0.1 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 it will 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.

0.1.1 Authorship

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

0.1.2 Acknowledgements

The following people have contributed corrections or good suggestions: Ignacio Martin, David Porter, Daniele Paolo Scarpazza, Dimo Velev, Francois Audeon, Horst Schirmeier, and Roman Lakeev.

0.1.3 What Is A Kernel Module?

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

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

0.1.4 Kernel module package

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

On Ubuntu/Debian:

1sudo apt-get install build-essential kmod

On Arch Linux:

1sudo pacman -S gcc kmod

0.1.5 What Modules are in my Kernel?

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

1sudo lsmod

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

1sudo cat /proc/modules

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

1sudo lsmod | grep fat

0.1.6 Do I need to download and compile the kernel?

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

0.1.7 Before We Begin

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

1. Modversioning. A module compiled for one kernel will not load if you boot a different kernel unless you enable CONFIG_MODVERSIONS in the kernel. We will not go into module versioning until later in this guide. Until we cover modversions, the examples in the guide may not work if you are running a kernel with modversioning turned on. However, most stock Linux distribution kernels come with it turned on. If you are having trouble loading the modules because of versioning errors, compile a kernel with modversioning turned off.

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

   Modules can not print to the screen like printf() can, but they can log information and warnings, which ends up being printed on your screen, but only on a console. If you insmod a module from an xterm, the information and warnings will be logged, but only to your systemd journal. You will not see it unless you look through your journalctl . See 0.4 for details. To have immediate access to this information, do all your work from the console.

0.2 Headers

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

On Ubuntu/Debian:

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

On Arch Linux:

1sudo pacman -S linux-libre-headers

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

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

0.3 Examples

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

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

0.4 Hello World

0.4.1 The Simplest Module

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

Here is the simplest module possible.

Make a test directory:

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

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

1/* 
2 * hello-1.c - The simplest kernel module. 
3 */ 
4#include <linux/kernel.h> /* Needed for KERN_INFO */ 
5#include <linux/module.h> /* Needed by all modules */ 
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 
3all: 
4    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules 
5 
6clean: 
7    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

And finally just:

1make

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:

1sudo modinfo hello-1.ko

At this point the command:

1sudo lsmod | grep hello

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

1sudo insmod hello-1.ko

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

1sudo lsmod | grep hello

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

1sudo rmmod hello_1

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

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 0.4.2. 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/kernel.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?

0.4.2 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/init.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/kernel.h> /* Needed for KERN_INFO */ 
7#include <linux/module.h> /* Needed by all modules */ 
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 
4all: 
5    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules 
6 
7clean: 
8    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 get hardwired into the kernel (obj-y) but where are all those obj-m gone? Those familiar with shell scripts will easily be able to spot them. For those 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.

0.4.3 The __init and __exit Macros

This demonstrates a feature of kernel 2.2 and later. Notice the change in the definitions of the init and cleanup functions. 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/kernel.h> /* Needed for KERN_INFO */ 
6#include <linux/module.h> /* Needed by all modules */ 
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");

0.4.4 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/kernel.h> /* Needed for KERN_INFO */ 
6#include <linux/module.h> /* Needed by all modules */ 
7 
8MODULE_LICENSE("GPL"); 
9MODULE_AUTHOR("LKMPG"); 
10MODULE_DESCRIPTION("A sample driver"); 
11MODULE_SUPPORTED_DEVICE("testdevice"); 
12 
13static int __init init_hello_4(void) 
14{ 
15    pr_info("Hello, world 4\n"); 
16    return 0; 
17} 
18 
19static void __exit cleanup_hello_4(void) 
20{ 
21    pr_info("Goodbye, world 4\n"); 
22} 
23 
24module_init(init_hello_4); 
25module_exit(cleanup_hello_4);

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

I would recommend playing around with this code:

1$ sudo insmod hello-5.ko mystring="bebop" myintArray=-1 
2myshort is a short integer: 1 
3myint is an integer: 420 
4mylong is a long integer: 9999 
5mystring is a string: bebop 
6myintArray[0] = -1 
7myintArray[1] = 420 
8got 1 arguments for myintArray. 
9 
10$ sudo rmmod hello-5 
11Goodbye, world 5 
12 
13$ sudo insmod hello-5.ko mystring="supercalifragilisticexpialidocious" myintArray=-1,-1 
14myshort is a short integer: 1 
15myint is an integer: 420 
16mylong is a long integer: 9999 
17mystring is a string: supercalifragilisticexpialidocious 
18myintArray[0] = -1 
19myintArray[1] = -1 
20got 2 arguments for myintArray. 
21 
22$ sudo rmmod hello-5 
23Goodbye, world 5 
24 
25$ sudo insmod hello-5.ko mylong=hello 
26hello-5.o: invalid argument syntax for mylong: 'h'

0.4.6 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() 
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 
9all: 
10    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules 
11 
12clean: 
13    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.

