Introduction to Linux Kernel Module

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/modules.conf. If it finds an alias line like: alias char-major-10-30 softdog it knows that the generic identifier refers to the module softdog.o. 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.o requires the fat.o module to be already loaded into the kernel. The requested module has a dependancy 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/[2], 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. So for example, if you wanted to load the msdos module, you'd have to either run: insmod /lib/modules/2.5.1/kernel/fs/fat/fat.o insmod /lib/modules/2.5.1/kernel/fs/msdos/msdos.o or just run "modprobe -a msdos". Linux distros provide modprobe, insmod and depmod as a package called modutils or mod-utils. Before finishing this chapter, let's take a quick look at a piece of /etc/modules.conf: #This file is automatically generated by update-modules path[misc]=/lib/modules/2.4.?/local keep path[net]=~p/mymodules options mydriver irq=10 alias eth0 eepro Lines beginning with a '#' are comments. Blank lines are ignored. The path[misc] line tells modprobe to replace the search path for misc modules with the directory /lib/modules/2.4.?/local. As you can see, shell meta characters are honored. The path[net] line tells modprobe to look for net modules in the directory ~p/mymodules, however, the "keep" directive preceding the path[net]directive tells modprobe to add this directory to the standard search path of net modules as opposed to replacing the standard search path, as we did for the misc modules. The alias line says to load in eepro.o whenever kmod requests that the generic identifier `eth0' be loaded. You won't see lines like "alias block-major-2 floppy" in /etc/modules.conf because modprobe already knows about the standard drivers which will be used on most systems. 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. 1.2.1. 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. 1.2.1.1. Modversioning A module compiled for one kernel won't load if you boot a different kernel unless you enable CONFIG_MODVERSIONS in the kernel. We won't go into module versioning until later in this guide. Until we cover modversions, the examples in the guide may not work if you're running a kernel with modversioning turned on. However, most stock Linux distro kernels come with it turned on. If you're having trouble loading the modules because of versioning errors, compile a kernel with modversioning turned off. 1.2.1.2. Using X It is highly recommended that you type in, compile and load all the examples this guide discusses. It's also highly recommended you do this from a console. You should not be working on this stuff in X. Modules can't 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 log files. You won't see it unless you look through your log files. To have immediate access to this information, do all your work from console. 1.2.1.3. Compiling Issues and Kernel Version Very often, Linux distros will distribute kernel source that has been patched in various non-standard ways, which may cause trouble. A more common problem is that some Linux distros distribute incomplete kernel headers. You'll need to compile your code using various header files from the Linux kernel. Murphy's Law states that the headers that are missing are exactly the ones that you'll need for your module work. To avoid these two problems, I highly recommend that you download, compile and boot into a fresh, stock Linux kernel which can be downloaded from any of the Linux kernel mirror sites. See the Linux Kernel HOWTO for more details. Ironically, this can also cause a problem. By default, gcc on your system may look for the kernel headers in their default location rather than where you installed the new copy of the kernel (usually in /usr/src/. This can be fixed by using gcc's -I switch. By  James Thornton