Kernel Modules

Let's say I have a sweet ride, I invest a lot of time and money into it. I add a spoiler, hitch, bike rack and other random things. These components don't actually change the core functionality of the car and I can remove and add them very easily. The kernel uses the same concept with kernel modules.

The kernel in itself is a monolithic piece of software, when we want to add support for a new type of keyboard, we don't write this code directly into the kernel code. Just as we wouldn't meld a bike rack to our car (well maybe some people would do that). Kernel modules are pieces of code that can be loaded and unloaded into the kernel on demand. They allow us to extend the functionality of the kernel without actually adding to the core kernel code. We can also add modules and not have to reboot the system (in most cases).

View a list of currently loaded modules

$ lsmod

Load a module

$ sudo modprobe bluetooth

Modprobe loads tries the module from /lib/modules/(kernel version)/kernel/drivers. Kernel modules may also have dependencies, modprobe loads our module dependencies if they are not already loaded.

Remove a module

$ sudo modprobe -r bluetooth

Load on bootup

You can also load modules during system boot, instead of temporarily loading them with modprobe (which will be unloaded when you reboot). Just modify the /etc/modprobe.d directory and add a configuration file in it like so:

pete@icebox:~$ /etc/modprobe.d/peanutbutter.conf

options peanut_butter type=almond

A bit of a outlandish example, but if you had a module named peanut_butter and you wanted to add a kernel parameter for type=almond, you can have it load on startup using this configuration file. Also note that kernel modules have their own kernel parameters so you'll want to read about the module specifically to find out more.

Do not load on bootup

You can also make sure a module does not load on bootup by adding a configuration file like so:

pete@icebox:~$ /etc/modprobe.d/peanutbutter.conf

blacklist peanut_butter

Measured against its subject this is the shallowest section in the course. The kernel is tens of millions of lines of code and the thing every other lesson here has been standing on. What these six lessons give you is vocabulary: enough to know what a module is, what a system call is, and to follow a conversation or an error message that mentions them. The kernel's own documentation is the real thing, and it is written for people building kernels rather than using them, which is a fair indication of the distance between here and there.