The Developer's Guide to Building Linux Kernel 6.x

This blog post provides a step-by-step tutorial on compiling and booting from Linux Kernel 6.x. We will use a VM environment for safety. This blog post is ideal for developers and curious learners.

I assume reader has basic understanding of Linux OS.

Setting up a playground

I am using Oracle VirtualBox v7.0 on the Ubuntu (v24.04) x86_64 Host. Download Ubuntu 20.04 iso. In Virtualbox, create a new VM machine by providing the iso image. Select Skip Unattended Installation option. This is optional, but I prefer to choose this to observe what’s going on.

Minimum resources to allocate are -

• 2 vCPUs
• 2 GB RAM
• 100 GB o Hard Disk space.

Once finished, Ubuntu OS will boot and provide option to install it. During installation, choose minimal installation option and do not install extra third party softwares. This is because we simply don’t need these things atm.

Post starting the VM, check the kernel version. I see following at my end.

uname -r 
5.15.0-126-generic

uname -a
Linux kernel0 5.15.0-126-generic #136~20.04.1-Ubuntu SMP Thu Nov 14 16:38:05 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

Do update and upgrade

sudo apt update
sudo apt upgrade

Reboot the VM once above is completed.

Install SSH Server and enable it

sudo apt install -y openssh-server
systemctl enable --now ssh

Now install few useful tools

wget -4 https://raw.githubusercontent.com/simplyatul/vagrant-vms/main/tools-0-install.sh
chmod +x tools-0-install.sh
sudo ./tools-0-install.sh

Then install the packages required to build the kernel

wget -4 https://raw.githubusercontent.com/simplyatul/vagrant-vms/refs/heads/main/tools-0-kernel-dev.sh
chmod +x tools-0-kernel-dev.sh
sudo ./tools-0-kernel-dev.sh

Kernel code config

Let us now checkout stable kernel version

git clone -4 --depth=1 --branch v6.8.12 git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
cd linux-stable

To build the kernel, .config file is required. This file contains kernel configurations. I have used my host’s config file for this

# Copy /boot/config-6.8.0-48-generic from my host PC to VM's /tmp directory

cp /tmp/config-6.8.0-48-generic .config

Now do menuconfig

make menuconfig

# Make following changes
# General Setup -> Kernel .config support => Make it *
# General Setup -> Kernel .config support -> Enable access to .config through /proc/config.gz => Select it
# General Setup -> Local version - append to kernel release => Enter -at-bl0 (Or put some identifier of your choice)

# Exit and Save

Disable following options

scripts/config --disable SYSTEM_TRUSTED_KEYS
scripts/config --disable SYSTEM_REVOCATION_KEYS

# Note => while "make" command is running, if any certificate related question arises, then simply hit Enter.

Kernel build

Gr8, all config is done. Time to build the kernel

time make -j6 2>&1 | tee build-0.log

If all goes well, you should see a statement like below at the end

Kernel: arch/x86/boot/bzImage is ready  (#2)

Check the vmlinux file has created

ls -lh vmlinux
-rwxrwxr-x 1 kernel0 kernel0 469M Dec 17 16:57 vmlinux

Kernel install

On Ubuntu, you can run following to install newly built kernel

sudo make -d modules_install install 2>&1 | tee make-install-0.log

Above command installs the kernel to /boot/, installs modules to /lib/modules/X.Y.Z/ (where X.Y.Z is 6.8.12-at-bld0 in our case), and updates file /boot/grub/grub.conf.

Now update following configs in /etc/default/grub file

GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0

to

#GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=15

This ensures you see boot option for 15 sec before it automatically boots.

At end, update the grub

sudo update-grub2

Post reboot, you should see newly installed kernel is in action

uname -r
6.8.12-at-bld0

uname -a
Linux kernel0 6.8.12-at-bld0 #2 SMP PREEMPT_DYNAMIC Tue Dec 17 16:56:45 IST 2024 x86_64 x86_64 x86_64 GNU/Linux
# Note the version number 6.8.12-at-bld0 has suffix "-at-bld0" which we had entered during kernel code config step above.

Note the version number 6.8.12-at-bld0 has suffix -at-bld0 which we had entered during kernel code config step above. Every time you build the new kernel, ensure to update this suffix.

Notes

• I used -4 option in few of the commands above. This instruct to use IPv4. This is because I was facing few issues with IPv6.

References

1. https://kernelnewbies.org/KernelBuild
2. Book: Linux Kernel Programming (Second Edition) By Kaiwan N. Billimoria
3. Kernel Workspace Setup