0.4.7 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 inserting 'poet_atkm.ko': -1 Invalid module format

Less cryptical information are logged to the systemd journal:

Jun  4 22:07:54 localhost kernel: poet_atkm: version magic '2.6.5-1.358custom 686
REGPARM 4KSTACKS gcc-3.3' should be '2.6.5-1.358 686 REGPARM 4KSTACKS gcc-3.3'

In other words, your kernel refuses to accept your module because version strings (more precisely, version magics) do not match. Incidentally, version magics 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 init/vermagic.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 inacceptable 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 distribution include. Then, examine your Makefile, and make sure that the specified version information matches exactly the one used for your current kernel. For example, you makefile could start as follows:

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

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

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

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

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

$ make
CHK     include/linux/version.h
UPD     include/linux/version.h
SYMLINK include/asm -> include/asm-i386
SPLIT   include/linux/autoconf.h -> include/config/*
HOSTCC  scripts/basic/fixdep
HOSTCC  scripts/basic/split-include
HOSTCC  scripts/basic/docproc
HOSTCC  scripts/conmakehash
HOSTCC  scripts/kallsyms
CC      scripts/empty.o

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 will be 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.

0.5 Preliminaries

0.5.1 How modules begin and end

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

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

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

0.5.2 Functions available to modules

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

Kernel modules are different here, too. In the hello world example, you might have noticed that we used a function, pr_info() but did not include a standard I/O library. That is because modules are object files whose symbols get resolved upon insmod ’ing. 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 exectable 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! everytime someone tries to delete a file on your system.

0.5.3 User Space vs Kernel Space

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

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

0.5.4 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 a 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.

0.5.5 Code space

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

If you have not thought about what a segfault really means, you may be surprised to hear that pointers do not actually point to memory locations. Not real ones, anyway. When a process is created, the kernel sets aside a portion of real physical memory and hands it to the process to use for its executing code, variables, stack, heap and other things which a computer scientist would know about. This memory begins with 0x00000000 and extends up to whatever it needs to be. Since the memory space for any two processes do not overlap, every process that can access a memory address, say 0xbffff978, would be accessing a different location in real physical memory! The processes would be accessing an index named 0xbffff978 which points to some kind of offset into the region of memory set aside for that particular process. For the most part, a process like our Hello, World program can’t access the space of another process, although there are ways which we will talk about later.

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

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

0.5.6 Device Drivers

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

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

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

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

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

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

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

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

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

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

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

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

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

0.6 Character Device drivers

0.6.1 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 v5.6, the proc_ops structure was introduced to replace the use of the file_operations structure when registering proc handlers.

0.6.2 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.

0.6.3 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 .

0.6.4 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 of /proc/modules. If this number isn’t zero, rmmod will fail. Note that you do not have to check the counter from 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 use count.
• module_put(THIS_MODULE) : Decrement the use count.

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.

0.6.5 chardev.c

The next code sample creates a char driver named chardev. You can cat 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.

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

0.6.6 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 .

0.7 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 stats 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 inode number is a pointer to a disk location where the file’s index-node (inode for short) is located. The inode contains information about the file, for example the file’s permissions, together with a pointer to the disk location or locations where the file’s data can be found.

