March 31, 2015

Building custom Linux kernel the Debian way

Image by: shokunin
Building a Linux kernel is a nice way to learn some new skills, install a fresh and new version of your Operating System core software, and also make any optimizations you see fit for your machine. However, building a kernel is not a trivial task. From downloading, configuring, building and installing, you may get lost if you don't follow some tips.

In this blog post, we are going to understand some basic steps on how to build a custom kernel from sources, using some tools available in the Debian GNU/Linux operating system. If you use any other Debian derivative, like Ubuntu or Mint, this tutorial should work as well.

Why build a custom Kernel?

First and most important: because you can! Secondly: because it is fun! And, off course, because you may need a newer kernel to support hardware, or may need to rebuild the kernel to enable specific features.

It is also a good experience that allows you to hack into the biggest open source project, see all available configuration options and disable those that you may never use, making a slim and optimized kernel for your machine.

Understanding the process

In Debian and derivatives, all packages are installed by the dpkg tool, or via the apt command-line and it's friends. These ensures that every file installed by your package is tracked by the system, allowing you to properly upgrade or remove it later. If you install something in Debian without using dpkg/apt, then you risk compromising your system and any future upgrades from Debian.

Building a Debian package is a complex task by itself: you need to follow a set of rules in order to do it right. But for some commonly used software, Debian also provides a few tools that help you automate the process of generating a .deb installable file out of the source tarball. This is true for the JDK, as well as for the Kernel.

To build a kernel from sources and generate a Debian packge using the tools available in the repository, you need to install the make-kpkg package:

# apt-get install make-kpkg build-essential

The process to build the kernel is:
  1. Download the sources from http://www.kernel.org/
  2. Extract the source tarball: 
    • tar xf linux-3.19.3.tar.xz
  3. Configure the Kernel options enabling/disabling features:
    • cd linux-3.19.3 && make menuconfig
  4. Compile all sources to generate the kernel binary and modules:
    • make-kpkg --revision=~custom1 -j 4 binary_image
This process is simple, except from the configuration options: if you don't know how to configure the kernel, you will get lost with the make menuconfig script. Since configuring the kernel itself is a hard thing to properly understand and master, let's use another script to to that in a more automated way. Instead of make menuconfig, use: make olddefconfig. This will take the current running kernel options as a base, and update it to match the new kernel options, using the default values for new options.

Helper script

This process can be even further automated using a script I wrote called kernelbuild. The script is available under the set of scripts I host at the tools repository on my Bitbucket account. To build the newest stable kernel version, all you need to do is:

$ kernelbuild --fast --safe-config

That's it. The script downloads the latest stable kernel from www.kernel.org, unpacks the source in the current directory, runs a simple option to automatically configure the kernel based on your current running one and uses auomated defaults for newer build options, and then compiles the kernel using the make-kpkg tool. This process will generate all .deb files for you: the kernel image, a linux-headers package and a package with debug symbols. Super easy!

How the script works

The script uses wget to download the kernel package for you. It parses the kernel.org HTML output to find the latest stable version, and downloads that version tarball for you. You can avoid autodetecting the kernel version to download a specific one with the -k switch.

Once downloaded, the script unpacks the source tarball. After that step, the script runs the make olddefconfig kernel configuration script. Altought simple, this can be odd if you are using a very different kernel release than the one you are building. You may be prompted to set some options by hand, and if unsure, just press ENTER to use the default value.

The next step is to build the kernel, and the script passes a few parameters to make-kpkg, such as a custom version tag (~custom1). This version number uses a tilde, so your kernel package may be upgraded by the oficial Debian kernel release once it is available in the repositories. The script also uses some other build options, like the -j flag that enables parallell build and use all CPU cores available.

Installing your new Kernel

To install your debian kernel, all you need to do is to use the dpkg tool. Use ls to see the deb files built, and install the image and header packages. If you built the 3.19.3 kernel version, to install use:

# dpkg -i linux-image-3.19.3_3.19.3~custom1_amd64.deb
# dpkg -i linux-headers-3.19.3_3.19.3~custom1_amd64.deb

This installs the packages built by the script, and you can then reboot your machine to use the new kernel. The make-kpkg tool builds a package that contains all triggers that will update/generate the initrd file (necessary to boot), and appropriate Grub menu entries.

Conclusion

Building the Linux Kernel is a very interesting task, that can help you better understand how an operating system works and learn more about the options available to optimize and customize your machine software.

In this post, we saw some tips to build a Kernel suitable to use on a Debian-based distribution, and how you can make use of a helper script to generate a Debian package that integrates nicelly with the operating system software.

Disclaimer: I have tested the scripts and commands provided in this post on multiple machines without issues. The source code is provided as is, without any warranties, and I am not resposible for any damange that you cause to your software or hardware when using it. Use at your own risk.

Happy hacking!