This blog post provides a step-by-step tutorial on compiling and booting from Linux Kernel 6.x. We will use a virtual 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 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. Every time you build the new kernel, ensure to update this suffix.
Notes
- I used
-4option in few of the commands above. This instruct to use IPv4. This is because I was facing few issues with IPv6.
References
Book: Linux Kernel Programming (Second Edition) By Kaiwan N. Billimoria