driver devicemodel_driver = { 
63    .driver = 
64        { 
65            .name = "devicemodel_example", 
66            .pm = &devicemodel_pm_ops, 
67        }, 
68    .probe = devicemodel_probe, 
69    .remove = devicemodel_remove, 
70}; 
71 
72static int devicemodel_init(void) 
73{ 
74    int ret; 
75 
76    pr_info("devicemodel initn"); 
77 
78    ret = platform_driver_register(&devicemodel_driver); 
79 
80    if (ret) { 
81        pr_err("Unable to register drivern"); 
82        return ret; 
83    } 
84 
85    return 0; 
86} 
87 
88static void devicemodel_exit(void) 
89{ 
90    pr_info("devicemodel exitn"); 
91    platform_driver_unregister(&devicemodel_driver); 
92} 
93 
94module_init(devicemodel_init); 
95module_exit(devicemodel_exit); 
96 
97MODULE_LICENSE("GPL"); 
98MODULE_DESCRIPTION("Linux Device Model example");

19 Optimizations

19.1 Likely and Unlikely conditions

Sometimes you might want your code to run as quickly as possible,
especially if it is handling an interrupt or doing something which might
cause noticeable latency. If your code contains boolean conditions and if
you know that the conditions are almost always likely to evaluate as either
true
or false
, then you can allow the compiler to optimize for this using the
likely
and unlikely
macros. For example, when allocating memory you are almost always expecting this
to succeed.

1bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx); 
2if (unlikely(!bvl)) { 
3    mempool_free(bio, bio_pool); 
4    bio = NULL; 
5    goto out; 
6}

When the unlikely
macro is used, the compiler alters its machine instruction output, so that it
continues along the false branch and only jumps if the condition is true. That
avoids flushing the processor pipeline. The opposite happens if you use the
likely
macro.

19.2 Static keys

Static keys allow us to enable or disable kernel code paths based on the runtime state
of key. Its APIs have been available since 2010 (most architectures are already
supported), use self-modifying code to eliminate the overhead of cache and branch
prediction. The most typical use case of static keys is for performance-sensitive kernel
code, such as tracepoints, context switching, networking, etc. These hot paths of the
kernel often contain branches and can be optimized easily using this technique.
Before we can use static keys in the kernel, we need to make sure that gcc supports
asm goto
inline assembly, and the following kernel configurations are set:

1CONFIG_JUMP_LABEL=y 
2CONFIG_HAVE_ARCH_JUMP_LABEL=y 
3CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE=y

To declare a static key, we need to define a global variable using the
DEFINE_STATIC_KEY_FALSE
or DEFINE_STATIC_KEY_TRUE
macro defined in include/linux/jump_label.h. This macro initializes the key with
the given initial value, which is either false or true, respectively. For example, to
declare a static key with an initial value of false, we can use the following
code:

1DEFINE_STATIC_KEY_FALSE(fkey);

Once the static key has been declared, we need to add branching code to the
module that uses the static key. For example, the code includes a fastpath, where a
no-op instruction will be generated at compile time as the key is initialized to false
and the branch is unlikely to be taken.

1pr_info("fastpath 1n"); 
2if (static_branch_unlikely(&fkey)) 
3    pr_alert("do unlikely thingn"); 
4pr_info("fastpath 2n");

If the key is enabled at runtime by calling
static_branch_enable(&fkey)
, the fastpath will be patched with an unconditional jump instruction to the slowpath
code pr_alert
, so the branch will always be taken until the key is disabled again.

The following kernel module derived from chardev.c, demostrates how the static
key works.

