Kernel Compile

Posted on 12:48 PM by Bharathvn


Building and using a custom kernel will make it very difficult to get support for your system. While it is a learning experience to compile your own kernel, you will not be allowed to file bugs on the custom-built kernel (if you do, they will be Rejected without further explanation).

If you have a commercial support contract with Ubuntu/Canonical, this will void such support.

Also note that this page describes how to do things for the Edgy (2.6.17) kernel and newer! Until this kernel source, we did not have any mechanisms in place that would allow people to build their own kernels easily. This was intentional.

This page does NOT describe how to build upstream kernels from This is how to rebuild the actual Ubuntu kernel source.

Reasons for compiling a custom kernel

  • You are a kernel developer.
  • You need the kernel compiled in a special way, that the official kernel is not compiled in (for example, with some experimental feature enabled).
  • You are attempting to debug a problem in the stock Ubuntu kernel for which you have filed or will file a bug report.
  • You have hardware the stock Ubuntu kernel does not support.

Reasons for NOT compiling a custom kernel

  • You merely need to compile a special driver. For this, you only need to install the linux-headers packages.
  • You have no idea what you are doing, and if you break something, you'll need help fixing it. Depending on what you do wrong, you might end up having to reinstall your system from scratch.
  • You got to this page by mistake, and checked it out because it looked interesting, but you don't really want to learn a lot about kernels.

If you want to install a new kernel without compilation, you can use Synaptic, search for linux-image and select the kernel version you want to install.

An easier way is Click on System > Administration > Update Manager > Click on Check button > Apply all updates including kernel.

Tools you'll need

To start, you will need to install a few packages.

sudo apt-get install linux-kernel-devel fakeroot build-essential makedumpfile

Note: the linux-kernel-devel package does not exist in intrepid. To compile the kernel on intrepid, you'll also need to do:

sudo apt-get build-dep linux

This will install the compiler related packages and kernel packaging tools. It will also install the git-core package, which is the best way to interact with the Ubuntu kernel source.

Get the kernel source

There are a few ways to obtain the Ubuntu kernel source:

  1. Use git (detailed instructions on it can be found in the Kernel Git Guide) - This is for users who always want to stay in sync with the latest Ubuntu kernel source.

  2. Download the source archive - This is for users who want to rebuild the standard Ubuntu packages with additional patches. Note that this will almost always be out of date compared to the latest development source, so you should use git (option #1) if you need the latest patches. To install the build dependencies and extract the source (to the current directory):
    sudo apt-get build-dep linux-source
    apt-get source linux-source
    Starting from Hardy (8.04) this has changed to:
    sudo apt-get build-dep linux-image-$(uname -r)
    apt-get source linux-image-$(uname -r)
    Ubuntu modules source may also be needed if you plan to enable PAE and 64GB support in the kernel for 32bit Hardy (8.04). The Ubuntu supplied modules may not be compatible with PAE enabled kernel.
    sudo apt-get build-dep linux-ubuntu-modules-$(uname -r)
    apt-get source linux-ubuntu-modules-$(uname -r)
  3. Download the source package (detailed instructions are further down this page under Alternate Build Method: The Old-Fashioned Debian Way) - This is for users who simply want to modify, or play around with, the Ubuntu-patched kernel source. Again, this will not be the most up-to-date (use option #1/git if you need the latest source). Please be aware this is NOT the same as option #2

Modify the source for your needs

For most people, simply modifying the configs is enough. If you need to install a patch, read the instructions from the patch provider for how to apply.

The stock Ubuntu configs are located in debian/config/ARCH/ where ARCH is the architecture you are building for. In this directory are several files. The config file is the base for all targets in that architecture. Then there are several config.FLAVOUR files that contain options specific to that target. For example, here are the files for 2.6.20, i386:

ls -l debian/config/i386/
total 108
-rw-r--r-- 1 root src 73962 2007-08-13 01:29 config
-rw-r--r-- 1 root root 1369 2007-08-13 01:29 config.386
-rw-r--r-- 1 root root 1330 2007-08-13 01:29 config.generic
-rw-r--r-- 1 root root 1395 2007-08-13 01:29 config.server
-rw-r--r-- 1 root root 1756 2007-08-13 01:29 config.server-bigiron
-rw-r--r-- 1 root root 8 2007-08-13 01:25 lowlatency
-rw-r--r-- 1 root root 194 2007-08-13 01:25 vars.386
-rw-r--r-- 1 root root 218 2007-08-13 01:25 vars.server-bigiron

If you do not find the config files under debian/config, you may find them in your /boot directory, for instance, /boot/config-2.6.22-14-generic.

