Creating a basic Linux kernel development setup

Linux kernel development is something I’ve been wanting to get into for a long time now. I’ve tried and failed to get started over the years, but I finally think I’m in a place where I can do some real kernel work. But before we can dive into kernel development, we have to have a working development setup.

This document will be a rough outline into

  1. Compiling the kernel
  2. Setting up a initramfs for the kernel to boot into
  3. Booting into our setup

Compiling the kernel

Compiling the kernel is a relatively straightforward task. First, do as you would any other open source project and fork Linus’ kernel repo on Github. Alternatively, you could clone his kernel.org repo, but it doesn’t really matter.

cd into your cloned repo and run make defconfig; make x86_64_defconfig; make kvmconfig. This will generate a pretty good set of default configs (located in .config).

Finally run make to compile the kernel. Hint: use make -jN, where N is the number of logical cores you have on your CPU for faster compilation.

Setting up initramfs

initramfs is the initial memory only filesystem the Linux kernel uses during boot. It is necessary to solve some of the boot time chicken-or-egg type problems that may arise. Read more from the link if curious. If we were to try and boot into a bare kernel, we would get a kernel panic about not being able to mount a rootfs. This is expected because of the previous reasons. Thus, we need to create our own initramfs.

There are a number of ways to create an initial ramdisk. We are going to roll our own busybox ramdisk. Although it’s not the easiest way, it’s rather transparent and somewhat easy to understand.

First, clone the busybox repo. Then, cd into the directory and run make defconfig. Sounds familiar, right? Next, run make menuconfig and make sure the static binary option is ticked. You may need to dig around in that interface, but it’s there. Finding it is left as an exercise to the reader. Finally, run make; make install. This will put all the symlinked busybox “binaries” into _install of the current directory.

As for the initramfs, create a folder for holding all the intermediate files. For this guide, we’ll assume it’s in ~/dev/initramfs.

mkdir -pv ~/dev/initramfs/x86-busybox
cd !$
mkdir -pv {bin,sbin,etc,proc,sys,usr/{bin,sbin}}
cp -av ~/dev/busybox/_install/* .

What this does is create a basic filesystem layout for our custom Linux kernel to boot into. Obviously some utilities that rely on a more complicated filesystem layout won’t work. Finding out which ones is another exercise left to the reader.

Now we need an init script. All Linux systems need an inital process to startup all the userspace fun, and this is no different. Plop this simple init script into ~/dev/initramfs:

#!/bin/sh

mount -t proc none /proc
mount -t sysfs none /sys

echo -e "\nBoot took $(cut -d' ' -f1 /proc/uptime) seconds\n"

exec /bin/sh

Afterwards, run chmod +x init to give the init script execution bits. After this, we’re ready to cpio the whole initramfs up. This cpio command will (g)zip up all of our files and folders up into something the kernel can work with:

find . -print0 \
    | cpio --null -ov --format=newc \
    | gzip -9 > ~/dev/initramfs/initramfs-busybox-x86.cpio.gz

Booting

Ah, for the moment of truth. Truth to be told, it’s a rather underwhelming command. But the results are exciting!

qemu-system-x86_64 \
    -kernel ~/dev/linux/arch/x86_64/boot/bzImage \
    -initrd ~/dev/initramfs/initramfs-busybox-x86.cpio.gz \
    -enable-kvm

Notice how we use those fancy kvm settings after all. The qemu command should pop open a small window with a small Linux environment running inside. All your classic userspace utilities should be working courtesy of busybox.

Addendums

I used a lot of resources on the internet to arrive at this process, but one resource stands out in particular.