1/* 
2 * static_key.c 
3 */ 
4 
5#include  
6#include  
7#include  
8#include  /* for sprintf() */ 
9#include  
10#include  
11#include  
12#include  /* for get_user and put_user */ 
13 
14#include  
15 
16static int device_open(struct inode *inode, struct file *file); 
17static int device_release(struct inode *inode, struct file *file); 
18static ssize_t device_read(struct file *file, char __user *buf, size_t count, 
19                           loff_t *ppos); 
20static ssize_t device_write(struct file *file, const char __user *buf, 
21                            size_t count, loff_t *ppos); 
22 
23#define SUCCESS 0 
24#define DEVICE_NAME "key_state" 
25#define BUF_LEN 10 
26 
27static int major; 
28 
29enum { 
30    CDEV_NOT_USED = 0, 
31    CDEV_EXCLUSIVE_OPEN = 1, 
32}; 
33 
34static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
35 
36static char msg[BUF_LEN + 1]; 
37 
38static struct class *cls; 
39 
40static DEFINE_STATIC_KEY_FALSE(fkey); 
41 
42static struct file_operations chardev_fops = { 
43    .owner = THIS_MODULE, 
44    .open = device_open, 
45    .release = device_release, 
46    .read = device_read, 
47    .write = device_write, 
48}; 
49 
50static int __init chardev_init(void) 
51{ 
52    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
53    if (major < 0) { 
54        pr_alert("Registering char device failed with %dn", major); 
55        return major; 
56    } 
57 
58    pr_info("I was assigned major number %dn", major); 
59 
60    cls = class_create(THIS_MODULE, DEVICE_NAME); 
61 
62    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
63 
64    pr_info("Device created on /dev/%sn", DEVICE_NAME); 
65 
66    return SUCCESS; 
67} 
68 
69static void __exit chardev_exit(void) 
70{ 
71    device_destroy(cls, MKDEV(major, 0)); 
72    class_destroy(cls); 
73 
74    /* Unregister the device */ 
75    unregister_chrdev(major, DEVICE_NAME); 
76} 
77 
78/* Methods */ 
79 
80/** 
81 * Called when a process tried to open the device file, like 
82 * cat /dev/key_state 
83 */ 
84static int device_open(struct inode *inode, struct file *file) 
85{ 
86    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
87        return -EBUSY; 
88 
89    sprintf(msg, static_key_enabled(&fkey) ? "enabledn" : "disabledn"); 
90 
91    pr_info("fastpath 1n"); 
92    if (static_branch_unlikely(&fkey)) 
93        pr_alert("do unlikely thingn"); 
94    pr_info("fastpath 2n"); 
95 
96    try_module_get(THIS_MODULE); 
97 
98    return SUCCESS; 
99} 
100 
101/** 
102 * Called when a process closes the device file 
103 */ 
104static int device_release(struct inode *inode, struct file *file) 
105{ 
106    /* We are now ready for our next caller. */ 
107    atomic_set(&already_open, CDEV_NOT_USED); 
108 
109    /** 
110     * Decrement the usage count, or else once you opened the file, you will 
111     * never get rid of the module. 
112     */ 
113    module_put(THIS_MODULE); 
114 
115    return SUCCESS; 
116} 
117 
118/** 
119 * Called when a process, which already opened the dev file, attempts to 
120 * read from it. 
121 */ 
122static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ 
123                           char __user *buffer, /* buffer to fill with data */ 
124                           size_t length, /* length of the buffer */ 
125                           loff_t *offset) 
126{ 
127    /* Number of the bytes actually written to the buffer */ 
128    int bytes_read = 0; 
129    const char *msg_ptr = msg; 
130 
131    if (!*(msg_ptr + *offset)) { /* We are at the end of the message */ 
132        *offset = 0; /* reset the offset */ 
133        return 0; /* signify end of file */ 
134    } 
135 
136    msg_ptr += *offset; 
137 
138    /* Actually put the date into the buffer */ 
139    while (length && *msg_ptr) { 
140        /** 
141         * The buffer is in the user data segment, not the kernel 
142         * segment so "*" assignment won't work. We have to use 
143         * put_user which copies data from the kernel data segment to 
144         * the user data segment. 
145         */ 
146        put_user(*(msg_ptr++), buffer++); 
147        length--; 
148        bytes_read++; 
149    } 
150 
151    *offset += bytes_read; 
152 
153    /* Most read functions return the number of bytes put into the buffer. */ 
154    return bytes_read; 
155} 
156 
157/* Called when a process writes to dev file; echo "enable" > /dev/key_state */ 
158static ssize_t device_write(struct file *filp, const char __user *buffer, 
159                            size_t length, loff_t *offset) 
160{ 
161    char command[10]; 
162 
163    if (length > 10) { 
164        pr_err("command exceeded 10 charn"); 
165        return -EINVAL; 
166    } 
167 
168    if (copy_from_user(command, buffer, length)) 
169        return -EFAULT; 
170 
171    if (strncmp(command, "enable", strlen("enable")) == 0) 
172        static_branch_enable(&fkey); 
173    else if (strncmp(command, "disable", strlen("disable")) == 0) 
174        static_branch_disable(&fkey); 
175    else { 
176        pr_err("Invalid command: %sn", command); 
177        return -EINVAL; 
178    } 
179 
180    /* Again, return the number of input characters used. */ 
181    return length; 
182} 
183 
184module_init(chardev_init); 
185module_exit(chardev_exit); 
186 
187MODULE_LICENSE("GPL");

To check the state of the static key, we can use the /dev/key_state
interface.

1cat /dev/key_state

This will display the current state of the key, which is disabled by default.

To change the state of the static key, we can perform a write operation on the
file:

1echo enable > /dev/key_state

This will enable the static key, causing the code path to switch from the fastpath
to the slowpath.

In some cases, the key is enabled or disabled at initialization and never changed,
we can declare a static key as read-only, which means that it can only be toggled in
the module init function. To declare a read-only static key, we can use the
DEFINE_STATIC_KEY_FALSE_RO
or DEFINE_STATIC_KEY_TRUE_RO
macro instead. Attempts to change the key at runtime will result in a page fault. For
more information, see Static keys

20 Common Pitfalls

20.1 Using standard libraries

You can not do that. In a kernel module, you can only use kernel functions which are
the functions you can see in /proc/kallsyms.

20.2 Disabling interrupts

You might need to do this for a short time and that is OK, but if you do not enable
them afterwards, your system will be stuck and you will have to power it
off.

21 Where To Go From Here?

For people seriously interested in kernel programming, I recommend kernelnewbies.org
and the Documentation subdirectory within the kernel source code which is not
always easy to understand but can be a starting point for further investigation. Also,
as Linus Torvalds said, the best way to learn the kernel is to read the source code
yourself.

If you would like to contribute to this guide or notice anything glaringly wrong,
please create an issue at https://github.com/sysprog21/lkmpg. Your pull requests
will be appreciated.

Happy hacking!

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