How to recompile linux kernel

Kernel/Traditional compilation

This article is an introduction to building custom kernels from kernel.org sources. This method of compiling kernels is the traditional method common to all distributions. It can be, depending on your background, more complicated than using the Kernels/Arch Build System. Consider the Arch Build System tools are developed and maintained to make repeatable compilation tasks efficient and safe.

Contents

Preparation

It is not necessary (or recommended) to use the root account or root privileges (i.e. via Sudo) for kernel preparation.

Install the core packages

Install the base-devel package group, which contains necessary packages such as make and gcc . It is also recommended to install the following packages, as listed in the default Arch kernel PKGBUILD: xmlto , kmod , inetutils , bc , libelf , git , cpio , perl , tar , xz .

Create a kernel compilation directory

It is recommended to create a separate build directory for your kernel(s). In this example, the directory kernelbuild will be created in the home directory:

Download the kernel source

Download the kernel source from https://www.kernel.org. This should be the tarball ( tar.xz ) file for your chosen kernel.

It can be downloaded by simply right-clicking the tar.xz link in your browser and selecting Save Link As. , or any other number of ways via alternative graphical or command-line tools that utilise HTTP, TFTP, Rsync, or Git.

In the following command-line example, wget has been installed and is used inside the

/kernelbuild directory to obtain kernel 4.8.6:

You should also verify the correctness of the download before trusting it. First grab the signature, then use that to grab the fingerprint of the signing key, then use the fingerprint to obtain the actual signing key:

Note the signature was generated for the tar archive (i.e. extension .tar ), not the compressed .tar.xz file that you have downloaded. You need to decompress the latter without untarring it. Verify that you have xz installed, then you can proceed like so:

Do not proceed if this does not result in output that includes the string «Good signature».

If wget was not used inside the build directory, it will be necessary to move the tarball into it, e.g.

Unpack the kernel source

Within the build directory, unpack the kernel tarball:

To finalise the preparation, ensure that the kernel tree is absolutely clean; do not rely on the source tree being clean after unpacking. To do so, first change into the new kernel source directory created, and then run the make mrproper command:

Kernel configuration

This is the most crucial step in customizing the default kernel to reflect your computer’s precise specifications. Kernel configuration is set in its .config file, which includes the use of Kernel modules. By setting the options in .config properly, your kernel and computer will function most efficiently.

You can do a mixture of two things:

• Use the default Arch settings from an official kernel (recommended)
• Manually configure the kernel options (optional, advanced and not recommended)

Default Arch configuration

This method will create a .config file for the custom kernel using the default Arch kernel settings. If a stock Arch kernel is running, you can use the following command inside the custom kernel source directory:

Otherwise, the default configuration can be found online in the official Arch Linux kernel package.

Advanced configuration

There are several tools available to fine-tune the kernel configuration, which provide an alternative to otherwise spending hours manually configuring each and every one of the options available during compilation.

Those tools are:

• make menuconfig : Command-line ncurses interface superseded by nconfig
• make nconfig : Newer ncurses interface for the command-line
• make xconfig : User-friendly graphical interface that requires packagekit-qt5 to be installed as a dependency. This is the recommended method — especially for less experienced users — as it is easier to navigate, and information about each option is also displayed.
• make gconfig : Graphical configuration similar to xconfig but using gtk. This requires gtk2 , glib2 and libgladeAUR .

The chosen method should be run inside the kernel source directory, and all will either create a new .config file, or overwrite an existing one where present. All optional configurations will be automatically enabled, although any newer configuration options (i.e. with an older kernel .config ) may not be automatically selected.

Once the changes have been made save the .config file. It is a good idea to make a backup copy outside the source directory. You may need to do this multiple times before you get all the options right.

If unsure, only change a few options between compilations. If you cannot boot your newly built kernel, see the list of necessary config items here.

Running lspci -k # from liveCD lists names of kernel modules in use. Most importantly, you must maintain cgroups support. This is necessary for systemd. For more detailed information, see Gentoo:Kernel/Gentoo Kernel Configuration Guide and Gentoo:Intel#Kernel or Gentoo:Ryzen#Kernel for Intel or AMD Ryzen processors.

