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 <jserv@ccns.ncku.edu.tw>. 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 initially authored by Ori Pomerantz for Linux v2.2. As the Linux kernel evolved, Ori’s availability to maintain the document diminished. Consequently, Peter Jay Salzman assumed the role of maintainer and updated the guide for Linux v2.4. Similar constraints arose for Peter when tracking developments in Linux v2.6, leading to Michael Burian joining as a co-maintainer to bring the guide up to speed with Linux v2.6. Bob Mottram contributed to the guide by updating examples for Linux v3.8 and later. Jim Huang then undertook the task of updating the guide for recent Linux versions (v5.0 and beyond), along with revising the LaTeX document.

1.2 Acknowledgements

The following people have contributed corrections or good suggestions:

1.3 What Is A Kernel Module?

Involvement in the development of Linux kernel modules requires a foundation in the C programming language and a track record of creating conventional programs intended for process execution. This pursuit delves into a domain where an unregulated pointer, if disregarded, may potentially trigger the total elimination of an entire file system, resulting in a scenario that necessitates a complete system reboot.

A Linux kernel module is precisely defined as a code segment capable of dynamic loading and unloading within the kernel as needed. These modules enhance kernel capabilities without necessitating a system reboot. A notable example is seen in the device driver module, which facilitates kernel interaction with hardware components linked to the system. In the absence of modules, the prevailing approach leans toward monolithic kernels, requiring direct integration of new functionalities into the kernel image. This approach leads to larger kernels and necessitates kernel rebuilding and subsequent system rebooting when new functionalities are desired.

1.4 Kernel module package

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

On Ubuntu/Debian GNU/Linux:

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 .

1lsmod

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

1cat /proc/modules

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

1lsmod | grep fat

1.6 Is there a need to download and compile the kernel?

To effectively follow this guide, there is no obligatory requirement for performing such actions. Nonetheless, a prudent approach involves executing the examples within a test distribution on a virtual machine, thus mitigating any potential risk of disrupting the system.

1.7 Before We Begin

Before delving into code, certain matters require attention. Variances exist among individuals’ systems, and distinct personal approaches are evident. The achievement of successful compilation and loading of the inaugural “hello world” program may, at times, present challenges. It is reassuring to note that overcoming the initial obstacle in the first attempt paves the way for subsequent endeavors to proceed seamlessly.

1. Modversioning. A module compiled for one kernel will not load if a different kernel is booted, unless CONFIG_MODVERSIONS is enabled in the kernel. Module versioning will be discussed later in this guide. Until module versioning is covered, the examples in this guide may not work correctly if running a kernel with modversioning turned on. However, most stock Linux distribution kernels come with modversioning enabled. If difficulties arise when loading the modules due to versioning errors, consider compiling a kernel with modversioning turned off.

2. Using X Window System. It is highly recommended to extract, compile, and load all the examples discussed in this guide from a console. Working on these tasks within the X Window System is discouraged.

   Modules cannot directly print to the screen like printf() can, but they can log information and warnings that are eventually displayed on the screen, specifically within a console. If a module is loaded from an xterm , the information and warnings will be logged, but solely within the systemd journal. These logs will not be visible unless consulting the journalctl . Refer to 4 for more information. For instant access to this information, it is advisable to perform all tasks from the console.

3. SecureBoot. Numerous modern computers arrive pre-configured with UEFI SecureBoot enabled—an essential security standard ensuring booting exclusively through trusted software endorsed by the original equipment manufacturer. Certain Linux distributions even ship with the default Linux kernel configured to support SecureBoot. In these cases, the kernel module necessitates a signed security key.

   Failing this, an attempt to insert your first “hello world” module would result in the message: “ERROR: could not insert module”. If this message Lockdown: insmod: unsigned module loading is restricted; see man kernel lockdown.7 appears in the dmesg output, the simplest approach involves disabling UEFI SecureBoot from the boot menu of your PC or laptop, allowing the successful insertion of “hello world” module. Naturally, an alternative involves undergoing intricate procedures such as generating keys, system key installation, and module signing to achieve functionality. However, this intricate process is less appropriate for beginners. If interested, more detailed steps for SecureBoot can be explored and followed.

2 Headers

Before building anything, it is necessary to install the header files for the kernel.

