Compile your custom linux kernel

Date: 17 Sept. 2021

Preparation

Install Dependencies

sudo pacman -S base-devel xmlto kmod inetutils bc libelf git --needed

Downloading source and local setup

It is recommended to create a separate build directory for your kernel(s). In this example, the directory kernelbuild will be created in the home directory:

mkdir ~/kernelbuild
cd ~/kernelbuild

Goto kernel.org and download kernel source

wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.14.5.tar.xz

Note: you can verify signature of the downloaded tarball if you want

Extract tarball

tar -xvJf linux-5.14.5.tar.xz

Check

ls command output

Kernel configuration

To finalise the preparation, ensure that the kernel tree is absolutely clean; do not rely on the source tree being clean after unpacking. To do so, first change into the new kernel source directory created, and then run the make mrproper command:

cd linux-5.14.5

# cean up the tree
make mrproper

copy config file

zcat /proc/config.gz > .config

make sure all configuration options have values that are required

make olddefconfig

Another option(not recommended)

automatically detect the hardware and creates config based on just that downside is that its not gonna detect anything which is not currently connected

make localmodconfig

make menuconfig

This should look somewhat like:

make menuconfig

Here is the main part of research and knowing your hardware, Good Luck 👍 with that. When all the required configurations are done, save your configurations in .config file.

Build the kernel

We can use -j flag to use multiple cores of cpu while compiling

make -j4

This process will normally take some time so grab a cup of coffee(it took around 1 hour for me)

Installation

Install the modules

Once the kernel has been compiled, the modules for it must follow. First build the modules:

make modules -j4

Change to root user sudo su

Then install the modules.

make modules_install -j4

This will copy the compiled modules into /lib/modules/<kernel_version>-<config_local_version>. For example, for kernel version 5.14 installed above, they would be copied to /lib/modules/5.14.5-ARCH. This keeps the modules for individual kernels used separated.

Copy the kernel to /boot directory

The kernel compilation process will generate a compressed bzImage (big zImage) of that kernel, which must be copied to the /boot directory and renamed in the process. Provided the name is prefixed with vmlinuz-, you may name the kernel as you wish.

# cp -v arch/x86_64/boot/bzImage /boot/vmlinuz-<kernel_name>

cp -v arch/x86_64/boot/bzImage /boot/vmlinuz-linux514

Make initial RAM disk

Generate initramfs image file You are free to name the initramfs image file whatever you wish when generating it. However, it is recommended to use the linux<major_revision><minor_revision> convention. For example, the name ‘linux514’ was given as ‘5’ is the major revision and ‘14’ is the minor revision of the 5.14 kernel. This convention will make it easier to maintain multiple kernels, regularly use mkinitcpio, and build third-party modules.

# mkinitcpio -k <kernel_version> -g /boot/initramfs-<kernel_name>.img

mkinitcpio -k 5.14.5 -g /boot/initramfs-linux514.img

Copy System.map file

The System.map file is not required for booting Linux. It is a type of “phone directory” list of functions in a particular build of a kernel. The System.map contains a list of kernel symbols (i.e function names, variable names etc) and their corresponding addresses.

Copy System.map to /boot, appending your kernel’s name to the destination file. Then create a symlink from /boot/System.map to point to /boot/System.map-<kernel_name>:

cp System.map /boot/System.map-<kernel_name>
ln -sf /boot/System.map-<kernel_name> /boot/System.map

Summary

After completing all steps above, you should have the following 3 files and 1 soft symlink in your /boot directory along with any other previously existing files:

Bootloader configuration

For grub bootloader

grub-mkconfig -o /boot/grub/grub.cfg

Refrences

  1. https://wiki.archlinux.org/title/Kernel/Traditional_compilation
  2. https://www.kernel.org/