Compilation

Compilation time will vary from as little as fifteen minutes to over an hour, depending on your kernel configuration and processor capability. Once the .config file has been set for the custom kernel, within the source directory run the following command to compile:

Installation

Install the modules

Once the kernel has been compiled, the modules for it must follow. First build the modules:

Then install the modules. As root or with root privileges, run the following command to do so:

This will copy the compiled modules into /lib/modules/ — . For example, for kernel version 4.8 installed above, they would be copied to /lib/modules/4.8.6-ARCH . This keeps the modules for individual kernels used separated.

Copy the kernel to /boot directory

The kernel compilation process will generate a compressed bzImage (big zImage) of that kernel, which must be copied to the /boot directory and renamed in the process. Provided the name is prefixed with vmlinuz- , you may name the kernel as you wish. In the examples below, the installed and compiled 4.8 kernel has been copied over and renamed to vmlinuz-linux48 :

Make initial RAM disk

If you do not know what making an initial RAM disk is, see Initramfs on Wikipedia and mkinitcpio.

Automated preset method

An existing mkinitcpio preset can be copied and modified so that the custom kernel initramfs images can be generated in the same way as for an official kernel. This is useful where intending to recompile the kernel (e.g. where updated). In the example below, the preset file for the stock Arch kernel will be copied and modified for kernel 4.8, installed above.

First, copy the existing preset file, renaming it to match the name of the custom kernel specified as a suffix to /boot/vmlinuz- when copying the bzImage (in this case, linux48 ):

Second, edit the file and amend for the custom kernel. Note (again) that the ALL_kver= parameter also matches the name of the custom kernel specified when copying the bzImage :

Finally, generate the initramfs images for the custom kernel in the same way as for an official kernel:

Manual method

Rather than use a preset file, mkinitcpio can also be used to generate an initramfs file manually. The syntax of the command is:

• -k ( —kernel ): Specifies the modules to use when generating the initramfs image. The name will be the same as the name of the custom kernel source directory (and the modules directory for it, located in /usr/lib/modules/ ).
• -g ( —generate ): Specifies the name of the initramfs file to generate in the /boot directory. Again, using the naming convention mentioned above is recommended.

For example, the command for the 4.8 custom kernel installed above would be:

Copy System.map

The System.map file is not required for booting Linux. It is a type of «phone directory» list of functions in a particular build of a kernel. The System.map contains a list of kernel symbols (i.e function names, variable names etc) and their corresponding addresses. This «symbol-name to address mapping» is used by:

• Some processes like klogd, ksymoops, etc.
• By OOPS handler when information has to be dumped to the screen during a kernel crash (i.e info like in which function it has crashed).

If your /boot is on a filesystem which supports symlinks (i.e., not FAT32), copy System.map to /boot , appending your kernel’s name to the destination file. Then create a symlink from /boot/System.map to point to /boot/System.map- :

After completing all steps above, you should have the following 3 files and 1 soft symlink in your /boot directory along with any other previously existing files:

• Kernel: vmlinuz-
• Initramfs: Initramfs- .img
• System Map: System.map-
• System Map kernel symlink

Bootloader configuration

Add an entry for your new kernel in your bootloader’s configuration file. See Arch boot process#Feature comparison for possible boot loaders, their wiki articles and other information.

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Ubuntu Documentation

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Disclaimer

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).

Note: This page would need significant cleaning. You may want to refer to Kernel/BuildYourOwnKernel page in Ubuntu wiki instead which is a cleaner and more up-to-date guide to (simple) kernel building

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 kernel.org. This is how to rebuild the actual Ubuntu kernel starting from 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.
• You love computers and are curious and interested in hacking on your own GNU/Linux system to learn more about how it works (with the understanding that you’ll need to fix anything you break).

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 to click on System > Administration > Update Manager, then click on the Check button, and finally click on Apply all updates including the kernel.

Tools you’ll need

To start, you will need to install a few packages. Use a following command line to install precisely the packages needed for the release you are using:

Hardy (8.04):

Note: The package makedumpfile is not available in Hardy.

Lucid (10.04):

Raring (13.04):

Get the kernel source

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

Option A) Use git

Use git — This is for users who always want to stay in sync with the latest Ubuntu kernel source. For your information, detailed instructions on it can be found in the Kernel Git Guide

The git repository does not include necessary control files, so you must build them by:

Option B) Download the source archive

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 A) if you need the latest patches.

Use a follow command to install the build dependencies and extract the source (to the current directory):

Ubuntu Hardy (8.04)

• Ubuntu modules source may also be needed if you plan to enable PAE and 64 GiB support in the kernel for 32-bit Hardy (8.04). The Ubuntu supplied modules may not be compatible with a PAE enabled kernel.