On Ubuntu/Debian GNU/Linux:

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

The following command provides information on the available kernel header files. Then for example:

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

On Arch Linux:

1sudo pacman -S linux-headers

On Fedora:

1sudo dnf install kernel-devel kernel-headers

3 Examples

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

Should compile errors occur, it may be due to a more recent kernel version being in use, or there might be a need to install the corresponding kernel header files.

4 Hello World

4.1 The Simplest Module

Most individuals beginning their programming journey typically start with some variant of a hello world example. It is unclear what the outcomes are for those who deviate from this tradition, but it seems prudent to adhere to it. The learning process will begin with a series of hello world programs that illustrate various fundamental aspects of writing a kernel module.

Presented next is the simplest possible module.

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 <linux/module.h> /* Needed by all modules */ 
5#include <linux/printk.h> /* 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:

1lsmod | 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:

1lsmod | 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:

1journalctl --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 <linux/module.h>. We needed to include <linux/printk.h> 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 <linux/init.h> /* Needed for the macros */ 
6#include <linux/module.h> /* Needed by all modules */ 
7#include <linux/printk.h> /* Needed for pr_info() */ 
8 
9static int __init hello_2_init(void) 
10{ 
11    pr_info("Hello, world 2\n"); 
12    return 0; 
13} 
14 
15static void __exit hello_2_exit(void) 
16{ 
17    pr_info("Goodbye, world 2\n"); 
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 <linux/init.h> /* Needed for the macros */ 
5#include <linux/module.h> /* Needed by all modules */ 
6#include <linux/printk.h> /* 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 %d\n", hello3_data); 
13    return 0; 
14} 
15 
16static void __exit hello_3_exit(void) 
17{ 
18    pr_info("Goodbye, world 3\n"); 
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 <linux/init.h> /* Needed for the macros */ 
5#include <linux/module.h> /* Needed by all modules */ 
6#include <linux/printk.h> /* 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 4\n"); 
15    return 0; 
16} 
17 
18static void __exit cleanup_hello_4(void) 
19{ 
20    pr_info("Goodbye, world 4\n"); 
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 <linux/init.h> 
5#include <linux/kernel.h> /* for ARRAY_SIZE() */ 
6#include <linux/module.h> 
7#include <linux/moduleparam.h> 
8#include <linux/printk.h> 
9#include <linux/stat.h> 
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 parameter's 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 5\n=============\n"); 
50    pr_info("myshort is a short integer: %hd\n", myshort); 
51    pr_info("myint is an integer: %d\n", myint); 
52    pr_info("mylong is a long integer: %ld\n", mylong); 
53    pr_info("mystring is a string: %s\n", mystring); 
54 
55    for (i = 0; i < ARRAY_SIZE(myintarray); i++) 
56        pr_info("myintarray[%d] = %d\n", 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 5\n"); 
65} 
66 
67module_init(hello_5_init); 
68module_exit(hello_5_exit);

It is recommended to experiment with the following 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 <linux/kernel.h> /* We are doing kernel work */ 
6#include <linux/module.h> /* Specifically, a module */ 
7 
8int init_module(void) 
9{ 
10    pr_info("Hello, world - this is the kernel speaking\n"); 
11    return 0; 
12} 
13 
14MODULE_LICENSE("GPL");

The next file:

1/* 
2 * stop.c - Illustration of multi filed modules 
3 */ 
4 
5#include <linux/kernel.h> /* We are doing kernel work */ 
6#include <linux/module.h> /* Specifically, a module  */ 
7 
8void cleanup_module(void) 
9{ 
10    pr_info("Short is the life of a kernel module\n"); 
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 keeping 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 typical program starts with a main() function, executes a series of instructions, and terminates after completing these instructions. Kernel modules, however, follow a different pattern. A module always begins with either the init_module function or a function designated by the module_init call. This function acts as the module’s entry point, informing the kernel of the module’s functionalities and preparing the kernel to utilize the module’s functions when necessary. After performing these tasks, the entry function returns, and the module remains inactive until the kernel requires its code.

All modules conclude by invoking either cleanup_module or a function specified through the module_exit call. This serves as the module’s exit function, reversing the actions of the entry function by unregistering the previously registered functionalities.

It is mandatory for every module to have both an entry and an exit function. While there are multiple methods to define these functions, the terms “entry function” and “exit function” are generally used. However, they may occasionally be referred to as init_module and cleanup_module , which are understood to mean the same.

