1.1. What Is A Kernel Module?

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

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

1.2. How Do Modules Get Into The Kernel?

You can see what modules are already loaded into the kernel by running lsmod, which gets its information by reading the file /proc/modules.

How do these modules find their way into the kernel? When the kernel needs a feature that is not resident in the kernel, the kernel module daemon kmod[1] execs modprobe to load the module in. modprobe is passed a string in one of two forms:

• A module name like softdog or ppp.

• A more generic identifier like char-major-10-30.

If modprobe is handed a generic identifier, it first looks for that string in the file /etc/modprobe.conf.[2] If it finds an alias line like:

alias char-major-10-30 softdog
	

it knows that the generic identifier refers to the module softdog.ko.

Next, modprobe looks through the file /lib/modules/version/modules.dep, to see if other modules must be loaded before the requested module may be loaded. This file is created by depmod -a and contains module dependencies. For example, msdos.ko requires the fat.ko module to be already loaded into the kernel. The requested module has a dependency on another module if the other module defines symbols (variables or functions) that the requested module uses.

Lastly, modprobe uses insmod to first load any prerequisite modules into the kernel, and then the requested module. modprobe directs insmod to /lib/modules/version/[3], the standard directory for modules. insmod is intended to be fairly dumb about the location of modules, whereas modprobe is aware of the default location of modules, knows how to figure out the dependencies and load the modules in the right order. So for example, if you wanted to load the msdos module, you'd have to either run:

insmod /lib/modules/2.6.11/kernel/fs/fat/fat.ko
insmod /lib/modules/2.6.11/kernel/fs/msdos/msdos.ko
	

or:

modprobe msdos
	

What we've seen here is: insmod requires you to pass it the full pathname and to insert the modules in the right order, while modprobe just takes the name, without any extension, and figures out all it needs to know by parsing /lib/modules/version/modules.dep.

Linux distros provide modprobe, insmod and depmod as a package called module-init-tools. In previous versions that package was called modutils. Some distros also set up some wrappers that allow both packages to be installed in parallel and do the right thing in order to be able to deal with 2.4 and 2.6 kernels. Users should not need to care about the details, as long as they're running recent versions of those tools.

Now you know how modules get into the kernel. There's a bit more to the story if you want to write your own modules which depend on other modules (we calling this `stacking modules'). But this will have to wait for a future chapter. We have a lot to cover before addressing this relatively high-level issue.

2.1. Hello, World (part 1): The Simplest Module

When the first caveman programmer chiseled the first program on the walls of the first cave computer, it was a program to paint the string `Hello, world' in Antelope pictures. Roman programming textbooks began with the `Salut, Mundi' program. I don't know what happens to people who break with this tradition, but I think it's safer not to find out. We'll start with a series of hello world programs that demonstrate the different aspects of the basics of writing a kernel module.

Here's the simplest module possible. Don't compile it yet; we'll cover module compilation in the next section.

>

Kernel modules must have at least two functions: a "start" (initialization) function called init_module() which is called when the module is insmoded into the kernel, and an "end" (cleanup) function called cleanup_module() which is called just before it is rmmoded. 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'll learn how to do this in Section 2.3. 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 printk() log level, KERN_ALERT, which you'll learn about in Section 2.1.1.

2.2. Compiling Kernel Modules

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'll find in this guide), see file linux/Documentation/kbuild/modules.txt.

So, let's look at a simple Makefile for compiling a module named hello-1.c:

>

From a technical point of view just the first line is really necessary, the "all" and "clean" targets were added for pure convenience.