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

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

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

Each time, everytime 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 
17struct proc_dir_entry *Our_Proc_File; 
18 
19 
20ssize_t procfile_read(struct file *filePointer, 
21                      char *buffer, 
22                      size_t buffer_length, 
23                      loff_t *offset) 
24{ 
25    char s[13] = "HelloWorld!\n"; 
26    int len = sizeof(s); 
27    ssize_t ret = len; 
28 
29    if (*offset >= len || copy_to_user(buffer, s, len)) { 
30        pr_info("copy_to_user failed\n"); 
31        ret = 0; 
32    } else { 
33        pr_info("procfile read %s\n", filePointer->f_path.dentry->d_name.name); 
34        *offset += len; 
35    } 
36 
37    return ret; 
38} 
39 
40#ifdef HAVE_PROC_OPS 
41static const struct proc_ops proc_file_fops = { 
42    .proc_read = procfile_read, 
43}; 
44#else 
45static const struct file_operations proc_file_fops = { 
46    .read = procfile_read, 
47}; 
48#endif 
49 
50static int __init procfs1_init(void) 
51{ 
52    Our_Proc_File = proc_create(procfs_name, 0644, NULL, &proc_file_fops); 
53    if (NULL == Our_Proc_File) { 
54        proc_remove(Our_Proc_File); 
55        pr_alert("Error:Could not initialize /proc/%s\n", procfs_name); 
56        return -ENOMEM; 
57    } 
58 
59    pr_info("/proc/%s created\n", procfs_name); 
60    return 0; 
61} 
62 
63static void __exit procfs1_exit(void) 
64{ 
65    proc_remove(Our_Proc_File); 
66    pr_info("/proc/%s removed\n", procfs_name); 
67} 
68 
69module_init(procfs1_init); 
70module_exit(procfs1_exit); 
71 
72MODULE_LICENSE("GPL");

0.7.1 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.

0.7.2 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 */ 
28ssize_t procfile_read(struct file *filePointer, 
29                      char *buffer, 
30                      size_t buffer_length, 
31                      loff_t *offset) 
32{ 
33    char s[13] = "HelloWorld!\n"; 
34    int len = sizeof(s); 
35    ssize_t ret = len; 
36 
37    if (*offset >= len || copy_to_user(buffer, s, len)) { 
38        pr_info("copy_to_user failed\n"); 
39        ret = 0; 
40    } else { 
41        pr_info("procfile read %s\n", filePointer->f_path.dentry->d_name.name); 
42        *offset += len; 
43    } 
44 
45    return ret; 
46} 
47 
48/* This function is called with the /proc file is written. */ 
49static ssize_t procfile_write(struct file *file, 
50                              const char *buff, 
51                              size_t len, 
52                              loff_t *off) 
53{ 
54    procfs_buffer_size = len; 
55    if (procfs_buffer_size > PROCFS_MAX_SIZE) 
56        procfs_buffer_size = PROCFS_MAX_SIZE; 
57 
58    if (copy_from_user(procfs_buffer, buff, procfs_buffer_size)) 
59        return -EFAULT; 
60 
61    procfs_buffer[procfs_buffer_size] = '\0'; 
62    return procfs_buffer_size; 
63} 
64 
65#ifdef HAVE_PROC_OPS 
66static const struct proc_ops proc_file_fops = { 
67    .proc_read = procfile_read, 
68    .proc_write = procfile_write, 
69}; 
70#else 
71static const struct file_operations proc_file_fops = { 
72    .read = procfile_read, 
73    .write = procfile_write, 
74}; 
75#endif 
76 
77static int __init procfs2_init(void) 
78{ 
79    Our_Proc_File = proc_create(PROCFS_NAME, 0644, NULL, &proc_file_fops); 
80    if (NULL == Our_Proc_File) { 
81        proc_remove(Our_Proc_File); 
82        pr_alert("Error:Could not initialize /proc/%s\n", PROCFS_NAME); 
83        return -ENOMEM; 
84    } 
85 
86    pr_info("/proc/%s created\n", PROCFS_NAME); 
87    return 0; 
88} 
89 
90static void __exit procfs2_exit(void) 
91{ 
92    proc_remove(Our_Proc_File); 
93    pr_info("/proc/%s removed\n", PROCFS_NAME); 
94} 
95 
96module_init(procfs2_init); 
97module_exit(procfs2_exit); 
98 
99MODULE_LICENSE("GPL");

0.7.3 Manage /proc file with standard filesystem

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

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

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

In /proc, whenever we register a new file, we’re allowed to specify which struct inode_operations will be used to access to it. This is the mechanism we use, a struct inode_operations which includes a pointer to a struct proc_ops which includes pointers to our procf_read and procfs_write functions.