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 ensures 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 <stdio.h> 
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

The kernel primarily manages access to resources, be it a video card, hard drive, or memory. Programs frequently vie for the same resources. For instance, as a document is saved, updatedb might commence updating the locate database. Sessions in editors like vim and processes like updatedb can simultaneously utilize the hard drive. The kernel’s role is to maintain order, ensuring that users do not access resources indiscriminately.

To manage this, CPUs operate in different modes, each offering varying levels of system control. The Intel 80386 architecture, for example, featured four such modes, known as rings. Unix, however, utilizes only two of these rings: the highest ring (ring 0, also known as “supervisor mode”, where all actions are permissible) and the lowest ring, referred to as “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.

It should be noted that the aforementioned discussion applies to any operating system utilizing a monolithic kernel. This concept differs slightly from “building all your modules into the kernel”, although the underlying principle is similar. In contrast, there are microkernels, where modules are allocated their own code space. Two notable examples of microkernels include the GNU Hurd and the Zircon kernel of Google’s Fuchsia.

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.

A few final points, although implicit in the previous discussion, are worth stating explicitly for clarity. When a device file is accessed, the kernel utilizes the file’s major number to identify the appropriate driver for handling the access. This indicates that the kernel does not necessarily rely on or need to be aware of the minor number. It is the driver that concerns itself with the minor number, using it to differentiate between various pieces of hardware.

It is important to note that when referring to “hardware”, the term is used in a slightly more abstract sense than just a physical PCI card that can be held in hand. Consider the following 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 int, unsigned long); 
13    long (*compat_ioctl) (struct file *, unsigned int, unsigned 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) (int, struct file *, int); 
21    int (*lock) (struct file *, int, struct file_lock *); 
22    ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); 
23    unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); 
24    int (*check_flags)(int); 
25    int (*flock) (struct file *, int, struct file_lock *); 
26    ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); 
27    ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); 
28    int (*setlease)(struct file *, long, struct 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_t, unsigned 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 dynamically-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 an 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 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 <linux/atomic.h> 
7#include <linux/cdev.h> 
8#include <linux/delay.h> 
9#include <linux/device.h> 
10#include <linux/fs.h> 
11#include <linux/init.h> 
12#include <linux/kernel.h> /* for sprintf() */ 
13#include <linux/module.h> 
14#include <linux/printk.h> 
15#include <linux/types.h> 
16#include <linux/uaccess.h> /* for get_user and put_user */ 
17#include <linux/version.h> 
18 
19#include <asm/errno.h> 
20 
21/*  Prototypes - this would normally go in a .h file */ 
22static int device_open(struct inode *, struct file *); 
23static int device_release(struct inode *, struct file *); 
24static ssize_t device_read(struct file *, char __user *, size_t, loff_t *); 
25static ssize_t device_write(struct file *, const char __user *, size_t, 
26                            loff_t *); 
27 
28#define SUCCESS 0 
29#define DEVICE_NAME "chardev" /* Dev name as it appears in /proc/devices   */ 
30#define BUF_LEN 80 /* Max length of the message from the device */ 
31 
32/* Global variables are declared as static, so are global within the file. */ 
33 
34static int major; /* major number assigned to our device driver */ 
35 
36enum { 
37    CDEV_NOT_USED, 
38    CDEV_EXCLUSIVE_OPEN, 
39}; 
40 
41/* Is device open? Used to prevent multiple access to device */ 
42static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
43 
44static char msg[BUF_LEN + 1]; /* The msg the device will give when asked */ 
45 
46static struct class *cls; 
47 
48static struct file_operations chardev_fops = { 
49    .read = device_read, 
50    .write = device_write, 
51    .open = device_open, 
52    .release = device_release, 
53}; 
54 
55static int __init chardev_init(void) 
56{ 
57    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
58 
59    if (major < 0) { 
60        pr_alert("Registering char device failed with %d\n", major); 
61        return major; 
62    } 
63 
64    pr_info("I was assigned major number %d.\n", major); 
65 
66#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 4, 0) 
67    cls = class_create(DEVICE_NAME); 
68#else 
69    cls = class_create(THIS_MODULE, DEVICE_NAME); 
70#endif 
71    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
72 
73    pr_info("Device created on /dev/%s\n", DEVICE_NAME); 
74 
75    return SUCCESS; 
76} 
77 
78static void __exit chardev_exit(void) 
79{ 
80    device_destroy(cls, MKDEV(major, 0)); 
81    class_destroy(cls); 
82 
83    /* Unregister the device */ 
84    unregister_chrdev(major, DEVICE_NAME); 
85} 
86 
87/* Methods */ 
88 
89/* Called when a process tries to open the device file, like 
90 * "sudo cat /dev/chardev" 
91 */ 
92static int device_open(struct inode *inode, struct file *file) 
93{ 
94    static int counter = 0; 
95 
96    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
97        return -EBUSY; 
98 
99    sprintf(msg, "I already told you %d times Hello world!\n", counter++); 
100    try_module_get(THIS_MODULE); 
101 
102    return SUCCESS; 
103} 
104 
105/* Called when a process closes the device file. */ 
106static int device_release(struct inode *inode, struct file *file) 
107{ 
108    /* We're now ready for our next caller */ 
109    atomic_set(&already_open, CDEV_NOT_USED); 
110 
111    /* Decrement the usage count, or else once you opened the file, you will 
112     * never get rid of the module. 
113     */ 
114    module_put(THIS_MODULE); 
115 
116    return SUCCESS; 
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 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 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 data into the buffer */ 
139    while (length && *msg_ptr) { 
140        /* The buffer is in the user data segment, not the kernel 
141         * segment so "*" assignment won't work.  We have to use 
142         * put_user which copies data from the kernel data segment to 
143         * the user data segment. 
144         */ 
145        put_user(*(msg_ptr++), buffer++); 
146        length--; 
147        bytes_read++; 
148    } 
149 
150    *offset += bytes_read; 
151 
152    /* Most read functions return the number of bytes put into the buffer. */ 
153    return bytes_read; 
154} 
155 
156/* Called when a process writes to dev file: echo "hi" > /dev/hello */ 
157static ssize_t device_write(struct file *filp, const char __user *buff, 
158                            size_t len, loff_t *off) 
159{ 
160    pr_alert("Sorry, this operation is not supported.\n"); 
161    return -EINVAL; 
162} 
163 
164module_init(chardev_init); 
165module_exit(chardev_exit); 
166 
167MODULE_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 .

