Chapter 4. Common kernel-related tasks

To get the Debian kernel source at the current maximum patchlevel, it is sufficient to install the latest linux-source-version package and unpack the source, for example:

# apt-get install linux-source-4.3
$ tar xaf /usr/src/linux-source-4.3.tar.xz

The unpacked source tree then will be available in linux-source-4.3 directory.

You can build all or selected kernel packages by following these instructions. You may be asked to do this in order to test a potential bug fix.

# apt-get install devscripts
$ bash debian/bin/test-patches ../fix-bug123456.patch ../add-foo-driver.patch

$ bash debian/bin/test-patches

$ fakeroot make -f debian/rules.gen setup_i386_none_686-pae
$ make -C debian/build/build_i386_none_686-pae nconfig

$ fakeroot debian/rules binary

$ fakeroot debian/rules binary-arch

$ fakeroot debian/rules binary-indep

$ fakeroot debian/rules source
$ fakeroot make -f debian/rules.gen binary-arch_i386_none_686-pae

$ fakeroot debian/rules source
$ fakeroot make -f debian/rules.gen binary-arch_i386_none_real

To build a kernel image based on the kernel team's unreleased development version:

First you must add a changelog entry for the new upstream version. If the new version is a release candidate, change the string -rc to ~rc. (In Debian package versions, a suffix beginning with ~ indicates a pre-release.)

The 'orig' tarball is generated by the genorig.py script. It takes either a tarball and optional patch from kernel.org, or a git repository. First ensure you have the necessary tools installed:

# apt-get install unifdef

Then if you have a tarball, run a command such as:

$ debian/bin/genorig.py ../linux-4.3.tar.xz ../patch-4.4-rc1.xz

Or if you have a git repository, pass the name of its directory:

$ debian/bin/genorig.py ~/src/linux

Either of these will generate a file such as ../orig/linux_3.5~rc1.orig.tar.xz. You can then combine this tarball with the Debian packaging by running:

$ debian/rules orig

This section describes the simplest possible procedure to build a custom kernel the "Debian way". It is assumed that user is somewhat familiar with kernel configuration and build process. If that's not the case, it is recommended to consult the kernel documentation and many excellent online resources dedicated to it.

The easiest way to build a custom kernel (the kernel with the configuration different from the one used in the official packages) from the Debian kernel source is to use the linux-source package and the make deb-pkg target. First, prepare the kernel tree:

# apt-get install linux-source-4.3
$ tar xaf /usr/src/linux-source-4.3.tar.xz
$ cd linux-source-4.3

The kernel now needs to be configured, that is you have to set the kernel options and select the drivers which are going to be included, either as built-in, or as external modules.

It is possible to reuse an old configuration file by placing it as a .config file in the top-level directory. Alternately, you can use the default configuration for the architecture (make defconfig) or generate a configuration based on the running kernel and the currently loaded modules (make localmodconfig).

If you reuse a Debian kernel config file, you may need to disable module signing (scripts/config --disable MODULE_SIG) or enable signing with an ephemeral key (scripts/config --set-str MODULE_SIG_KEY certs/signing_key.pem). The build will use less time and disk space (see Section 4.2.1.1, “Disk space requirements”) if debug information is disabled (scripts/config --disable DEBUG_INFO). Debuginfo is only needed if you plan to use binary object tools like crash, kgdb, and SystemTap on the kernel.

The kernel build infrastructure offers a number of targets, which invoke different configuration frontends. For example, one can use console-based menu configuration by invoking the command

$ make nconfig

Instead of nconfig one can use oldconfig (text-based line-by-line configuration frontend) or xconfig (graphical configuration frontend). Note that different frontends may require different additional libraries and utilities to be installed to function properly. For example, the nconfig frontend requires the ncurses library, which at time of writing is provided by the libncurses5-dev package.