The source will be downloaded to a subdirectory inside the current directory.

Ubuntu Karmic Koala (9.10) and newer releases

The source will be downloaded to the current directory as a trio of files (for Lucid, at least) (.orig.tar.gz, .diff.gz, and .dsc) and a sub-directory. For instance, if uname -r returns 2.6.32-25-generic, you’ll obtain linux_2.6.32.orig.tar.gz, linux_2.6.32-25.44.diff.gz, linux_2.6.32-25.44.dsc and the sub-directory linux-2.6.32.

Raring (13.04):

Option C) Download the source package

Download the source package (detailed instructions are further down this page under Alternate Build Method (B): 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 A/git if you need the latest source). Please be aware this is NOT the same as Option B/Download the source archive.

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 to learn how to apply it.

The stock Ubuntu configs are located in debian/config/ARCH/ where ARCH is the architecture you are building for (Starting with Jaunty this is debian.master/config/ARCH/). In this directory there 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:

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) otherwise you should check to see if an alternate location has been specified within debian/debian.env of your kernel source directory.

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:

If you just want to update one architecture, run:

Note: If you don’t have the debian/ directory after using apt-get source, use dpkg-source -x *dsc to extract the sources properly.

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

Build the Kernel(s)

There are two listed ways to build the Ubuntu kernel:

Build Method A: 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: Though these outside instructions include making a separate and unique branch of the kernel, unlike here, they include thorough explanations 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 kernel.org

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

debian/rules clean creates debian/control, debian/changelog, and so on from debian.
/* (e.g. debian.master). It is necessary in git trees following git commit 3ebd3729ce35b784056239131408b9a72b0288ef «UBUNTU: [Config] Abstract the debian directory».

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. (Note that in Intrepid and newer, you will need skipabi=true to skip ABI checks.)

To build a specific target, use this command:

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:

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).

If you get ABI errors, you can avoid the ABI check with skipabi=true. For example,

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 the parent directory of the kernel source directory.

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

Alternatively, if you need to specify a different kernel than the running one, use

If you get an error, try running this in the kerneldir: (example for the generic flavour)

Alternate Build Method (B): 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:

The last command in the sequence brings you into 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

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:

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:

Since the 2.6.32 kernel, a new feature allows you to update the configuration to only compile modules that are actually used in your system:

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

What about this. ? (which is from the Kernel/BuildYourOwnKernel Page in the section «Modifying the configuration»)

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 the config’s «Kernel hacking» and turn OFF «Compile the kernel with debug info».

Now you can compile the kernel and create the packages:

You can enable parallel make use make -j). Try 1+number of processor cores, e.g. 3 if you have a dual core processor:

On a newer kernel, if you only need binary packages and want several builds (while editing the source) to not cause everything to be rebuilt, use:

The *.deb packages will be created in the parent directory of your Linux source directory (in this example, they would be placed in

/src because our Linux source directory is

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:

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

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

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

Note: In response to the various comments in the remainder of this section: On Ubuntu Precise (12.04) it appears that postinst DOES take care of the initramfs stuff. After installing the package my new kernel booted just fine without following any of the methods below. Someone please correct me if I’m mistaken.

Since Ubuntu Lucid (10.04) the image postinst no longer runs the initramfs creation commands. Instead, there are example scripts provided that will perform the task. These scripts will work for official kernel images as well. For example:

Note: I couldn’t get the above scripts to help in generating an initrd for the kernel — and so the built kernel couldn’t boot; the only thing that worked for me was the recommendation in http://www.debian-administration.org/article/How_Do_I_Make_an_initrd_image, «use initramfs command. It is real solution.»; what I used (after the custom-built kernel’s *.deb’s were installed), was:

Note (Michael): that is because you need to include the right package scripts to build the initrd at package install time. The make-kpkg option is --overlay-dir. By default, make-kpkg uses /usr/share/kernel-package as an overlay directory, which contains the default, uncustomised scripts for a Debian distribution, and not the ones needed for building a Ubuntu kernel.

First copy the default overlay directory to your home directory:

Then install the source of the kernel you are using currently, using the exact package name, e.g.

which will unpack the sources to $HOME/linux-2.6.32. Now copy the control scripts into your new overlay:

And now you can execute make-kpkg with the additional command line option --overlay-dir=$HOME/kernel-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 restricted modules.

Further note: There are no l-r-m or linux-restricted-modules packages in Lucid.

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