Now you can compile the module by issuing the command make . You should obtain an output which resembles the following:

hostname:~/lkmpg-examples/02-HelloWorld# make
make -C /lib/modules/2.6.11/build M=/root/lkmpg-examples/02-HelloWorld modules
make[1]: Entering directory `/usr/src/linux-2.6.11'
  CC [M]  /root/lkmpg-examples/02-HelloWorld/hello-1.o
 Building modules, stage 2.
  MODPOST
  CC      /root/lkmpg-examples/02-HelloWorld/hello-1.mod.o
  LD [M]  /root/lkmpg-examples/02-HelloWorld/hello-1.ko
make[1]: Leaving directory `/usr/src/linux-2.6.11'
hostname:~/lkmpg-examples/02-HelloWorld#
	

Note that kernel 2.6 introduces a new file naming convention: kernel modules now have a .ko extension (in place of the old .o extension) which easily distinguishes them from conventional object files. The reason for this is that they contain an additional .modinfo section that where additional information about the module is kept. We'll soon see what this information is good for.

Use modinfo hello-*.ko to see what kind of information it is.

hostname:~/lkmpg-examples/02-HelloWorld# modinfo hello-1.ko
filename:       hello-1.ko
vermagic:       2.6.11 preempt PENTIUMII 4KSTACKS gcc-3.3
depends:

Nothing spectacular, so far. That changes once we're using modinfo on one of our the later examples, hello-5.ko .

hostname:~/lkmpg-examples/02-HelloWorld# modinfo hello-5.ko
filename:       hello-5.ko
license:        GPL
author:         Peter Jay Salzman
vermagic:       2.6.11 preempt PENTIUMII 4KSTACKS gcc-3.3
depends:
parm:           myintArray:An array of integers (array of int)
parm:           mystring:A character string (charp)
parm:           mylong:A long integer (long)
parm:           myint:An integer (int)
parm:           myshort:A short integer (short)
hostname:~/lkmpg-examples/02-HelloWorld# 

Lot's of useful information to see here. An author string for bugreports, license information, even a short description of the parameters it accepts.

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

Now it is time to insert your freshly-compiled module it into the kernel with insmod ./hello-1.ko (ignore anything you see about tainted kernels; we'll cover that shortly).

All modules loaded into the kernel are listed in /proc/modules. Go ahead and cat that file to see that your module is really a part of the kernel. Congratulations, you are now the author of Linux kernel code! When the novelty wears off, remove your module from the kernel by using rmmod hello-1. Take a look at /var/log/messages just to see that it got logged to your system logfile.

Here's 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?

2.3. Hello World (part 2)

As of Linux 2.4, you can rename the init and cleanup functions of your modules; they no longer have to be called init_module() and cleanup_module() respectively. This is done with the module_init() and module_exit() macros. These macros are defined in linux/init.h. The only caveat is that your init and cleanup functions must be defined before calling the macros, otherwise you'll get compilation errors. Here's an example of this technique:

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

>

Now have a look at linux/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 linux/.config file, the last time when you said make menuconfig or something like that.

2.4. Hello World (part 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 __exit, has no effect for loadable modules. Again, if you consider when the cleanup function runs, this makes complete sense; built-in drivers don't need a cleanup function, while loadable modules do.

These macros are defined in 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.

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2.5. Hello World (part 4): Licensing and Module Documentation

If you're running kernel 2.4 or later, you might have noticed something like this when you loaded proprietary modules:

# insmod xxxxxx.o
Warning: loading xxxxxx.ko will taint the kernel: no license
  See http://www.tux.org/lkml/#export-tainted for information about tainted modules
Module xxxxxx loaded, with warnings
	

In kernel 2.4 and later, a mechanism was devised to identify code licensed under the GPL (and friends) so people can be warned that the code is non open-source. This is accomplished by the MODULE_LICENSE() macro which is demonstrated in the next piece of code. By setting the license to GPL, you can keep the warning from being printed. This license mechanism is defined and documented in linux/module.h:

