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're 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'll 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'll know what I mean.
3.1.2. Functions available to modules
Programmers use functions they don't 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 don't 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, printk() but didn't include a standard I/O library. That's 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's easy! Compile the
following program:
#include <stdio.h>
int main(void)
{ printf("hello"); return 0; }
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[1] 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 what it returns. It's an invaluable tool for
figuring out things like what files a program is trying to access. Towards the end, you'll 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's 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'll 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.
3.1.3. 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 has 4 of these modes, which are 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.
3.1.4. 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're 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 don't want to declare
everything as static, another option is to declare a symbol table and register it with a
kernel. We'll 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.
3.1.5. Code space
Memory management is a very complicated subject---the majority of O'Reilly's `Understanding The Linux Kernel' is
just on memory management! We're 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 haven't thought about what a segfault really means, you may be surprised to hear that pointers don't 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[2].
This memory begins with $0$ and extends up to whatever it needs to be. Since the memory space for any two processes don't
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'll 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 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[3]. There are things called microkernels which have modules which get their own
codespace. The GNU Hurd and QNX Neutrino are two examples of a microkernel.
3.1.6. Device Drivers
One class of module is the device driver, which provides functionality for hardware like a TV card or 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.o 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.
3.1.6.1. Major and Minor Numbers
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's 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's `b' then it's a block device,
and if it's `c' then it's a character device. The devices you see above are block devices. Here are some character
devices (the serial ports):
If you want to see which major numbers have been assigned, you can look at
/usr/src/linux/Documentation/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 don't have to put your device files
into /dev, but it's done by convention. Linus put his device files in
/dev, and so should you. However, when creating a device file for testing purposes, it's
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'd 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:
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 2). You might even be aware that these both represent your floppy drive,
even if you only have one floppy drive. Why two files? One represents the floppy drive with 1.44MB of storage. The other is the same floppy drive with
1.68MB of storage, and corresponds to what some people call a `superformatted'
disk. One that holds more data than a standard formatted floppy. So here's a case where two device files with
different minor number actually represent the same piece of physical hardware. So just be aware that the word
`hardware' in our discussion can mean something very abstract.
It's an invaluable tool for
figuring out things like what files a program is trying to access. Ever have a program bail silently because it
couldn't find a file? It's a PITA!
