Building a custom kernel

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This document provides instructions for advanced users who want to rebuild the kernel from some source. Note, however, that when building or running any such kernel, one should NOT expect support from the Fedora kernel team; you're pretty much on your own here if something doesn't work as you'd hoped or expected. But hey, you're an advanced user, so you can handle it, right? Anyway, advanced users build custom kernels for a variety of reasons:

  • To apply patches for testing that they either generated or obtained from another source
  • To reconfigure the existing kernel
  • To learn more about the kernel and kernel development


Contents

Dependencies for building kernels

Not all of these will apply to all methods but this provides a good dependency list of items to install

# yum install fedpkg fedora-packager rpmdevtools yum-utils ncurses-devel

if you plan to run 'make xconfig'

# yum install qt3-devel libXi-devel gcc-c++

Building a Kernel from the Fedora source tree

Make sure you have installed all dependencies

$ fedpkg clone kernel

you may need to checkout anonymously

$ fedpkg clone -a kernel

As of the time of this wiki writing (Apr2015), the kernel was managed using git. Each fedora release is a separate branch. rawhide tracks master. To get the tree for a particular release, you can use git checkout.

e.g. for fedora 22,

$ git checkout origin/f22

You can now make whatever changes you need. When finished, you can use

$ fedpkg local

to generate appropriate rpms. The rpms will be generated in a subdirectory $ARCH which can then be installed:

$ yum localinstall --nogpgcheck ./x86_64/kernel-$version.rpm

Building a non-debugging kernel

Branched kernels are built with debugging enabled by default in the early stages of the release to assist developers. To make a kernel with debugging information disabled, you can follow the above instructions to check out and do:

$ make release

$ fedpkg local

Updating

  • $ cd kernel
  • kernel $ git status
    • your tree will be dirty in the configs and kernel.spec
  • kernel $ git stash
    • puts aside your changes so your tree will be clean
  • kernel $ git pull origin
    • update to the latest tree from fedpkg git

Now you can run whatever other commands you want (e.g. make release)

Building a Kernel from the source RPM

Make sure you have installed all dependencies

Get the Source

Warning (medium size).png
Do Not Build Packages as root.
Configuring and building packages as root is inherently dangerous and not required, even for the kernel. The following instructions allow any normal user to configure and build kernels from the source packages.
  1. Prepare an RPM package-building environment in your (non-root) home directory by running the following command:
    rpmdev-setuptree
    This command creates a new directory ~/rpmbuild, with several empty subdirectories including SPECS, SOURCES, BUILD and others. It also creates an initial ~/.rpmmacros customization file for you.
  2. Download the kernel-<version>.src.rpm file. It is not necessary to enable the Fedora source repositories -- the yumdownloader command will do that for you automatically:
    yumdownloader --source kernel
  3. Install build dependencies for the kernel source with the yum-builddep command (root is required to install these packages):
    su -c 'yum-builddep kernel-<version>.src.rpm'
  4. Install kernel-<version>.src.rpm with the following command:
    rpm -Uvh kernel-<version>.src.rpm
    This command writes the RPM contents into ${HOME}/rpmbuild/SOURCES and ${HOME}/rpmbuild/SPECS, where ${HOME} is your home directory. It is safe to ignore any messages similar to the following:
warning: user kojibuilder does not exist - using root
warning: group kojibuilder does not exist - using root
Warning (medium size).png
Space Required. The full kernel building process requires several gigabytes of extra space on the file system containing your home directory.

Prepare the Kernel Source Tree

This step expands all of the source code files for the kernel. This is required to view the code, edit the code, or to generate a patch. (Again, all of this should be done as a regular user, not as root.)

1. Prepare the kernel source tree using the following commands:

cd ~/rpmbuild/SPECS
rpmbuild -bp --target=$(uname -m) kernel.spec

The unloaded and patched kernel source tree is now located in the ~/rpmbuild/BUILD/kernel-<version>/linux-<version>.<arch> directory.

Copy the Source Tree and Generate a Patch

This step is for applying a patch to the kernel source. If a patch is not needed, proceed to "Configure Kernel Options".