/*
 * The following license idents are currently accepted as indicating free
 * software modules
 *
 *	"GPL"				[GNU Public License v2 or later]
 *	"GPL v2"			[GNU Public License v2]
 *	"GPL and additional rights"	[GNU Public License v2 rights and more]
 *	"Dual BSD/GPL"			[GNU Public License v2
 *					 or BSD license choice]
 *	"Dual MIT/GPL"			[GNU Public License v2
 *					 or MIT license choice]
 *	"Dual MPL/GPL"			[GNU Public License v2
 *					 or Mozilla license choice]
 *
 * The following other idents are available
 *
 *	"Proprietary"			[Non free products]
 *
 * There are dual licensed components, but when running with Linux it is the
 * GPL that is relevant so this is a non issue. Similarly LGPL linked with GPL
 * is a GPL combined work.
 *
 * This exists for several reasons
 * 1.	So modinfo can show license info for users wanting to vet their setup 
 *	is free
 * 2.	So the community can ignore bug reports including proprietary modules
 * 3.	So vendors can do likewise based on their own policies
 */

Similarly, MODULE_DESCRIPTION() is used to describe what the module does, MODULE_AUTHOR() declares the module's author, and MODULE_SUPPORTED_DEVICE() declares what types of devices the module supports.

These macros are all defined in linux/module.h and aren't used by the kernel itself. They're simply for documentation and can be viewed by a tool like objdump. As an exercise to the reader, try and search fo these macros in linux/drivers to see how module authors use these macros to document their modules.

I'd recommend to use something like grep -inr MODULE_AUTHOR * in /usr/src/linux-2.6.x/ . People unfamiliar with command line tools will probably like some web base solution, search for sites that offer kernel trees that got indexed with LXR. (or setup it up on your local machine).

Users of traditional Unix editors, like emacs or vi will also find tag files useful. They can be generated by make tags or make TAGS in /usr/src/linux-2.6.x/ . Once you've got such a tagfile in your kerneltree you can put the cursor on some function call and use some key combination to directly jump to the definition function.

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2.6. 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 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().

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

Arrays are supported too, but things are a bit different now than they were in the 2.4. 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:

int myintarray[2];
module_param_array(myintarray, int, NULL, 0); /* not interested in count */

int myshortarray[4];
int count;
module_parm_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's 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.

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I would recommend playing around with this code:

satan# insmod hello-5.ko mystring="bebop" mybyte=255 myintArray=-1
mybyte is an 8 bit integer: 255
myshort is a short integer: 1
myint is an integer: 20
mylong is a long integer: 9999
mystring is a string: bebop
myintArray is -1 and 420

satan# rmmod hello-5
Goodbye, world 5

satan# insmod hello-5.ko mystring="supercalifragilisticexpialidocious" \
> mybyte=256 myintArray=-1,-1
mybyte is an 8 bit integer: 0
myshort is a short integer: 1
myint is an integer: 20
mylong is a long integer: 9999
mystring is a string: supercalifragilisticexpialidocious
myintArray is -1 and -1

satan# rmmod hello-5
Goodbye, world 5

satan# insmod hello-5.ko mylong=hello
hello-5.o: invalid argument syntax for mylong: 'h'

2.7. Modules Spanning Multiple Files

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

Here's an example of such a kernel module.

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The next file:

>

And finally, the makefile:

>

This is the complete makefile for all the examples we've seen so far. The first five lines are nothing special, but for the last example we'll 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.

2.8. 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 rmmod -f module command. This option can save you a lot of time and a number of reboots during the development of a module.

Nevertheless, there is 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 /var/log/messages:

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 modinfo module.ko command:

[root@pcsenonsrv 02-HelloWorld]# modinfo hello-4.ko 
license:        GPL
author:         Peter Jay Salzman <p@dirac.org>
description:    A sample driver
vermagic:       2.6.5-1.358 686 REGPARM 4KSTACKS gcc-3.3
depends:        
	

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-2.6.x. You may just want to copy it to your kernel source tree: cp /boot/config-`uname -r` /usr/src/linux-`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 /usr/src/linux/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 = 2
PATCHLEVEL = 6
SUBLEVEL = 5
EXTRAVERSION = -1.358custom
...
	