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

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

1/* 
2 * procfs3.c 
3 */ 
4 
5#include <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 
12#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0) 
13#define HAVE_PROC_OPS 
14#endif 
15 
16#define PROCFS_MAX_SIZE 2048 
17#define PROCFS_ENTRY_FILENAME "buffer2k" 
18 
19struct proc_dir_entry *Our_Proc_File; 
20static char procfs_buffer[PROCFS_MAX_SIZE]; 
21static unsigned long procfs_buffer_size = 0; 
22 
23static ssize_t procfs_read(struct file *filp, 
24                           char *buffer, 
25                           size_t length, 
26                           loff_t *offset) 
27{ 
28    static int finished = 0; 
29    if (finished) { 
30        pr_debug("procfs_read: END\n"); 
31        finished = 0; 
32        return 0; 
33    } 
34    finished = 1; 
35    if (copy_to_user(buffer, procfs_buffer, procfs_buffer_size)) 
36        return -EFAULT; 
37    pr_debug("procfs_read: read %lu bytes\n", procfs_buffer_size); 
38    return procfs_buffer_size; 
39} 
40static ssize_t procfs_write(struct file *file, 
41                            const char *buffer, 
42                            size_t len, 
43                            loff_t *off) 
44{ 
45    if (len > PROCFS_MAX_SIZE) 
46        procfs_buffer_size = PROCFS_MAX_SIZE; 
47    else 
48        procfs_buffer_size = len; 
49    if (copy_from_user(procfs_buffer, buffer, procfs_buffer_size)) 
50        return -EFAULT; 
51    pr_debug("procfs_write: write %lu bytes\n", procfs_buffer_size); 
52    return procfs_buffer_size; 
53} 
54int procfs_open(struct inode *inode, struct file *file) 
55{ 
56    try_module_get(THIS_MODULE); 
57    return 0; 
58} 
59int procfs_close(struct inode *inode, struct file *file) 
60{ 
61    module_put(THIS_MODULE); 
62    return 0; 
63} 
64 
65#ifdef HAVE_PROC_OPS 
66static struct proc_ops File_Ops_4_Our_Proc_File = { 
67    .proc_read = procfs_read, 
68    .proc_write = procfs_write, 
69    .proc_open = procfs_open, 
70    .proc_release = procfs_close, 
71}; 
72#else 
73static const struct file_operations File_Ops_4_Our_Proc_File = { 
74    .read = procfs_read, 
75    .write = procfs_write, 
76    .open = procfs_open, 
77    .release = procfs_close, 
78}; 
79#endif 
80 
81static int __init procfs3_init(void) 
82{ 
83    Our_Proc_File = proc_create(PROCFS_ENTRY_FILENAME, 0644, NULL, 
84                                &File_Ops_4_Our_Proc_File); 
85    if (Our_Proc_File == NULL) { 
86        remove_proc_entry(PROCFS_ENTRY_FILENAME, NULL); 
87        pr_debug("Error: Could not initialize /proc/%s\n", 
88                 PROCFS_ENTRY_FILENAME); 
89        return -ENOMEM; 
90    } 
91    proc_set_size(Our_Proc_File, 80); 
92    proc_set_user(Our_Proc_File, GLOBAL_ROOT_UID, GLOBAL_ROOT_GID); 
93 
94    pr_debug("/proc/%s created\n", PROCFS_ENTRY_FILENAME); 
95    return 0; 
96} 
97 
98static void __exit procfs3_exit(void) 
99{ 
100    remove_proc_entry(PROCFS_ENTRY_FILENAME, NULL); 
101    pr_debug("/proc/%s removed\n", PROCFS_ENTRY_FILENAME); 
102} 
103 
104module_init(procfs3_init); 
105module_exit(procfs3_exit); 
106 
107MODULE_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.

0.7.4 Manage /proc file with seq_file

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

A sequence begins with the call of the function start() . If the return is a non NULL value, the function next() is called. This function is an iterator, the goal is to go through all the data. Each time next() is called, the function show() is also called. It writes data values in the buffer read by the user. The function next() is called until it returns NULL . The sequence ends when next() returns NULL , then the function stop() is called.