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 pointer to 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 <linux/kernel.h> 
6#include <linux/module.h> 
7#include <linux/proc_fs.h> 
8#include <linux/uaccess.h> 
9#include <linux/version.h> 
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 failed\n"); 
28        ret = 0; 
29    } else { 
30        pr_info("procfile read %s\n", 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        pr_alert("Error:Could not initialize /proc/%s\n", procfs_name); 
52        return -ENOMEM; 
53    } 
54 
55    pr_info("/proc/%s created\n", procfs_name); 
56    return 0; 
57} 
58 
59static void __exit procfs1_exit(void) 
60{ 
61    proc_remove(our_proc_file); 
62    pr_info("/proc/%s removed\n", procfs_name); 
63} 
64 
65module_init(procfs1_init); 
66module_exit(procfs1_exit); 
67 
68MODULE_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 <linux/kernel.h> /* We're doing kernel work */ 
6#include <linux/module.h> /* Specifically, a module */ 
7#include <linux/proc_fs.h> /* Necessary because we use the proc fs */ 
8#include <linux/uaccess.h> /* for copy_from_user */ 
9#include <linux/version.h> 
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 failed\n"); 
37        ret = 0; 
38    } else { 
39        pr_info("procfile read %s\n", 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 - 1; 
53 
54    if (copy_from_user(procfs_buffer, buff, procfs_buffer_size)) 
55        return -EFAULT; 
56 
57    procfs_buffer[procfs_buffer_size] = '\0'; 
58    *off += procfs_buffer_size; 
59    pr_info("procfile write %s\n", 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        pr_alert("Error:Could not initialize /proc/%s\n", PROCFS_NAME); 
81        return -ENOMEM; 
82    } 
83 
84    pr_info("/proc/%s created\n", PROCFS_NAME); 
85    return 0; 
86} 
87 
88static void __exit procfs2_exit(void) 
89{ 
90    proc_remove(our_proc_file); 
91    pr_info("/proc/%s removed\n", PROCFS_NAME); 
92} 
93 
94module_init(procfs2_init); 
95module_exit(procfs2_exit); 
96 
97MODULE_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 procfs_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 <linux/kernel.h> 
6#include <linux/module.h> 
7#include <linux/proc_fs.h> 
8#include <linux/sched.h> 
9#include <linux/uaccess.h> 
10#include <linux/version.h> 
11#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 10, 0) 
12#include <linux/minmax.h> 
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: END\n"); 
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 bytes\n", 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 bytes\n", 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        pr_debug("Error: Could not initialize /proc/%s\n", 
86                 PROCFS_ENTRY_FILENAME); 
87        return -ENOMEM; 
88    } 
89    proc_set_size(our_proc_file, 80); 
90    proc_set_user(our_proc_file, GLOBAL_ROOT_UID, GLOBAL_ROOT_GID); 
91 
92    pr_debug("/proc/%s created\n", PROCFS_ENTRY_FILENAME); 
93    return 0; 
94} 
95 
96static void __exit procfs3_exit(void) 
97{ 
98    remove_proc_entry(PROCFS_ENTRY_FILENAME, NULL); 
99    pr_debug("/proc/%s removed\n", PROCFS_ENTRY_FILENAME); 
100} 
101 
102module_init(procfs3_init); 
103module_exit(procfs3_exit); 
104 
105MODULE_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 writing /proc file, there is an API named seq_file that helps formatting 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; otherwise, the stop() function is called directly. 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.