In this case, you need to restore the value of symbol EXTRAVERSION to -1.358. We suggest to keep a backup copy of the makefile used to compile your kernel available in /lib/modules/2.6.5-1.358/build. A simple cp /lib/modules/`uname -r`/build/Makefile /usr/src/linux-`uname -r` should suffice. Additionally, if you already started a kernel build with the previous (wrong) Makefile, you should also rerun make, or directly modify symbol UTS_RELEASE in file /usr/src/linux-2.6.x/include/linux/version.h according to contents of file /lib/modules/2.6.x/build/include/linux/version.h, or overwrite the latter with the first.

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

[root@pcsenonsrv linux-2.6.x]# 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 your current kernel settings, and it will load into it without any errors.

3.1. Modules vs Programs

#include <stdio.h>
int main(void)
{ printf("hello"); return 0; }
				

# 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
					

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
					

% ls -l /dev/fd0 /dev/fd0u1680
brwxrwxrwx   1 root  floppy   2,  0 Jul  5  2000 /dev/fd0
brw-rw----   1 root  floppy   2, 44 Jul  5  2000 /dev/fd0u1680
					

4.1. Character Device Drivers

struct file_operations {
	struct module *owner;
	 loff_t(*llseek) (struct file *, loff_t, int);
	 ssize_t(*read) (struct file *, char __user *, size_t, loff_t *);
	 ssize_t(*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
	 ssize_t(*write) (struct file *, const char __user *, size_t, loff_t *);
	 ssize_t(*aio_write) (struct kiocb *, const char __user *, size_t,
			      loff_t);
	int (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	int (*ioctl) (struct inode *, struct file *, unsigned int,
		      unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, struct dentry *, int datasync);
	int (*aio_fsync) (struct kiocb *, int datasync);
	int (*fasync) (int, struct file *, int);
	int (*lock) (struct file *, int, struct file_lock *);
	 ssize_t(*readv) (struct file *, const struct iovec *, unsigned long,
			  loff_t *);
	 ssize_t(*writev) (struct file *, const struct iovec *, unsigned long,
			   loff_t *);
	 ssize_t(*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
			     void __user *);
	 ssize_t(*sendpage) (struct file *, struct page *, int, size_t,
			     loff_t *, int);
	unsigned long (*get_unmapped_area) (struct file *, unsigned long,
					    unsigned long, unsigned long,
					    unsigned long);
};
			

struct file_operations fops = {
	read: device_read,
	write: device_write,
	open: device_open,
	release: device_release
};
			

struct file_operations fops = {
	.read = device_read,
	.write = device_write,
	.open = device_open,
	.release = device_release
};
			

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

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5.1. 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.

The reason we use proc_register_dynamic[8] is because we don't want to determine the inode number used for our file in advance, but to allow the kernel to determine it to prevent clashes. 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 try_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 procfs_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 create_proc_entry. 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 procfs_read is called. Two parameters of this function are very important: the buffer (the first parameter) and the offset (the third 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 isn't 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!
        

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5.2. 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's 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's 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 caracter, you can handle several caracters 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.

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5.3. Manage /proc file with standard filesystem

We have seen how to read and write a /proc file with the /proc interface. But it's also possible to manage /proc file with inodes. The main interest is to use advanced function, 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[9], there is a special structure to hold pointers to all those functions, struct inode_operations, which includes a pointer to struct file_operations. 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 file_operations which includes pointers to our procfs_read and procfs_write functions.

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

It's 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.

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Still hungry for procfs examples? Well, first of all keep in mind, there are rumors around, claiming that procfs is on it's way out, consider using sysfs instead. Second, if you really can't get enough, there's a highly recommendable bonus level for procfs below linux/Documentation/DocBook/ . Use make help in your toplevel kernel directory for instructions about how to convert it into your favourite format. Example: make htmldocs . Consider using this mechanism, in case you want to document something kernel related yourself.