If you need to change a config option, simply modify the file that contains the option. If you modify just the config file, it will affect all targets for this architecture. If you modify one of the target files, it only affects that target.

After applying a patch, or adjusting the configs, it is always best to regenerate the config files to ensure they are consistent. There is a helper command for this. To regenerate all architectures run:

debian/rules updateconfigs

If you just want to update one architecture, run:

debian/scripts/misc/oldconfig ARCH

For these two commands to work, you may need to give the scripts in the debian/scripts/misc directory execute permission with the following command:

chmod a+x *

Build the kernel (when source is from git repository, or from apt-get source)

To build the kernel(s) is very simple. Depending on your needs, you may want to build all the kernel targets, or just one specific to your system. However, you also want to make sure that you do not clash with the stock kernels.

Note: for an outside reference, see: Though these outside instructions include making a separate and unique branch of the kernel, unlike here, it is a thorough explanation of all necessary steps from start to finish.

These instructions are specific to the git-tree and for the source downloaded via apt-get source, not when downloading the linux-source package from

Use this command to build all targets for the architecture you are building on:

AUTOBUILD=1 fakeroot debian/rules binary-debs

The AUTOBUILD environment variable triggers special features in the kernel build. First, it skips normal ABI checks (ABI is the binary compatibility). It can do this because it also creates a unique ABI ID. If you used a git repo, this unique ID is generated from the git HEAD SHA. If not, it is generated from the uuidgen program (which means every time you execute the debian/rules build, the UUID will be different!). Your packages will be named using this ID.

To build a specific target, use this command:

AUTOBUILD=1 NOEXTRAS=1 fakeroot debian/rules binary-FLAVOUR

Where FLAVOUR is one of the main flavours of the kernel (e.g. generic)

To build one of the custom flavours (found in debian/binary-custom.d/), use:

AUTOBUILD=1 NOEXTRAS=1 fakeroot debian/rules custom-binary-FLAVOUR

As of this documentation, custom flavours include xen and rt.

If you have a more than one processor or more than one core, you can speed things up by running concurrent compile commands. Prepend CONCURRENCY_LEVEL=2 for two processors or two cores; replace '2' with whatever number suits your hardware setup (for Gutsy and later, you can alternatively use DEB_BUILD_OPTIONS=parallel=2).

CONCURRENCY_LEVEL=2 AUTOBUILD=1 NOEXTRAS=1 fakeroot debian/rules binary-generic

To trigger a rebuild, remove the appropriate stamp file from debian/stamps (e.g. stamp-build-server for the server flavour, etc.).

The debs are placed in your kernel directory's parent directory.

If needed, the Ubuntu modules source for Hardy (8.04) can be built in similar way.

cd linux-ubuntu-modules-2.6.24-2.6.24
AUTOBUILD=1 fakeroot debian/rules binary-debs

Alternate Build Method: The Old-Fashioned Debian Way

The new Ubuntu build system is great for developers, for people who need the absolute latest bleeding-edge kernel, and people who need to build a diverse set of kernels (several "flavours"). However it can be a little complex for ordinary users. If you don't need the latest development sources, there is a simpler way to compile your kernel from the linux-source package. As suggested above, all you need for this is:

sudo apt-get install linux-source device-tree-compiler # device-tree-compiler is only needed if you are targeting the PowerPC architecture
mkdir ~/src
cd ~/src
tar xjvf /usr/src/linux-source-.tar.bz2
cd linux-source-

