LKM

LKM stands for "Linux Kernel Module" or "Loadable Kernel Module". As the name implies, it is a way to allow code to interact directly with the kernel, in the kernel. In essence, it is a way to extend the kernel and add functionality to it. Although it goes without saying that you need root to modify the kernel, LKMs can be very powerful if used correctly, since the kernel operates under significantly elevated privileges compared to userland. In particular, the functionality provided by extending the kernel can be used to great effect in the development of Rootkits.

LKMs interact with your system on the kernel level, executing with the highest possible level of privilege. A poorly-designed kernel module may make your OS unstable, corrupt your filesystem and even brick your computer. You have been warned.

You can see a list of currently loaded kernel modules in two ways:

$ lsmod $ cat /proc/modules

You can (as root) add new modules to your kernel with the insmod and rmmod commands:

$ insmod modname.ko $ rmmod modname

These two utilities provide a simple, clean way to insert or remove modules from the kernel. If you need more advanced control over the insertion, removal and alteration of modules in the kernel, use the more fully-featured modprobe utility instead.

Writing a Basic LKM

Linux kernel modules are written in C and compiled from one or more source files into a kernel object (.ko) file. In order to write an LKM, you will need a strong grasp of the fundamentals of C programming and at least a basic understanding of the way linux manages files, processes and devices.

See Also

The Linux Kernel Module Programming Guide- an outdated but solid tutorial covering many of the concepts that will help you to understand the linux kernel.