Note.png
Advanced Users
There are tools such as 'quilt' that allow you to avoid copying the source tree. For advanced users, such tools are often a time saver over the steps below

Copy the source tree to preserve the original tree while making changes to the


export arch=x86_64 # replace x86_64 with your arch
export ver=3.7 # replace 3.1 with your kernel version
export subver=4-204
export fedver=fc16 # replace fc16 with your fedora version 
cp -r ~/rpmbuild/BUILD/kernel-$ver.$subver.$fedver ~/rpmbuild/BUILD/kernel-$ver.$fedver.orig
cp -al ~/rpmbuild/BUILD/kernel-$ver.$subver.$fedver.orig ~/rpmbuild/BUILD/kernel-$ver.$fedver.new
Warning (medium size).png
The second cp command hardlinks the .orig and .new trees to make diff run faster. Most text editors know how to break the hardlink correctly to avoid problems. Vim does not break hard links by default, so don't use 'cp -al' use 'cp -r', or look into the 'breakhardlink' setting.

Using vim on FC14, it treated the hard link as a hard link and thus the above technique failed. It was necessary to repeat the original copy used for the .orig directory for the .new directory. Note that this uses twice the space.

Make changes directly to the code in the .new source tree, or copy in a modified file. This file might come from a developer who has requested a test, from the upstream kernel sources, or from a different distribution.

After the .new source tree is modified, generate a patch. To generate the patch, run diff against the entire .new and .orig source trees with the following command:

cd ~/rpmbuild/BUILD
diff -uNrp kernel-$ver.$subver.$fedver.orig kernel-$ver.$subver.$fedver.new > ../SOURCES/linux-$ver.$subver.$fedver-mynewpatch.patch

Replace 'linux-$ver.$subver.$fedver-mynewpatch.patch' with the desired name for the new patch.

Lastly edit the patch file to remove the leading directory, this is required because the kernel spec file applies patches with '-p1' only.

For example the following:

--- kernel-3.8.fc18.orig/linux-3.8.11-200.bz708406.fc18.x86_64/kernel/kexec.c        2013-02-18 18:58:34.000000000 -0500
+++ kernel-3.8.fc18.new/linux-3.8.11-200.bz708406.fc18.x86_64/kernel/kexec.c    1969-12-31 19:00:00.000000000 -0500

Should be changed to:

--- linux-3.8.11-200.bz708406.fc18.x86_64/kernel/kexec.c        2013-02-18 18:58:34.000000000 -0500
+++ linux-3.8.11-200.bz708406.fc18.x86_64/kernel/kexec.c    1969-12-31 19:00:00.000000000 -0500


Warning (medium size).png
For more information on patching refer to the man pages for diff(1) and patch(1).

Configure Kernel Options

This step is for modifying the options the kernel is configured with. This step is optional. If no configuration changes are needed, proceed to "Prepare Build Files".

Important.png
Small changes
If you only want to make a small number of configuration changes, you should simply set the options as desired in the config-local file. This will be sourced and override the remaining config-* files and avoids a lot of unnecessary work. You can skip the steps below if you use config-local
  1. Change to the kernel source tree directory:
    cd ~/rpmbuild/BUILD/kernel-$ver.$fedver/linux-$ver.$subver.$fedver.$arch/
    If you only want to make minor changes to the default fedora kernel, skip to step 4., and use one of the two configuration tools to edit those minor changes into the default config file.
  2. Select the desired configuration file from ~/rpmbuild/BUILD/kernel-$ver.$fedver/linux-$ver.$subver.$fedver.$arch/configs. Copy the desired config file to ~/rpmbuild/BUILD/kernel-$ver.$fedver/linux-$ver.$subver.$fedver.$arch/.config:
    cp configs/<desired-config-file> .config
  3. Run the following command:
    make oldconfig
  4. Then run the following command, selecting and saving the desired kernel options from the text-based UI:
    make menuconfig
    • For a graphical UI, instead run:
      make xconfig
  5. Add a new line to the top of the config file that contains the hardware platform the kernel is built for (the output of uname -i). The line is preceded by a # sign. For example, an x86_64 machine would have the following line added to the top of the config file:
    # x86_64
  6. Copy the config file to ~/rpmbuild/SOURCES/:
    cp .config ~/rpmbuild/SOURCES/config-`uname -m`-generic
Important.png
32-bit x86 kernels
The 32-bit PAE kernel uses the config-i686-PAE configuration file. If you are building a PAE kernel, you will need to copy your config file to ~/rpmbuild/SOURCES/:
 cp .config ~/rpmbuild/SOURCES/config-i686-PAE
If you are building a non-PAE kernel, you will need to copy your config file to
 cp .config ~/rpmbuild/SOURCES/config-x86-32-generic
. Again, the use of config-local is encouraged unless you are making a large number of configuration changes.