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

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    /* no => it is the end of the sequence, return end to stop reading */ 
33    *pos = 0; 
34    return NULL; 
35} 
36 
37/* This function is called after the beginning of a sequence. 
38 * It is called untill the return is NULL (this ends the sequence). 
39 */ 
40static void *my_seq_next(struct seq_file *s, void *v, loff_t *pos) 
41{ 
42    unsigned long *tmp_v = (unsigned long *) v; 
43    (*tmp_v)++; 
44    (*pos)++; 
45    return NULL; 
46} 
47 
48/* This function is called at the end of a sequence. */ 
49static void my_seq_stop(struct seq_file *s, void *v) 
50{ 
51    /* nothing to do, we use a static value in start() */ 
52} 
53 
54/* This function is called for each "step" of a sequence. */ 
55static int my_seq_show(struct seq_file *s, void *v) 
56{ 
57    loff_t *spos = (loff_t *) v; 
58 
59    seq_printf(s, "%Ld\n", *spos); 
60    return 0; 
61} 
62 
63/* This structure gather "function" to manage the sequence */ 
64static struct seq_operations my_seq_ops = { 
65    .start = my_seq_start, 
66    .next = my_seq_next, 
67    .stop = my_seq_stop, 
68    .show = my_seq_show, 
69}; 
70 
71/* This function is called when the /proc file is open. */ 
72static int my_open(struct inode *inode, struct file *file) 
73{ 
74    return seq_open(file, &my_seq_ops); 
75}; 
76 
77/* This structure gather "function" that manage the /proc file */ 
78#ifdef HAVE_PROC_OPS 
79static const struct proc_ops my_file_ops = { 
80    .proc_open = my_open, 
81    .proc_read = seq_read, 
82    .proc_lseek = seq_lseek, 
83    .proc_release = seq_release, 
84}; 
85#else 
86static const struct file_operations my_file_ops = { 
87    .open = my_open, 
88    .read = seq_read, 
89    .llseek = seq_lseek, 
90    .release = seq_release, 
91}; 
92#endif 
93 
94static int __init procfs4_init(void) 
95{ 
96    struct proc_dir_entry *entry; 
97 
98    entry = proc_create(PROC_NAME, 0, NULL, &my_file_ops); 
99    if (entry == NULL) { 
100        remove_proc_entry(PROC_NAME, 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:

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

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

0.8 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

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, 
18                               char *buf) 
19{ 
20    return sprintf(buf, "%d\n", myvariable); 
21} 
22 
23static ssize_t myvariable_store(struct kobject *kobj, 
24                                struct kobj_attribute *attr, 
25                                char *buf, 
26                                size_t count) 
27{ 
28    sscanf(buf, "%du", &myvariable); 
29    return count; 
30} 
31 
32 
33static struct kobj_attribute myvariable_attribute = 
34    __ATTR(myvariable, 0660, myvariable_show, (void *) myvariable_store); 
35 
36static int __init mymodule_init(void) 
37{ 
38    int error = 0; 
39 
40    pr_info("mymodule: initialised\n"); 
41 
42    mymodule = kobject_create_and_add("mymodule", kernel_kobj); 
43    if (!mymodule) 
44        return -ENOMEM; 
45 
46    error = sysfs_create_file(mymodule, &myvariable_attribute.attr); 
47    if (error) { 
48        pr_info( 
49            "failed to create the myvariable file " 
50            "in /sys/kernel/mymodule\n"); 
51    } 
52 
53    return error; 
54} 
55 
56static void __exit mymodule_exit(void) 
57{ 
58    pr_info("mymodule: Exit success\n"); 
59    kobject_put(mymodule); 
60} 
61 
62module_init(mymodule_init); 
63module_exit(mymodule_exit); 
64 
65MODULE_LICENSE("GPL");

Make and install the module:

1make 
2sudo insmod hello-sysfs.ko

Check that it exists:

1sudo lsmod | grep hello_sysfs

What is the current value of myvariable ?

1cat /sys/kernel/mymodule/myvariable

Set the value of myvariable and check that it changed.

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

Finally, remove the test module:

1sudo rmmod hello_sysfs

0.9 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 send 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.

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 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/driver-api/ioctl.rst.