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 <linux/kernel.h> /* We are doing kernel work */ 
7#include <linux/module.h> /* Specifically, a module */ 
8#include <linux/proc_fs.h> /* Necessary because we use proc fs */ 
9#include <linux/seq_file.h> /* for seq_file */ 
10#include <linux/version.h> 
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 until 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, "%Ld\n", *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        pr_debug("Error: Could not initialize /proc/%s\n", PROC_NAME); 
102        return -ENOMEM; 
103    } 
104 
105    return 0; 
106} 
107 
108static void __exit procfs4_exit(void) 
109{ 
110    remove_proc_entry(PROC_NAME, NULL); 
111    pr_debug("/proc/%s removed\n", PROC_NAME); 
112} 
113 
114module_init(procfs4_init); 
115module_exit(procfs4_exit); 
116 
117MODULE_LICENSE("GPL");

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

• https://lwn.net/Articles/22355/
• https://kernelnewbies.org/Documents/SeqFileHowTo

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 <linux/fs.h> 
5#include <linux/init.h> 
6#include <linux/kobject.h> 
7#include <linux/module.h> 
8#include <linux/string.h> 
9#include <linux/sysfs.h> 
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, "%d\n", myvariable); 
20} 
21 
22static ssize_t myvariable_store(struct kobject *kobj, 
23                                struct kobj_attribute *attr, const char *buf, 
24                                size_t count) 
25{ 
26    sscanf(buf, "%d", &myvariable); 
27    return count; 
28} 
29 
30static struct kobj_attribute myvariable_attribute = 
31    __ATTR(myvariable, 0660, myvariable_show, myvariable_store); 
32 
33static int __init mymodule_init(void) 
34{ 
35    int error = 0; 
36 
37    pr_info("mymodule: initialized\n"); 
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        kobject_put(mymodule); 
46        pr_info("failed to create the myvariable file " 
47                "in /sys/kernel/mymodule\n"); 
48    } 
49 
50    return error; 
51} 
52 
53static void __exit mymodule_exit(void) 
54{ 
55    pr_info("mymodule: Exit success\n"); 
56    kobject_put(mymodule); 
57} 
58 
59module_init(mymodule_init); 
60module_exit(mymodule_exit); 
61 
62MODULE_LICENSE("GPL");

Make and install the module:

1make 
2sudo insmod hello-sysfs.ko

Check that it exists:

