Compiling Kernel Modules

Kernel modules need to be compiled with certain gcc options to make them work. In addition, they also need to be compiled with certain symbols defined. 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 (as ours), see file linux/Documentation/kbuild/modules.txt.

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

obj-m += hello-1.o

Now you can compile the module by issuing the command make -C /usr/src/linux-`uname -r` SUBDIRS=$PWD modules . You should obtain an output which resembles the following:

[root@pcsenonsrv test_module]# make -C /usr/src/linux-`uname -r` SUBDIRS=$PWD modules
make: Entering directory `/usr/src/linux-2.6.x
  CC [M]  /root/test_module/hello-1.o
  Building modules, stage 2.
  MODPOST
  CC      /root/test_module/hello-1.mod.o
  LD [M]  /root/test_module/hello-1.ko
make: Leaving directory `/usr/src/linux-2.6.x
	

Please 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. 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 dig into Makefiles.

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 wares 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 to the reader. See that comment above the return statement in init_module()? Change the return value to something non-zero, recompile and load the module again. What happens?