Now you are in the top directory of a kernel source tree. Before building the kernel, you must configure it. If you wish to re-use the configuration of your currently-running kernel, start with

cp -vi /boot/config-`uname -r` .config

Before you run 'make menuconfig' or 'make xconfig' (which is what the next step tells you to do), make sure you have the necessary packages

sudo apt-get install qt3-dev-tools libqt3-mt-dev # if you plan to use 'make xconfig'
sudo apt-get install libncurses5 libncurses5-dev # if you plan to use 'make menuconfig'

If you would like to see what is different between your original kernel config and the new one (and decide whether you want any of the new features), you can run:

make oldconfig

Then, regardless of whether you're re-using existing configuration or starting from scratch:

make menuconfig # or "make xconfig" if you prefer

If you re-used the existing configuration, note that Ubuntu kernels build with debugging information on, which makes the resulting kernel modules (*.ko files) much larger than they would otherwise be. To turn this off, go into "Kernel hacking"; then, under "Kernel debugging", turn OFF "Compile the kernel with debug info".

Now you can compile the kernel and create the packages:

make-kpkg clean # only needed if you want to do a "clean" build
fakeroot make-kpkg --initrd --append-to-version=-some-string-here kernel-image kernel-headers

You can enable parallel make (just like make -j) by defining the CONCURRENCY_LEVEL environment variable. Don't try to add the -j option to make-kpkg. Try 1+number of processor cores, e.g. 3 if you have a dual core processor.


The *.deb packages will be created in the parent directory of your Linux source directory (in this example, it would be placed in ~/src because our Linux source directory is ~/src/linux-source-).

Install the new kernel

If you want to see the ubuntu splash screen (or use text mode) before you get to X instead of just a black screen, you'll want to make sure the framebuffer driver loads:

echo vesafb | sudo tee -a /etc/initramfs-tools/modules
echo fbcon | sudo tee -a /etc/initramfs-tools/modules

Now that you've told initramfs-tools what modules it should include and the build is complete, you can install the generated debs using dpkg:

sudo dpkg -i linux-image-2.6.20-16-2be-k7_2.6.20-16_i386.deb
sudo dpkg -i linux-headers-2.6.20-16-2be-k7_2.6.20-16_i386.deb

Similarly, if you have build the Ubuntu module for Hardy (8.04) earlier, install them as follows:

sudo dpkg -i linux-ubuntu-modules-2.6.24-16-generic_2.6.24-16.23_i386.deb
sudo dpkg -i linux-headers-lum-2.6.24-16-generic_2.6.24-16.23_i386.deb

If you use modules from linux-restricted-modules, you will need to recompile this against your new linux-headers package.

Rebuilding ''linux-restricted-modules''

The Linux-Restricted-Modules (l-r-m) package contains a number of non-DFSG-free drivers (as well as some firmware and the ipw3945 wireless networking daemon) which, in a perfect world, wouldn't have to be packaged separately, but which unfortunately are not available under a GPL-compatible license. If you use any of the hardware supported by the l-r-m package, you will likely find that your system does not work as well after switching to a custom kernel. In this case you should try to compile the l-r-m package.

See CustomRestrictedModules on how to rebuild l-r-m (if you use nvidia or ATi binary drivers, you do).

Note: you will need around 8 hours of compilation time and around 10 Gb of hard drive space to compile all kernel flavours and a restricted modules.

Speeding Up The Build

Use distcc and, if you're rebuilding often, ccache. A good overview of using distcc on a debian-based system is available at If you have AMD64 machines available on your local net, they can still participate in building 32-bit code; distcc seems to handle that automatically. However, with distcc taking over all compiles by default, you will need to set HOSTCC so that when kernel builds want to use the compiler on the host itself, they don't end up distributing jobs to the 64-bit server. If you fail to do that, you'll get link-compatibility failures between 64-bit and 32-bit code. My make-kpkg command, with /usr/lib/ccache at the head of my $PATH, looks like:

MAKEFLAGS="HOSTCC=/usr/bin/gcc CCACHE_PREFIX=distcc" make-kpkg --rootcmd fakeroot --initrd --append-to-version=-suspend2 kernel-image kernel-headers kernel-source

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