1lsmod | 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" | sudo tee /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 <linux/cdev.h> 
5#include <linux/fs.h> 
6#include <linux/init.h> 
7#include <linux/ioctl.h> 
8#include <linux/module.h> 
9#include <linux/slab.h> 
10#include <linux/uaccess.h> 
11#include <linux/version.h> 
12 
13struct ioctl_arg { 
14    unsigned int val; 
15}; 
16 
17/* Documentation/userspace-api/ioctl/ioctl-number.rst */ 
18#define IOC_MAGIC '\x66' 
19 
20#define IOCTL_VALSET _IOW(IOC_MAGIC, 0, struct ioctl_arg) 
21#define IOCTL_VALGET _IOR(IOC_MAGIC, 1, struct ioctl_arg) 
22#define IOCTL_VALGET_NUM _IOR(IOC_MAGIC, 2, int) 
23#define IOCTL_VALSET_NUM _IOW(IOC_MAGIC, 3, int) 
24 
25#define IOCTL_VAL_MAXNR 3 
26#define DRIVER_NAME "ioctltest" 
27 
28static unsigned int test_ioctl_major = 0; 
29static unsigned int num_of_dev = 1; 
30static struct cdev test_ioctl_cdev; 
31static int ioctl_num = 0; 
32 
33struct test_ioctl_data { 
34    unsigned char val; 
35    rwlock_t lock; 
36}; 
37 
38static long test_ioctl_ioctl(struct file *filp, unsigned int cmd, 
39                             unsigned long arg) 
40{ 
41    struct test_ioctl_data *ioctl_data = filp->private_data; 
42    int retval = 0; 
43    unsigned char val; 
44    struct ioctl_arg data; 
45    memset(&data, 0, sizeof(data)); 
46 
47    switch (cmd) { 
48    case IOCTL_VALSET: 
49        if (copy_from_user(&data, (int __user *)arg, sizeof(data))) { 
50            retval = -EFAULT; 
51            goto done; 
52        } 
53 
54        pr_alert("IOCTL set val:%x .\n", data.val); 
55        write_lock(&ioctl_data->lock); 
56        ioctl_data->val = data.val; 
57        write_unlock(&ioctl_data->lock); 
58        break; 
59 
60    case IOCTL_VALGET: 
61        read_lock(&ioctl_data->lock); 
62        val = ioctl_data->val; 
63        read_unlock(&ioctl_data->lock); 
64        data.val = val; 
65 
66        if (copy_to_user((int __user *)arg, &data, sizeof(data))) { 
67            retval = -EFAULT; 
68            goto done; 
69        } 
70 
71        break; 
72 
73    case IOCTL_VALGET_NUM: 
74        retval = __put_user(ioctl_num, (int __user *)arg); 
75        break; 
76 
77    case IOCTL_VALSET_NUM: 
78        ioctl_num = arg; 
79        break; 
80 
81    default: 
82        retval = -ENOTTY; 
83    } 
84 
85done: 
86    return retval; 
87} 
88 
89static ssize_t test_ioctl_read(struct file *filp, char __user *buf, 
90                               size_t count, loff_t *f_pos) 
91{ 
92    struct test_ioctl_data *ioctl_data = filp->private_data; 
93    unsigned char val; 
94    int retval; 
95    int i = 0; 
96 
97    read_lock(&ioctl_data->lock); 
98    val = ioctl_data->val; 
99    read_unlock(&ioctl_data->lock); 
100 
101    for (; i < count; i++) { 
102        if (copy_to_user(&buf[i], &val, 1)) { 
103            retval = -EFAULT; 
104            goto out; 
105        } 
106    } 
107 
108    retval = count; 
109out: 
110    return retval; 
111} 
112 
113static int test_ioctl_close(struct inode *inode, struct file *filp) 
114{ 
115    pr_alert("%s call.\n", __func__); 
116 
117    if (filp->private_data) { 
118        kfree(filp->private_data); 
119        filp->private_data = NULL; 
120    } 
121 
122    return 0; 
123} 
124 
125static int test_ioctl_open(struct inode *inode, struct file *filp) 
126{ 
127    struct test_ioctl_data *ioctl_data; 
128 
129    pr_alert("%s call.\n", __func__); 
130    ioctl_data = kmalloc(sizeof(struct test_ioctl_data), GFP_KERNEL); 
131 
132    if (ioctl_data == NULL) 
133        return -ENOMEM; 
134 
135    rwlock_init(&ioctl_data->lock); 
136    ioctl_data->val = 0xFF; 
137    filp->private_data = ioctl_data; 
138 
139    return 0; 
140} 
141 
142static struct file_operations fops = { 
143#if LINUX_VERSION_CODE < KERNEL_VERSION(6, 4, 0) 
144    .owner = THIS_MODULE, 
145#endif 
146    .open = test_ioctl_open, 
147    .release = test_ioctl_close, 
148    .read = test_ioctl_read, 
149    .unlocked_ioctl = test_ioctl_ioctl, 
150}; 
151 
152static int __init ioctl_init(void) 
153{ 
154    dev_t dev; 
155    int alloc_ret = -1; 
156    int cdev_ret = -1; 
157    alloc_ret = alloc_chrdev_region(&dev, 0, num_of_dev, DRIVER_NAME); 
158 
159    if (alloc_ret) 
160        goto error; 
161 
162    test_ioctl_major = MAJOR(dev); 