1/* 
2 * chardev2.c - Create an input/output character device 
3 */ 
4 
5#include <linux/cdev.h> 
6#include <linux/delay.h> 
7#include <linux/device.h> 
8#include <linux/fs.h> 
9#include <linux/init.h> 
10#include <linux/irq.h> 
11#include <linux/kernel.h> /* We are doing kernel work */ 
12#include <linux/module.h> /* Specifically, a module */ 
13#include <linux/poll.h> 
14 
15#include "chardev.h" 
16#define SUCCESS 0 
17#define DEVICE_NAME "char_dev" 
18#define BUF_LEN 80 
19 
20/* Is the device open right now? Used to prevent concurrent access into 
21 * the same device 
22 */ 
23static int Device_Open = 0; 
24 
25/* The message the device will give when asked */ 
26static char Message[BUF_LEN]; 
27 
28/* How far did the process reading the message get? Useful if the message 
29 * is larger than the size of the buffer we get to fill in device_read. 
30 */ 
31static char *Message_Ptr; 
32 
33/* Major number assigned to our device driver */ 
34static int Major; 
35static struct class *cls; 
36 
37/* This is called whenever a process attempts to open the device file */ 
38static int device_open(struct inode *inode, struct file *file) 
39{ 
40    pr_info("device_open(%p)\n", file); 
41 
42    /* We don't want to talk to two processes at the same time. */ 
43    if (Device_Open) 
44        return -EBUSY; 
45 
46    Device_Open++; 
47    /* Initialize the message */ 
48    Message_Ptr = Message; 
49    try_module_get(THIS_MODULE); 
50    return SUCCESS; 
51} 
52 
53static int device_release(struct inode *inode, struct file *file) 
54{ 
55    pr_info("device_release(%p,%p)\n", inode, file); 
56 
57    /* We're now ready for our next caller */ 
58    Device_Open--; 
59 
60    module_put(THIS_MODULE); 
61    return SUCCESS; 
62} 
63 
64/* This function is called whenever a process which has already opened the 
65 * device file attempts to read from it. 
66 */ 
67static ssize_t device_read(struct file *file,   /* see include/linux/fs.h   */ 
68                           char __user *buffer, /* buffer to be filled  */ 
69                           size_t length,       /* length of the buffer     */ 
70                           loff_t *offset) 
71{ 
72    /* Number of bytes actually written to the buffer */ 
73    int bytes_read = 0; 
74 
75    pr_info("device_read(%p,%p,%ld)\n", file, buffer, length); 
76 
77    /* If at the end of message, return 0 (which signifies end of file). */ 
78    if (*Message_Ptr == 0) 
79        return 0; 
80 
81    /* Actually put the data into the buffer */ 
82    while (length && *Message_Ptr) { 
83        /* Because the buffer is in the user data segment, not the kernel 
84         * data segment, assignment would not work. Instead, we have to 
85         * use put_user which copies data from the kernel data segment to 
86         * the user data segment. 
87         */ 
88        put_user(*(Message_Ptr++), buffer++); 
89        length--; 
90        bytes_read++; 
91    } 
92 
93    pr_info("Read %d bytes, %ld left\n", bytes_read, length); 
94 
95    /* Read functions are supposed to return the number of bytes actually 
96     * inserted into the buffer. 
97     */ 
98    return bytes_read; 
99} 
100 
101/* called when somebody tries to write into our device file. */ 
102static ssize_t device_write(struct file *file, 
103                            const char __user *buffer, 
104                            size_t length, 
105                            loff_t *offset) 
106{ 
107    int i; 
108 
109    pr_info("device_write(%p,%s,%ld)", file, buffer, length); 
110 
111    for (i = 0; i < length && i < BUF_LEN; i++) 
112        get_user(Message[i], buffer + i); 
113 
114    Message_Ptr = Message; 
115 
116    /* Again, return the number of input characters used. */ 
117    return i; 
118} 
119 
120/* This function is called whenever a process tries to do an ioctl on our 
121 * device file. We get two extra parameters (additional to the inode and file 
122 * structures, which all device functions get): the number of the ioctl called 
123 * and the parameter given to the ioctl function. 
124 * 
125 * If the ioctl is write or read/write (meaning output is returned to the 
126 * calling process), the ioctl call returns the output of this function. 
127 */ 
128long device_ioctl(struct file *file,      /* ditto */ 
129                  unsigned int ioctl_num, /* number and param for ioctl */ 
130                  unsigned long ioctl_param) 
131{ 
132    int i; 
133    char *temp; 
134    char ch; 
135 
136    /* Switch according to the ioctl called */ 
137    switch (ioctl_num) { 
138    case IOCTL_SET_MSG: 
139        /* Receive a pointer to a message (in user space) and set that to 
140         * be the device's message.  Get the parameter given to ioctl by 
141         * the process. 
142         */ 
143        temp = (char *) ioctl_param; 
144 
145        /* Find the length of the message */ 
146        get_user(ch, temp); 
147        for (i = 0; ch && i < BUF_LEN; i++, temp++) 
148            get_user(ch, temp); 
149 
150        device_write(file, (char *) ioctl_param, i, 0); 
151        break; 
152 
153    case IOCTL_GET_MSG: 
154        /* Give the current message to the calling process - the parameter 
155         * we got is a pointer, fill it. 
156         */ 
157        i = device_read(file, (char *) ioctl_param, 99, 0); 
158 
159        /* Put a zero at the end of the buffer, so it will be properly 
160         * terminated. 
161         */ 
162        put_user('\0', (char *) ioctl_param + i); 
163        break; 
164 
165    case IOCTL_GET_NTH_BYTE: 
166        /* This ioctl is both input (ioctl_param) and output (the return 
167         * value of this function). 
168         */ 
169        return Message[ioctl_param]; 
170        break; 
171    } 
172 
173    return SUCCESS; 
174} 
175 
176/* Module Declarations */ 
177 
178/* This structure will hold the functions to be called when a process does 
179 * something to the device we created. Since a pointer to this structure 
180 * is kept in the devices table, it can't be local to init_module. NULL is 
181 * for unimplemented functions. 
182 */ 
183struct file_operations Fops = { 
184    .read = device_read, 
185    .write = device_write, 
186    .unlocked_ioctl = device_ioctl, 
187    .open = device_open, 
188    .release = device_release, /* a.k.a. close */ 
189}; 
190 
191/* Initialize the module - Register the character device */ 
192static int __init chardev2_init(void) 
193{ 
194    /* Register the character device (atleast try) */ 
195    int ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &Fops); 
196 
197    /* Negative values signify an error */ 
198    if (ret_val < 0) { 
199        pr_alert("%s failed with %d\n", 
200                 "Sorry, registering the character device ", ret_val); 
201        return ret_val; 
202    } 
203 
204    Major = ret_val; 
205 
206    cls = class_create(THIS_MODULE, DEVICE_FILE_NAME); 
207    device_create(cls, NULL, MKDEV(Major, MAJOR_NUM), NULL, DEVICE_FILE_NAME); 
208 
209    pr_info("Device created on /dev/%s\n", DEVICE_FILE_NAME); 
210 
211    return 0; 
212} 
213 
214/* Cleanup - unregister the appropriate file from /proc */ 
215static void __exit chardev2_exit(void) 
216{ 
217    device_destroy(cls, MKDEV(Major, 0)); 
218    class_destroy(cls); 
219 
220    /* Unregister the device */ 
221    unregister_chrdev(Major, DEVICE_NAME); 
222} 
223 
224module_init(chardev2_init); 
225module_exit(chardev2_exit); 
226 
227MODULE_LICENSE("GPL");