After the configuration process is finished, the new or updated kernel configuration will be stored in .config file in the top-level directory. The build is started using the commands

$ make clean
$ make deb-pkg

As a result of the build, a custom kernel package linux-image-3.2.19_3.2.19-1_i386.deb (name will reflect the version of the kernel and build number) will be created in the directory one level above the top of the tree. It may be installed using dpkg just as any other package:

# dpkg -i ../linux-image-3.2.19_3.2.19-1_i386.deb

This command will unpack the kernel, generate the initrd if necessary (see Chapter 7, Managing the initial ramfs (initramfs) archive for details), and configure the bootloader to make the newly installed kernel the default one. If this command completed without any problems, you can reboot using the

# shutdown -r now

command to boot the new kernel.

For much more information about bootloaders and their configuration please check their documentation, which can be accessed using the commands man lilo, man lilo.conf, man grub, and so on. You can also look for documentation in the /usr/share/doc/package directories, with package being the name of the package involved.

Building a kernel from the "pristine" (also sometimes called "vanilla") kernel source, distributed from www.kernel.org and its mirrors, may be occasionally useful for debugging or in the situations when a newer kernel version is desired. The procedure differs only in obtaining the kernel source: instead of unpacking the kernel source from Debian packages, the "pristine" source is downloaded using your favourite browser or using wget, as follows:

$ wget https://kernel.org/pub/linux/kernel/v4.x/linux-4.3.tar.xz

The integrity of the downloaded archive may be verified by fetching the corresponding cryptographic signature

$ wget https://kernel.org/pub/linux/kernel/v4.x/linux-4.3.tar.sign

and running this command (gnupg package must be installed):

$ unxz -c linux-4.3.tar.xz | gpg --verify linux-4.3.tar.sign -

Successful verification results in output similar to the one below:

gpg: Signature made Mon 21 May 2012 01:48:14 AM CEST using RSA key ID 00411886
gpg: Good signature from "Linus Torvalds <torvalds@linux-foundation.org>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: ABAF 11C6 5A29 70B1 30AB  E3C4 79BE 3E43 0041 1886

After that the archive may be unpacked using

$ tar xaf linux-4.3.tar.xz
$ cd linux-4.3

The unpacked kernel tree (in linux-4.3) can now be configured and built, in the same way described in the previous section.

Some kernel modules are not included in the upstream or Debian kernel source, but are provided as third-party source packages. For some of the most popular out-of-tree modules, the binary Debian packages with modules built against the stock Debian kernels are provided. For example, if you are running stock Debian kernel 3.2.0-2-686-pae (use the uname -r command to verify the version) from the linux-image-3.2.0-2-686-pae package, and would like to use the squash filesystem, all you need to do is install squashfs-modules-3.2.0-2-686-pae binary package, which provides the necessary binary kernel modules.

If you are not so lucky, and there are no binary module packages in the archive, there is a fair chance that the Debian archive contains the packaged source for the kernel modules. Names of such packages typically end in -source, for example squashfs-source, thinkpad-source, rt2x00-source and many others. These packages contain debianized source code of the kernel modules, suitable for building using the module-assistant (or m-a) script from the module-assistant package. Typical sequence to build a custom binary module package, matching a kernel 3.2.0-2-686-pae (as returned by uname -r) from the debianized source consists of the following steps:

Install a set of kernel headers, matching the kernel for which the modules are going to be built:

# apt-get install linux-headers-3.2.0-2-686-pae

Install the package containing the source:

# apt-get install squashfs-source

Invoke module-assistant (aka m-a) to do the heavy lifting:

# m-a build squashfs

As a result, a Debian package is going to be built and placed in /usr/src. It can be installed the usual way, using dpkg -i. Two last steps (building and installation) may be combined using the invocation

# m-a auto-install squashfs

Check out the module-assistant documentation (man module-assistant) for other options and much more information on how to use it.

Finally, in some rare circumstances, you might need to build the kernel modules from the upstream source packages. In that case, follow the documentation included with the package to build the modules. If the build process will require you to specify the directory with the kernel headers, matching the currently running kernel, for stock Debian kernels this directory is /usr/src/linux-headers-uname, provided by the linux-headers-uname package. Here uname is the output of the uname -r command. If you are building and running your own custom kernels, it is a good idea to keep the original build tree around, as it also can be used for out-of-tree module building.