163    cdev_init(&test_ioctl_cdev, &fops); 
164    cdev_ret = cdev_add(&test_ioctl_cdev, dev, num_of_dev); 
165 
166    if (cdev_ret) 
167        goto error; 
168 
169    pr_alert("%s driver(major: %d) installed.\n", DRIVER_NAME, 
170             test_ioctl_major); 
171    return 0; 
172error: 
173    if (cdev_ret == 0) 
174        cdev_del(&test_ioctl_cdev); 
175    if (alloc_ret == 0) 
176        unregister_chrdev_region(dev, num_of_dev); 
177    return -1; 
178} 
179 
180static void __exit ioctl_exit(void) 
181{ 
182    dev_t dev = MKDEV(test_ioctl_major, 0); 
183 
184    cdev_del(&test_ioctl_cdev); 
185    unregister_chrdev_region(dev, num_of_dev); 
186    pr_alert("%s driver removed.\n", DRIVER_NAME); 
187} 
188 
189module_init(ioctl_init); 
190module_exit(ioctl_exit); 
191 
192MODULE_LICENSE("GPL"); 
193MODULE_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 <linux/atomic.h> 
6#include <linux/cdev.h> 
7#include <linux/delay.h> 
8#include <linux/device.h> 
9#include <linux/fs.h> 
10#include <linux/init.h> 
11#include <linux/module.h> /* Specifically, a module */ 
12#include <linux/printk.h> 
13#include <linux/types.h> 
14#include <linux/uaccess.h> /* for get_user and put_user */ 
15#include <linux/version.h> 
16 
17#include <asm/errno.h> 
18 
19#include "chardev.h" 
20#define SUCCESS 0 
21#define DEVICE_NAME "char_dev" 
22#define BUF_LEN 80 
23 
24enum { 
25    CDEV_NOT_USED, 
26    CDEV_EXCLUSIVE_OPEN, 
27}; 
28 
29/* Is the device open right now? Used to prevent concurrent access into 
30 * the same device 
31 */ 
32static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
33 
34/* The message the device will give when asked */ 
35static char message[BUF_LEN + 1]; 
36 
37static struct class *cls; 
38 
39/* This is called whenever a process attempts to open the device file */ 
40static int device_open(struct inode *inode, struct file *file) 
41{ 
42    pr_info("device_open(%p)\n", file); 
43 
44    try_module_get(THIS_MODULE); 
45    return SUCCESS; 
46} 
47 
48static int device_release(struct inode *inode, struct file *file) 
49{ 
50    pr_info("device_release(%p,%p)\n", inode, file); 
51 
52    module_put(THIS_MODULE); 
53    return SUCCESS; 
54} 
55 
56/* This function is called whenever a process which has already opened the 
57 * device file attempts to read from it. 
58 */ 
59static ssize_t device_read(struct file *file, /* see include/linux/fs.h   */ 
60                           char __user *buffer, /* buffer to be filled  */ 
61                           size_t length, /* length of the buffer     */ 
62                           loff_t *offset) 
63{ 
64    /* Number of bytes actually written to the buffer */ 
65    int bytes_read = 0; 
66    /* How far did the process reading the message get? Useful if the message 
67     * is larger than the size of the buffer we get to fill in device_read. 
68     */ 
69    const char *message_ptr = message; 
70 
71    if (!*(message_ptr + *offset)) { /* we are at the end of message */ 
72        *offset = 0; /* reset the offset */ 
73        return 0; /* signify end of file */ 
74    } 
75 
76    message_ptr += *offset; 
77 
78    /* Actually put the data into the buffer */ 
79    while (length && *message_ptr) { 
80        /* Because the buffer is in the user data segment, not the kernel 
81         * data segment, assignment would not work. Instead, we have to 
82         * use put_user which copies data from the kernel data segment to 
83         * the user data segment. 
84         */ 
85        put_user(*(message_ptr++), buffer++); 
86        length--; 
87        bytes_read++; 
88    } 
89 
90    pr_info("Read %d bytes, %ld left\n", bytes_read, length); 
91 
92    *offset += bytes_read; 
93 
94    /* Read functions are supposed to return the number of bytes actually 
95     * inserted into the buffer. 
96     */ 
97    return bytes_read; 
98} 
99 
100/* called when somebody tries to write into our device file. */ 
101static ssize_t device_write(struct file *file, const char __user *buffer, 
102                            size_t length, loff_t *offset) 
103{ 
104    int i; 
105 
106    pr_info("device_write(%p,%p,%ld)", file, buffer, length); 
107 
108    for (i = 0; i < length && i < BUF_LEN; i++) 
109        get_user(message[i], buffer + i); 