1/* 
2 * chardev.h - the header file with the ioctl definitions. 
3 * 
4 * The declarations here have to be in a header file, because they need 
5 * to be known both to the kernel module (in chardev.c) and the process 
6 * calling ioctl (ioctl.c). 
7 */ 
8 
9#ifndef CHARDEV_H 
10#define CHARDEV_H 
11 
12#include <linux/ioctl.h> 
13 
14/* The major device number. We can not rely on dynamic registration 
15 * any more, because ioctls need to know it. 
16 */ 
17#define MAJOR_NUM 100 
18 
19/* Set the message of the device driver */ 
20#define IOCTL_SET_MSG _IOW(MAJOR_NUM, 0, char *) 
21/* _IOW means that we are creating an ioctl command number for passing 
22 * information from a user process to the kernel module. 
23 * 
24 * The first arguments, MAJOR_NUM, is the major device number we are using. 
25 * 
26 * The second argument is the number of the command (there could be several 
27 * with different meanings). 
28 * 
29 * The third argument is the type we want to get from the process to the 
30 * kernel. 
31 */ 
32 
33/* Get the message of the device driver */ 
34#define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *) 
35/* This IOCTL is used for output, to get the message of the device driver. 
36 * However, we still need the buffer to place the message in to be input, 
37 * as it is allocated by the process. 
38 */ 
39 
40/* Get the n'th byte of the message */ 
41#define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int) 
42/* The IOCTL is used for both input and output. It receives from the user 
43 * a number, n, and returns Message[n]. 
44 */ 
45 
46/* The name of the device file */ 
47#define DEVICE_FILE_NAME "char_dev" 
48 
49#endif

