Tools And Environment

Now you have your kernel, but might still not know where to start. So before you start your adventure into the kernel, you might want some appropriate gears.

A Cross Referencer

It's never fun to play with such a large project like Linux without knowing who's who. Knowing where each identifier is used and defined is really important. Such tool is called a cross referencer. Tons of cross referencers are available and any one of them works. But to get on board quicker without hours of code analysis, an online service might be prefered. Luckily enough, there exists such service.

My choice is https://elixir.bootlin.com. You can select the kernel version from the left and view each file and click identifiers on the page.

You may have your own favour. As long as it works, it's good.

QEMU

A pro tip from every software testers is that you should never put a software in test onto a machine that you care. Except for some hardware features you might want to test with, most kernel experiments could be done by using a virtual machine. The best VM for us that packs with features is QEMU.

Unlike commercial VM that are optimized for cloud environment and are designed to boot commercial OSes, QEMU provides support for switching kernels, loading up your own image. These are really wanted during the process of having fun with the kernel.

Debootstrap

A kernel is a never an OS until you have a minimum boot environment. In the past kernel researchers love Busybox, a really tiny set of UNIX tools in one binary. But everyone using it know that it's a piece of masterwork in term of small, but not anything else. Meet Debootstrap. Debootstrap is an environment provided by Debain to create a minimal Debain environment. You will get GNU Coreutils, you will get APT package manager, and best of all, systemd (Yes, that's not for granted. No systemd on Busybox.). A good example of using Debootstrap is provided by Google Syzkaller. See the Image section of https://github.com/google/syzkaller/blob/master/docs/linux/setup_ubuntu-host_qemu-vm_x86-64-kernel.md. They provided a script and you just follow that and it's done.

A First Boot

Now you have your kernel, your boot environment, time to boot your system for the very first time.

sudo qemu-system-x86_64 \
   -kernel your/build/dir/arch/x86/boot/bzImage \
   -append "root=/dev/sda debug earlyprintk=serial slub_debug=QUZ" \
   -hda your/debootstrap/dir/stretch.img \
   -net user,hostfwd=tcp::10021-:22 -net nic \
   -enable-kvm \
   -m 2G \
   -smp 2

Line 2 you are giving your kernel's path. Line 3 is the arguments for the kernel, so it only works with the -kernel option. If you packed your kernel into the image then you have to pass the arguments inside the image. That's why pack the kernel into the image is not suggested. Line 4 is the boot image. Line 5 gives the VM a network card. Line 6 tells QEMU to work under KVM so it would be accelerated. Line 7 means that the VM's memory is 2GB. Line 8 means that you are offering 2 cores to the VM.

Now a QEMU window will popup and you are good to go.

However, there are times when a GUI is not prefered or simply not available, like an SSH session. The following script gets you covered.

sudo qemu-system-x86_64 \
   -kernel your/build/dir/arch/x86/boot/bzImage \
   -append "console=ttyS0 root=/dev/sda debug earlyprintk=serial slub_debug=QUZ" \
   -hda your/debootstrap/dir/stretch.img \
   -net user,hostfwd=tcp::10021-:22 -net nic \
   -enable-kvm \
   -nographic \
   -m 2G \
   -smp 2

Line 2 added console=ttyS0 to forward the console output to ttyS0. Line 7's -nographic tells QEMU to forward ttyS0 to the terminal you are using. Now have fun in your terminal.