5.4. 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's 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 thought 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 CARREFUL: 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 "How seq_file works".

Seq_file provides basic functions for file_operations, 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.

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

• http://lwn.net/Articles/22355/

• http://www.kernelnewbies.org/documents/seq_file_howto.txt

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

6.1. TODO: Write a chapter about sysfs

This is just a placeholder for now. Finally I'd like to see a (yet to be written) chapter about sysfs instead here. If you are familiar with sysfs and would like to take part in writing this chapter, feel free to contact us (the LKMPG maintainers) for further details.

7.1. Talking to Device Files (writes and IOCTLs)

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), [10] both or neither. 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. [11]

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/ioctl-number.txt.

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8.1. System Calls

So far, the only thing we've done was to use well defined kernel mechanisms to register /proc files and device handlers. This is fine if you want to do something the kernel programmers thought you'd want, such as write a device driver. But what if you want to do something unusual, to change the behavior of the system in some way? Then, you're mostly on your own.

This is where kernel programming gets dangerous. While writing the example below, I killed the open() system call. This meant I couldn't open any files, I couldn't run any programs, and I couldn't shutdown the computer. I had to pull the power switch. Luckily, no files died. To ensure you won't lose any files either, please run sync right before you do the insmod and the rmmod.

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

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

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

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

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

The source code here is an example of such a kernel module. We want to `spy' on a certain user, and to printk() a message whenever that user opens a file. Towards this end, we replace the system call to open a file with our own function, called our_sys_open. This function checks the uid (user's id) of the current process, and if it's equal to the uid we spy on, it calls printk() to display the name of the file to be opened. Then, either way, it calls the original open() function with the same parameters, to actually open the file.

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

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

Note that all the related problems make syscall stealing unfeasiable for production use. In order to keep people from doing potential harmful things sys_call_table is no longer exported. This means, if you want to do something more than a mere dry run of this example, you will have to patch your current kernel in order to have sys_call_table exported. In the example directory you will find a README and the patch. As you can imagine, such modifications are not to be taken lightly. Do not try this on valueable systems (ie systems that you do not own - or cannot restore easily). You'll need to get the complete sourcecode of this guide as a tarball in order to get the patch and the README. Depending on your kernel version, you might even need to hand apply the patch. Still here? Well, so is this chapter. If Wyle E. Coyote was a kernel hacker, this would be the first thing he'd try. ;)

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9.1. Blocking Processes

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

This kernel module is an example of this. The file (called /proc/sleep) can only be opened by a single process at a time. If the file is already open, the kernel module calls wait_event_interruptible[12]. This function changes the status of the task (a task is the kernel data structure which holds information about a process and the system call it's in, if any) to TASK_INTERRUPTIBLE, which means that the task will not run until it is woken up somehow, and adds it to WaitQ, the queue of tasks waiting to access the file. Then, the function calls the scheduler to context switch to a different process, one which has some use for the CPU.

When a process is done with the file, it closes it, and module_close is called. That function wakes up all the processes in the queue (there's no mechanism to only wake up one of them). It then returns and the process which just closed the file can continue to run. In time, the scheduler decides that the process has had enough and gives control of the CPU to another process. Eventually, one of the processes which was in the queue will be given control of the CPU by the scheduler. It starts at the point right after the call to module_interruptible_sleep_on[13]. It can then proceed to set a global variable to tell all the other processes that the file is still open and go on with its life. When the other processes get a piece of the CPU, they'll see that global variable and go back to sleep.

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

To make our life more interesting, module_close doesn't have a monopoly on waking up the processes which wait to access the file. A signal, such as Ctrl+c (SIGINT) can also wake up a process. [14] In that case, we want to return with -EINTR immediately. This is important so users can, for example, kill the process before it receives the file.

