Going through Linux Device Drivers, 3rd Edition

I was introduced to Linux Device Drivers Book by Alessandro Rubini, Greg Kroah-Hartman, and Jonathan Corbet in a video by LiveOverflow from 2020. But I never got around to reading it. This post is just me jotting down my experiences going through this book.

Linux Device Drivers, Third Edition #

This book is freely available from lwn.net under the Creative Commons “Attribution-ShareAlike” license, Version 2.0. LiveOverflow goes through a bit of history in his video. I think I don’t need to reiterate it. Oh, and BTW, sub to lwn.net if you can!

I built the kernel with the existing config from my Arch Linux install a couple of years back. And custom kernels for building TWRP and some test ROMs for my previous android devices. But that was it. The kernel source is too scary for me 😅

So, in other words, this is another attempt to try and understand the kernel source, or at least some parts of it. Heard, device drivers are a good starting point? Well, the kernel can be thought of as a driver.

Chapter 1 is a lighter version of my Engineering Degree’s CS204 syllabus but a bit skewed towards Linux Kernel.

Also, since LDD3 was created with Linux 2.6 in mind, some coding examples will require some modifications. There is this repo ldd3 which has examples updated to work in recent kernels.

Chapter 2 #

So, the real meat starts from chapter 2 which contains a hello world kernel module!

I jumped on Vagrant to set up a temporary, quick VM. Just followed the Vagrant ArchWiki and used libvirt as the virtualization provider. Also had to install nfs-utils along with the libvirt dependencies.

Ran vagrant init and edited the Vagrantfile to include Debian Bullseye box debian/bullseye64. A sample config might look something like this.

Vagrant.configure("2") do |config|
  config.vm.box = "debian/bullseye64"

  config.vm.provider "libvirt" do |v|
    # Adjust these as required
    v.memory = 8162
    v.cpus = 4
  end
end

Now, just do vagrant up and if it’s successful, you can do vagrant ssh to get into the VM. You also need to install build-essentials and appropriate kernel headers for your host(VM) -> apt install build-essential linux-headers-`uname -r` .

Copied over the hello.c file from here to /vagrant/src directory on the guest (or to ./src directory on the host) and added the makefile. Make sure to remove all the modules that we are not building at this step -> Only keep hello.o in obj-m.

While copying, I’ve also noticed this placement of braces and spaces. It’s not something that I’m used to. So it might be worth looking at the very opinionated coding style for the kernel.

Now if you run make, it should hopefully build our very very complex kernel modules without errors :)

To insert our module to the kernel, we can use insmod; sudo insmod hello.ko. Note the .ko extension indicating that it’s a kernel object.

At this point we can inspect the kernel logs and we should see the function we defined in the module_init macro getting executed.

sudo tail /var/log/kern.log ->

Aug 24 09:04:43 bullseye kernel: [ 1143.598608] hello: loading out-of-tree module taints kernel.
Aug 24 09:04:43 bullseye kernel: [ 1143.599432] hello: module verification failed: signature and/or required key missing - tainting kernel
Aug 24 09:04:43 bullseye kernel: [ 1143.600865] Hello, world

How exciting is this! Something we wrote is getting executed in the kernel space, not in userspace!!

To remove this module, we can run sudo rmmod hello and see our poor module realizing the realities of the world :P

Aug 24 09:26:00 bullseye kernel: [ 2421.287445] Goodbye, cruel world

Just to spice things a bit I decided to calculate the factorial of 10 from the kernel space 😆

#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("Dual BSD/GPL");
static int hello_init(void)
{
printk(KERN_ALERT "Hello, world\n");
return 0;
}
static void hello_exit(void)
{
long fact = 1;
int i = 1;
while(i<=10)
        {
                fact*=i;
                i++;
        }
printk(KERN_INFO "Factorial: %ld\n", fact);
}

module_init(hello_init);
module_exit(hello_exit);

I’m pretty sure that I have broken 10 different kernel coding guidelines with these ^ lines. And is probably against the conventions, but, it’s fun!

Aug 28 10:04:26 bullseye kernel: [ 4727.115511] Hello, world
Aug 28 10:04:30 bullseye kernel: [ 4730.577740] Factorial: 3628800

I’ll be back with moarr sweet kernel juice.