8.10.2. Getting the Sources
Like anything that can be useful on a Debian system, the Linux kernel sources are available in a package. To retrieve them, just install the linux-source-version package. The apt-cache search ^linux-source command lists the various kernel versions packaged by Debian. The latest version is available in the Unstable distribution: you can retrieve them without much risk (especially if your APT is configured according to the instructions of Section 6.2.6, “Working with Several Distributions”). Note that the source code contained in these packages does not correspond precisely with that published by Linus Torvalds and the kernel developers; like all distributions, Debian applies a number of patches. These modifications include patches (some relevant to security problems) that are waiting to be included in the next version of the kernel, as well as some features that are specific to Debian (like cramfs, a filesystem specifically for the initrd image).
CULTURE Names of kernel packages
Historically, packages containing the Debian kernel were called kernel-image-*, but they all actually contained a Linux kernel. Since Debian works with other kernels (Hurd or FreeBSD, for example), this was confusing. Nowadays, the packages are called “linux-image-*"; the kernel-image-* packages are now empty shells and their only purpose is to facilitate the transition. Source packages for the Linux kernel are also called “linux-source-*”. As for packages containing patches, the transition is still in progress so both linux-patch-* packages and kernel-patch-* can still be found. kernel-package remains kernel-package, since it is not specific to Linux (it could, for example, prepare FreeBSD kernel packages).
The remainder of this section focuses on the 2.6.32 version of the Linux kernel, but the examples can, of course, be adapted to the particular version of the kernel that you want.
We assume the linux-source-2.6.32 package has been installed. It contains /usr/src/linux-source-2.6.32.tar.bz2, a compressed archive of the kernel sources. You must extract these files in a new directory (not directly under /usr/src/, since there is no need for special permissions to compile a Linux kernel): ~/kernel/ is appropriate.
$ mkdir ~/kernel; cd ~/kernel
$ tar -xjf /usr/src/linux-source-2.6.32.tar.bz2
CULTURE Location of kernel sources
Traditionally, Linux kernel sources would be placed in /usr/src/linux/ thus requiring root permissions for compilation. However, working with administrator rights should be avoided when not needed. There is a src group that allows members to work in this directory, but working in /usr/src/ should be avoided nevertheless. By keeping the kernel sources in a personal directory, you get security on all counts: no files in /usr/ not known to the packaging system, and no risk of misleading programs that read /usr/src/linux when trying to gather information on the used kernel.
8.10.3. Configuring the Kernel
The next step consists of configuring the kernel according to your needs. The exact procedure depends on the goals.
When recompiling a more recent version of the kernel (possibly with an additional patch), the configuration will most likely be kept as close as possible to that proposed by Debian. In this case, and rather than reconfiguring everything from scratch, it is sufficient to copy the /boot/config-version file (the version is that of the kernel currently used, which can be found with the uname -r command) into a .config file in the directory containing the kernel sources.
$ cp /boot/config-2.6.32-5-686 ~/kernel/linux-source-2.6.32/.config
Unless you need to change the configuration, you can stop here and skip to the next section. If you need to change it, on the other hand, or if you decide to reconfigure everything from scratch, you must take the time to configure your kernel. There are various dedicated interfaces in the kernel source directory that can be used by calling the make target command, where target is one of the values described below.
make menuconfig compiles and executes an text-mode interface (this is where the libncurses5-dev package is required) which allows navigating the options available in a hierarchical structure. Pressing the Space key changes the value of the selected option, and Enter validates the button selected at the bottom of the screen; Select returns to the selected sub-menu; Exit closes the current screen and move back up in the hierarchy; Help will display more detailed information on the role of the selected option. The arrows allow moving within the list of options and buttons. To exit the configuration program, choose Exit from the main menu. The program then offers to save the changes you've made; accept if you are satisfied with your choices.
Other interfaces have similar features, but they work within more modern graphical interfaces; such as make xconfig which uses a Qt graphical interface, and make gconfig which uses GTK+. The former requires libqt3-mt-dev, while the latter depends on libglade2-dev and libgtk2.0-dev.
The make-kpkg command, presented in the next paragraph, runs make oldconfig automatically to ensure the presence of a kernel configuration. This configuration method simply reuses the choices saved in the .config file. If there is no such file, it behaves like make config, a text interface that asks all questions (hundreds of them) one a time. If the .config file already exists but doesn't mention all the existing options, then this method will only ask questions for which the file has no saved answer.
TIP make-kpkg --config
make-kpkg can be told to use configuration methods other than make oldconfig, by indicating the target (menuconfig, xconfig or gconfig) in the make-kpkg invocation with the --config option.
8.10.4. Compiling and Building the Package
NOTE Clean up before rebuilding
If you have already compiled once in the directory and wish to recompile with new sources, you must run fakeroot make-kpkg clean. Additionally, this allows generating a package with a new name (different --append-to-version setting).
TIP Kernel package headers
make-kpkg uses information contained in the /etc/kernel-pkg.conf file to generate headers for the Debian kernel package. It is recommended to edit this file with correct information if you wish to publish your kernel package.
Once the kernel configuration is ready, the make-pkg command provided by Debian compiles the kernel, then generates the corresponding Debian package (or packages). Just like make, make-pkg takes the name of a target to execute as an argument: kernel-image generates a compiled kernel package, kernel-doc a package containing the documentation included with the kernel, kernel-headers a package of kernel header files (.h files for the kernel in the include/ directory, which is useful for compilation of some external modules), and kernel-source creates a package containing the kernel sources.