Become a kernel hacker: A guide to writing your first Linux kernel module

jeremy · 05-30-2014, 10:02 AM

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Ever wanted to start hacking the kernel? Don’t have a clue how to begin? Let us show you how it’s done…

Kernel programming is often seen as a black magic. In Arthur C Clarke’s sense, it probably is. The Linux kernel is quite different from its user space: many abstractions are waived, and you have to take extra care, as a bug in you code affects the whole system. There is no easy way to do floating-point maths, the stack is fixed and small, and the code you write is always asynchronous so you need to think about the concurrency. Despite all of this though, the Linux kernel is just a very large and complex C program that is open for everyone to read, learn and improve, and you too can be a part of it.

Probably the easiest way to start kernel programming is to write a module – a piece of code that can be dynamically loaded into the kernel and removed from it. There are limits to what modules can do – for example, they can’t add or remove fields to common data structures like process descriptors. But in all other ways they are full-fledged kernel-level code, and they can always be compiled into the kernel (thus removing all the restrictions) if needed. It is fully possible to develop and compile a module outside the Linux source tree (this is unsurprisingly called an out-of-tree build), which is very convenient if you just want to play a bit and do not wish to submit your changes for inclusion into the mainline kernel.

In this tutorial, we’ll develop a simple kernel module that creates a /dev/reverse device. A string written to this device is read back with the word order reversed (“Hello World” becomes “World Hello”). It is a popular programmer interview puzzle, and you are likely to get some bonus points when you show the ability to implement it at the kernel level as well. A word of warning before we start: a bug in your module may lead to a system crash and (unlikely, but possible) data loss. Be sure you’ve backed up all your important data before you start, or, even better, experiment in a virtual machine.

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--jeremy