Build the Linux kernel

Recently, I started reading the book Linux Device Drivers with the goal of writing my own driver someday. Although the book is based on a rather old kernel version (v2.6.10), I decided to follow its instructions to build and run an example driver module on the latest kernel release, v6.11.5. To my surprise, the instructions still worked perfectly!

Before I could perform the experiments in the book, I needed to build the kernel myself, as the book mentions that building modules for v2.6.x requires a complete built kernel tree on my system. However, the book does not provide any instructions for building a kernel, so I documented the entire process here.

Build and install the kernel

There are 5 steps:

• install the source
• install dependcies
• configure the kernel
• build the kernel
• install the kernel

Install the source

• Download the latest release from The Linux Kernel Archives as a compressed tar ball linux-6.11.5.tar.xz
• Uncompress it using tar tar xf <path-to-downloaded-source>
• cd into the kernel source directory and do make mrproper to make sure the directory is clean

Now, the source is properly installed on my system.

Install the dependcies

Here is the list of dependcies I installed on a freshly installed Debian 12.7.0:

• make
• gcc
• flex
• bison
• libncurses-dev (needed to run make menuconfig)
• pkg-config
• libssl-dev (build error if not installed)
• libelf-dev (build error if not installed)

Note that there are other dependcies not listed, as they come bundled with the Debian distribution. For a complete list, refer to the kernel documentation at Documentation/process/changes.rst.

Configure the kernel

Before building the kernel, you need to configure it first. Since there are thousands of configuration parameters, configuring them all from scratch is impratical. A more efficient approach is to copy the configuration from the currently running kernel and use that as a starting point.

mkdir -p ${HOME}/kernel-build/v6.11.5
cp -v /boot/config-$(uname -r) ${HOME}/kernel-build/v6.11.5/.config

Build the kernel

This step is straightforward but time consuming (it took me nearly 2 hours!). Run the following command:

make -j2 O=${HOME}/kernel-build/v6.11.5

To reduce the build time, I may need to further configure the kernel to exclude unnecessary components.

Install the kernel

To install the new kernel, run another make command with sudo:

sudo make O=${HOME}/kernel-build/v6.11.5 modules_install install

The modules_install target is necessary if any kernel components are configured as modules.

If everything goes well, reboot the system, and the bootloader (GRUB) should select the new kernel by default. Unfortunately, I encountered an issue: the initramfs image was too large and the reboot failed. While the Debian kernel’s initramfs is about 30 MB, the initramfs for my new kernel exceeded 400 MB! The initramfs of the Debian kernel is 30M, but the one of my new kernel is above 400M! To resolve this, I edited the configuration file for update-initramfs, changing the MODULES option from most to dep, and then regenerated the initramfs with sudo update-initramfs -u.

Summary of the Kernel Installation Process:

• make install triggers the /scripts/install.sh script
• On my system, install.sh calls installkernel, a Debian-provided kernel installation script
• installkenel executes all scripts in /etc/kernel/postinst.d/ using run-parts, which runs:
  • run-parts --verbose --exit-on-error --arg="$ver" --arg="$dir/$img_dest-$ver" /etc/kernel/postinst.d
  • the first script, initramfs-tools, has a command line at its last line: update-initramfs -c -k ${version}" "${bootopt}" >&2
  • the second script, zz-update-grub, runs exec update-grub, which updates the GRUB boot menu to set the new kernel as the default

The above sequence produces the following installation output:

  INSTALL /boot
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 6.11.5 /boot/vmlinuz-6.11.5
update-initramfs: Generating /boot/initrd.img-6.11.5
W: Possible missing firmware /lib/firmware/i915/mtl_gsc_1.bin for module i915
W: Possible missing firmware /lib/firmware/i915/dg2_huc_gsc.bin for module i915
W: Possible missing firmware /lib/firmware/i915/mtl_huc_gsc.bin for module i915
W: Possible missing firmware /lib/firmware/i915/mtl_guc_70.bin for module i915
W: Possible missing firmware /lib/firmware/i915/bmg_dmc.bin for module i915
W: Possible missing firmware /lib/firmware/i915/xe2lpd_dmc.bin for module i915
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 6.11.5 /boot/vmlinuz-6.11.5
Generating grub configuration file ...
Found background image: /usr/share/images/desktop-base/desktop-grub.png
Found linux image: /boot/vmlinuz-6.11.5
Found initrd image: /boot/initrd.img-6.11.5
Found linux image: /boot/vmlinuz-6.1.0-26-amd64
Found initrd image: /boot/initrd.img-6.1.0-26-amd64
Found linux image: /boot/vmlinuz-6.1.0-25-amd64
Found initrd image: /boot/initrd.img-6.1.0-25-amd64
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
done