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

hostname:~/lkmpg-examples/09-BlockingProcesses# insmod sleep.ko
hostname:~/lkmpg-examples/09-BlockingProcesses# cat_noblock /proc/sleep
Last input:
hostname:~/lkmpg-examples/09-BlockingProcesses# tail -f /proc/sleep &
Last input:
Last input:
Last input:
Last input:
Last input:
Last input:
Last input:
tail: /proc/sleep: file truncated
[1] 6540
hostname:~/lkmpg-examples/09-BlockingProcesses# cat_noblock /proc/sleep
Open would block
hostname:~/lkmpg-examples/09-BlockingProcesses# kill %1
[1]+  Terminated              tail -f /proc/sleep
hostname:~/lkmpg-examples/09-BlockingProcesses# cat_noblock /proc/sleep
Last input:
hostname:~/lkmpg-examples/09-BlockingProcesses#

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10.1. Replacing printk

In Section 1.2.1.2, I said that X and kernel module programming don't mix. That's true for developing kernel modules, but in actual use, you want to be able to send messages to whichever tty[15] the command to load the module came from.

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

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10.2. Flashing keyboard LEDs

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

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

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

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

11.1. Scheduling Tasks

Very often, we have "housekeeping" tasks which have to be done at a certain time, or every so often. If the task is to be done by a process, we do it by putting it in the crontab file. If the task is to be done by a kernel module, we have two possibilities. The first is to put a process in the crontab file which will wake up the module by a system call when necessary, for example by opening a file. This is terribly inefficient, however -- we run a new process off of crontab, read a new executable to memory, and all this just to wake up a kernel module which is in memory anyway.

Instead of doing that, we can create a function that will be called once for every timer interrupt. The way we do this is we create a task, held in a workqueue_struct structure, which will hold a pointer to the function. Then, we use queue_delayed_work to put that task on a task list called my_workqueue, which is the list of tasks to be executed on the next timer interrupt. Because we want the function to keep on being executed, we need to put it back on my_workqueue whenever it is called, for the next timer interrupt.

There's one more point we need to remember here. When a module is removed by rmmod, first its reference count is checked. If it is zero, module_cleanup is called. Then, the module is removed from memory with all its functions. Things need to be shut down properly, or bad things will happen. See the code below how this can be done in a safe way.

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12.1. Interrupt Handlers

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13.1. Symmetrical Multi-Processing

One of the easiest and cheapest ways to improve hardware performance is to put more than one CPU on the board. This can be done either making the different CPU's take on different jobs (asymmetrical multi-processing) or by making them all run in parallel, doing the same job (symmetrical multi-processing, a.k.a. SMP). Doing asymmetrical multi-processing effectively requires specialized knowledge about the tasks the computer should do, which is unavailable in a general purpose operating system such as Linux. On the other hand, symmetrical multi-processing is relatively easy to implement.

By relatively easy, I mean exactly that: not that it's really easy. In a symmetrical multi-processing environment, the CPU's share the same memory, and as a result code running in one CPU can affect the memory used by another. You can no longer be certain that a variable you've set to a certain value in the previous line still has that value; the other CPU might have played with it while you weren't looking. Obviously, it's impossible to program like this.

In the case of process programming this normally isn't an issue, because a process will normally only run on one CPU at a time[18]. The kernel, on the other hand, could be called by different processes running on different CPU's.

In version 2.0.x, this isn't a problem because the entire kernel is in one big spinlock. This means that if one CPU is in the kernel and another CPU wants to get in, for example because of a system call, it has to wait until the first CPU is done. This makes Linux SMP safe[19], but inefficient.

In version 2.2.x, several CPU's can be in the kernel at the same time. This is something module writers need to be aware of.

14.1. Common Pitfalls

Before I send you on your way to go out into the world and write kernel modules, there are a few things I need to warn you about. If I fail to warn you and something bad happens, please report the problem to me for a full refund of the amount I was paid for your copy of the book.