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 
12struct ioctl_arg { 
13    unsigned int reg; 
14    unsigned int val; 
15}; 
16 
17/* Documentation/ioctl/ioctl-number.txt */ 
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, 
39                             unsigned int cmd, 
40                             unsigned long arg) 
41{ 
42    struct test_ioctl_data *ioctl_data = filp->private_data; 
43    int retval = 0; 
44    unsigned char val; 
45    struct ioctl_arg data; 
46    memset(&data, 0, sizeof(data)); 
47 
48    switch (cmd) { 
49    case IOCTL_VALSET: 
50        if (copy_from_user(&data, (int __user *) arg, sizeof(data))) { 
51            retval = -EFAULT; 
52            goto done; 
53        } 
54 
55        pr_alert("IOCTL set val:%x .\n", data.val); 
56        write_lock(&ioctl_data->lock); 
57        ioctl_data->val = data.val; 
58        write_unlock(&ioctl_data->lock); 
59        break; 
60 
61    case IOCTL_VALGET: 
62        read_lock(&ioctl_data->lock); 
63        val = ioctl_data->val; 
64        read_unlock(&ioctl_data->lock); 
65        data.val = val; 
66 
67        if (copy_to_user((int __user *) arg, &data, sizeof(data))) { 
68            retval = -EFAULT; 
69            goto done; 
70        } 
71 
72        break; 
73 
74    case IOCTL_VALGET_NUM: 
75        retval = __put_user(ioctl_num, (int __user *) arg); 
76        break; 
77 
78    case IOCTL_VALSET_NUM: 
79        ioctl_num = arg; 
80        break; 
81 
82    default: 
83        retval = -ENOTTY; 
84    } 
85 
86done: 
87    return retval; 
88} 
89 
90ssize_t test_ioctl_read(struct file *filp, 
91                        char __user *buf, 
92                        size_t count, 
93                        loff_t *f_pos) 
94{ 
95    struct test_ioctl_data *ioctl_data = filp->private_data; 
96    unsigned char val; 
97    int retval; 
98    int i = 0; 
99    read_lock(&ioctl_data->lock); 
100    val = ioctl_data->val; 
101    read_unlock(&ioctl_data->lock); 
102 
103    for (; i < count; i++) { 
104        if (copy_to_user(&buf[i], &val, 1)) { 
105            retval = -EFAULT; 
106            goto out; 
107        } 
108    } 
109 
110    retval = count; 
111out: 
112    return retval; 
113} 
114 
115static int test_ioctl_close(struct inode *inode, struct file *filp) 
116{ 
117    pr_alert("%s call.\n", __func__); 
118 
119    if (filp->private_data) { 
120        kfree(filp->private_data); 
121        filp->private_data = NULL; 
122    } 
123 
124    return 0; 
125} 
126 
127static int test_ioctl_open(struct inode *inode, struct file *filp) 
128{ 
129    struct test_ioctl_data *ioctl_data; 
130    pr_alert("%s call.\n", __func__); 
131    ioctl_data = kmalloc(sizeof(struct test_ioctl_data), GFP_KERNEL); 
132 
133    if (ioctl_data == NULL) { 
134        return -ENOMEM; 
135    } 
136 
137    rwlock_init(&ioctl_data->lock); 
138    ioctl_data->val = 0xFF; 
139    filp->private_data = ioctl_data; 
140    return 0; 
141} 
142 
143struct file_operations fops = { 
144    .owner = THIS_MODULE, 
145    .open = test_ioctl_open, 
146    .release = test_ioctl_close, 
147    .read = test_ioctl_read, 
148    .unlocked_ioctl = test_ioctl_ioctl, 
149}; 
150 
151static int ioctl_init(void) 
152{ 
153    dev_t dev = MKDEV(test_ioctl_major, 0); 
154    int alloc_ret = 0; 
155    int cdev_ret = 0; 
156    alloc_ret = alloc_chrdev_region(&dev, 0, num_of_dev, DRIVER_NAME); 
157 
158    if (alloc_ret) { 
159        goto error; 
160    } 
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 
170    pr_alert("%s driver(major: %d) installed.\n", DRIVER_NAME, 
171             test_ioctl_major); 
172    return 0; 
173error: 
174 
175    if (cdev_ret == 0) { 
176        cdev_del(&test_ioctl_cdev); 
177    } 
178 
179    if (alloc_ret == 0) { 
180        unregister_chrdev_region(dev, num_of_dev); 
181    } 
182 
183    return -1; 
184} 
185 
186static void ioctl_exit(void) 
187{ 
188    dev_t dev = MKDEV(test_ioctl_major, 0); 
189    cdev_del(&test_ioctl_cdev); 
190    unregister_chrdev_region(dev, num_of_dev); 
191    pr_alert("%s driver removed.\n", DRIVER_NAME); 
192} 
193 
194module_init(ioctl_init); 
195module_exit(ioctl_exit); 
196 
197MODULE_LICENSE("GPL"); 
198MODULE_DESCRIPTION("This is test_ioctl module");