Getting Started with Kernel Development

Linux Kernel Versions

#

• Linux kernel comes in two flavors: stable and development.
• New stable kernel versions are released only to fix bugs and security issues. Development versions undergo rapid changes and include new features.

Kernel Versioning Scheme

#

Historically,

• Linux kernel versions are numbered in the form X.Y.Z, where:
  • X is the major release number.
  • Y is the minor release number.
  • Z is the revision number.
• Sometimes, an additional fourth number W is added, which is the stable version number.
• Minor releases also determine whether the kernel is a stable or development version:
  • If Y is even, it is a stable release.
  • If Y is odd, it is a development release. Example: 2.6.30.1 is a stable release, while 2.5.45 is a development release.
• Development kernels stablize over time, for example, series 1.3 stablized into 2.0, and 2.5 stablized into 2.6.

After 2.6 series,

• After an invite-only Kernel Development Summit in 2004, the versioning scheme was changed, and the kernel developers decided to prolong the 2.6 series and postpone the introduction of 2.7 development series.
• Instead, the development cycle of each 2.6 revision grew longer, each release incorporating a mini-development series.
• To compensate for the reduced frequency of releases, the kernel developers introduced the stable version number W as the fourth number in the versioning scheme, which contain only bug fixes and security patches. And these are often backported from the under-development kernel. Example: Release 2.6.32.8 is the eighth stable release based on the development kernel 2.6.33.
• This allows the previous releases to receive attention focused on stabilization.

Linux Mailing List(lkml)

#

• This is the mail forum where kernel development is discussed.
• It is a high-traffic list with thousands of messages per day.
• Subscriptions are free and open to anyone: https://vger.kernel.org.
• Here is an archive of the mailing list: https://lkml.org.
• Since, its a high-traffic list, Linus and other kernel maintainers often get thousands of emails per day. So, they are not open to dealing with nonsense.
• This is where you will find testers, developers, peer reviewers while developing new features or fixing bugs.

Kernel Setup and Configuration

#

• Get the latest code from Linus’s tree:

  git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
  

  or get the tarball from the website and uncompress it:

  • bzip2 format:

    tar xvjf linux-x.y.z.tar.bz2
    

  • GNU zip(modern):

  tar xvzf linux-x.y.z.tar.gz
  

NOTE: The kernel source lives in /usr/src/linux in your system. Avoid directly making any changes to it, even when installing new kernel.

• Configure the kernel:

  • Configure kernel before building it. The configuration lives in the .config file, in the root of the kernel source.

  • Configure options are either booleans(yes or no) or tristates(yes, no, or module).

  • A module setting in tristate represents a config option that is set but is to be compiled as a module(separate dynamically loadable object).

  • Drivers are usually represented by tristates.

  • These options do not control the build process, they just specify values that kernel source can access as a preprocessor macro.

  • Kernels such as those provided by Canonical for Ubuntu or Red Hat for Fedora, are precompiled as part of the distribution. Mostly these enable a good cross section of the needed kernel features and compile nearly all drivers as modules.

  • To configure the kernel one option at a time run:

    make config
    

    NOTE: This can get very tedious.

  • Better ways to config:

    • Use a TUI menu

    make menuconfig
    

    NOTE: This is what I use.

    • Or a gtk+ based graphical utility:

    make gconfig
    

  • Sometimes, you just want things to be up and running with a default config file:

    make defconfig
    

  • Validate config before proceeding:

    make oldconfig
    

• Build the Kernel

  • Simple

  make
  

  • Using multiple jobs to speed up compilation:

  make -jN
  

  where N is the number of threads to use.

  NOTE: Using make -j$(nproc) uses all available cores.

• Install the Kernel: Depends on architecture and bootloader. Here I will cover installation for x86 systems using GRUB bootloader.

  • Install modules first: Installs modules to /lib/modules

  sudo make modules_install
  

  • Install the kernel:

  sudo make install
  

  • This will copy the kernel image to /boot and update the GRUB config file located at /boot/grub/grub.cfg or /boot/grub2/grub.cfg depending on your distribution.
  • Reboot the system to use the new kernel.

References

#

• Linux Kernel Development (3rd edition) by Robert Love.