110 
111    /* Again, return the number of input characters used. */ 
112    return i; 
113} 
114 
115/* This function is called whenever a process tries to do an ioctl on our 
116 * device file. We get two extra parameters (additional to the inode and file 
117 * structures, which all device functions get): the number of the ioctl called 
118 * and the parameter given to the ioctl function. 
119 * 
120 * If the ioctl is write or read/write (meaning output is returned to the 
121 * calling process), the ioctl call returns the output of this function. 
122 */ 
123static long 
124device_ioctl(struct file *file, /* ditto */ 
125             unsigned int ioctl_num, /* number and param for ioctl */ 
126             unsigned long ioctl_param) 
127{ 
128    int i; 
129    long ret = SUCCESS; 
130 
131    /* We don't want to talk to two processes at the same time. */ 
132    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
133        return -EBUSY; 
134 
135    /* Switch according to the ioctl called */ 
136    switch (ioctl_num) { 
137    case IOCTL_SET_MSG: { 
138        /* Receive a pointer to a message (in user space) and set that to 
139         * be the device's message. Get the parameter given to ioctl by 
140         * the process. 
141         */ 
142        char __user *tmp = (char __user *)ioctl_param; 
143        char ch; 
144 
145        /* Find the length of the message */ 
146        get_user(ch, tmp); 
147        for (i = 0; ch && i < BUF_LEN; i++, tmp++) 
148            get_user(ch, tmp); 
149 
150        device_write(file, (char __user *)ioctl_param, i, NULL); 
151        break; 
152    } 
153    case IOCTL_GET_MSG: { 
154        loff_t offset = 0; 
155 
156        /* Give the current message to the calling process - the parameter 
157         * we got is a pointer, fill it. 
158         */ 
159        i = device_read(file, (char __user *)ioctl_param, 99, &offset); 
160 
161        /* Put a zero at the end of the buffer, so it will be properly 
162         * terminated. 
163         */ 
164        put_user('\0', (char __user *)ioctl_param + i); 
165        break; 
166    } 
167    case IOCTL_GET_NTH_BYTE: 
168        /* This ioctl is both input (ioctl_param) and output (the return 
169         * value of this function). 
170         */ 
171        ret = (long)message[ioctl_param]; 
172        break; 
173    } 
174 
175    /* We're now ready for our next caller */ 
176    atomic_set(&already_open, CDEV_NOT_USED); 
177 
178    return ret; 
179} 
180 
181/* Module Declarations */ 
182 
183/* This structure will hold the functions to be called when a process does 
184 * something to the device we created. Since a pointer to this structure 
185 * is kept in the devices table, it can't be local to init_module. NULL is 
186 * for unimplemented functions. 
187 */ 
188static struct file_operations fops = { 
189    .read = device_read, 
190    .write = device_write, 
191    .unlocked_ioctl = device_ioctl, 
192    .open = device_open, 
193    .release = device_release, /* a.k.a. close */ 
194}; 
195 
196/* Initialize the module - Register the character device */ 
197static int __init chardev2_init(void) 
198{ 
199    /* Register the character device (at least try) */ 
200    int ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &fops); 
201 
202    /* Negative values signify an error */ 
203    if (ret_val < 0) { 
204        pr_alert("%s failed with %d\n", 
205                 "Sorry, registering the character device ", ret_val); 
206        return ret_val; 
207    } 
208 
209#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 4, 0) 
210    cls = class_create(DEVICE_FILE_NAME); 
211#else 
212    cls = class_create(THIS_MODULE, DEVICE_FILE_NAME); 
213#endif 
214    device_create(cls, NULL, MKDEV(MAJOR_NUM, 0), NULL, DEVICE_FILE_NAME); 
215 
216    pr_info("Device created on /dev/%s\n", DEVICE_FILE_NAME); 
217 
218    return 0; 
219} 
220 
221/* Cleanup - unregister the appropriate file from /proc */ 
222static void __exit chardev2_exit(void) 
223{ 
224    device_destroy(cls, MKDEV(MAJOR_NUM, 0)); 
225    class_destroy(cls); 
226 
227    /* Unregister the device */ 
228    unregister_chrdev(MAJOR_NUM, DEVICE_NAME); 
229} 
230 
231module_init(chardev2_init); 
232module_exit(chardev2_exit); 
233 
234MODULE_LICENSE("GPL");