Prepare Build Files

This step makes the necessary changes to the kernel.spec file. This step is required for building a custom kernel.

1. Change to the ~/rpmbuild/SPECS directory:

cd ~/rpmbuild/SPECS

2. Open the kernel.spec file for editing.
3. Give the kernel a unique name. This is important to ensure the custom kernel is not confused with any released kernel. Add a unique string to the kernel name by changing the 'buildid' line. Optionally, change ".local" to your initials, a bug number, the date or any other unique string.

Change this line:

#% define buildid .local

To this (note the extra space is removed in addition to the pound sign):

%define buildid .<custom_text>

4. If you generated a patch, add the patch to the kernel.spec file, preferably at the end of all the existing patches and clearly commented.

# cputime accounting is broken, revert to 2.6.22 version
Patch2220: linux-2.6-cputime-fix-accounting.patch

Patch9999: linux-2.6-samfw-test.patch

The patch then needs to be applied in the patch application section of the spec file. Again, at the end of the existing patch applications and clearly commented.

ApplyPatch linux-2.6-cputime-fix-accounting.patch

ApplyPatch linux-2.6-samfw-test.patch


Build the New Kernel

This step actually generates the kernel RPM files. This step is required for building a custom kernel.

Use the rpmbuild utility to build the new kernel:

  • To build all kernel flavors:
rpmbuild -bb --target=`uname -m` kernel.spec
  • To disable specific kernel flavors from the build (for a faster build):
rpmbuild -bb --without <option> --target=`uname -m` kernel.spec

Valid values for "option" above include xen, smp, up, pae, kdump, debug and debuginfo. Specifying --without debug strips out some debugging code from the kernels, where specifying --without debuginfo disables the building of the kernel-debuginfo packages.

  • To specify that only a specific kernel should be built:
rpmbuild -bb --with <option> --target=`uname -m` kernel.spec

Valid values for "option" above include xenonly, smponly, and baseonly.

  • For example, to build just the kernel and kernel-devel packages, the command would be:
rpmbuild -bb --with baseonly --without debuginfo --target=`uname -m` kernel.spec

The build process takes a long time to complete. A lot of messages will be printed to the screen. These messages can be ignored, unless the build ends with an error. If the build completes successfully, the new kernel packages will be located in the ~/rpmbuild/RPMS directory.

Warning (medium size).png
TO DO
add a troubleshooting section

Following Generic Textbooks

Many of the tutorials, examples, and textbooks about Linux kernel development assume the kernel sources are installed under the /usr/src/linux/ directory. If you make a symbolic link, as shown below, you should be able to use those learning materials with the Fedora packages. Install the appropriate kernel sources, as shown earlier, and then run the following command:

su -c 'ln -s /usr/src/kernels/<version>.<release>-<arch> /usr/src/linux'

Enter the root password when prompted.

Install the New Kernel

This step actually installs the new kernel into the running system.

To install the new kernel, use the rpm -ivh command, not the -U or --upgrade options:

su -c "rpm -ivh $HOME/rpmbuild/RPMS/<arch>/kernel-<version>.<arch>.rpm"

This command will install your kernel in /boot, create a new initramfs to bootstrap your kernel, and automatically add your new kernel to your grub bootloader "menu.lst". At this point, you can reboot to give control to your new kernel.

Building Only Kernel Modules (Out Of Tree Modules)

Warning (medium size).png
This section needs to be updated and fleshed out

This section is for users who are only interested in working on a kernel module, and who do not wish to build an entire custom kernel. It is not necessary to download and rebuild the entire kernel in order to build a module. To build a module for the currently running kernel, only the matching kernel-devel package is required. Run the following command to install the kernel-devel package using yum.

su -c 'yum install kernel-devel'
Note.png
You may need to install 'kernel-PAE-devel' if you are using the PAE kernel

You can build against any kernel version, as long as you have kernel and kernel-devel packages installed for that version. The rest of this section assumes we're building for the running kernel; if not, replace `uname -r` with the desired version number.

Note.png
The kernel-doc package contains official Kbuild documentation - see files under Documentation/kbuild, in particular the modules.txt file.

As a simple example, to build the foo.ko module from foo.c, create the following Makefile in the directory containing the foo.c file:

obj-m := foo.o

KDIR  := /lib/modules/$(shell uname -r)/build
PWD   := $(shell pwd)

default:
[TAB]$(MAKE) -C $(KDIR) M=$(PWD) modules

[TAB] Denotes a tab character which must come first for makefile lines containing commands.

Then, issue the make command to build the foo.ko module.

The above is a helpful local Makefile wrapper invoking kbuild; in general you can simply do things like

# make -C /lib/modules/`uname -r`/build M=`pwd` modules
# make -C /lib/modules/`uname -r`/build M=`pwd` clean
# make -C /lib/modules/`uname -r`/build M=`pwd` modules_install

etc to build those targets.

Building Vanilla upstream kernel

Sometimes a Fedora developer may ask you to try building and installing an upstream kernel (possibly with a patch added) for testing. If there are multiple iterations, it may be quicker for you to do this than for the developer to turn around several RPMs.

Existing Fedora Vanilla packages

There is an effort underway for packaging vanilla kernels. See if this meets your needs first

Getting the sources

Clone a kernel tree from kernel.org

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git This will clone the entire upstream tree. This may take a while depending on your connection speed. (While the tree is syncing, why not take the time to update some steps on this wiki that are inevitably out of date?)

$ cd linux

Double check what baseline is being used and check out a new one if necessary:

$ git checkout v3.19

Applying patches

The patch method

If you were asked to apply any patches by the developer, this is the stage at which we would do so. These would typically be applied using a command something like..

$ cat ~/testpatch.diff | patch -p1

If you have to try multiple different patches individually, you can unapply the previous one after testing by adding -R on the end of the above command.

The git method

Most developers these days generate patches using git and you can use git to help apply patches. You can do:

$ git am -3 <patch file>

This will create a git commit of a single patch in your tree.

Configuring the kernel

Chances are that the kernel you are running is older than the one you are about to configure. This means there will be new options. There are several possibilities here.

  • If the developer has pointed you at a specific config file to use, save it in the linux directory with the filename .config
  • You can take your existing .config file by using the command cp /boot/config-<code>uname -r .config</code>

When you run the next step, you'll be asked (potentially lots of) questions about all the new options. Just hitting return 'should' always pick the safe decision for each option. However, it's worth taking care and reading each option, as this isn't always the case, and they may introduce new features your distro isn't capable of running, which may result in a non-booting system.

  • FIXME how to grab a rawhide config

With the config in place, you are now ready to move on to the next step.

Building the kernel

$EDITOR Makefile Change the EXTRAVERSION line to add something on the end. For example, if it reads "EXTRAVERSION = -rc5" change it to "EXTRAVERSION = -rc5-dave" (what you choose is only relevant for the final part of this procedure)

$ make oldconfig

$ make bzImage

$ make modules

(become root)

# make modules_install # make install

You have now built and installed a kernel. It will show up in the grub menu next time you reboot.

Rebuilding

If you have been asked to try several different things, the procedure once you have already built the tree once is mostly the same. A make clean is recommended between builds. This will leave the .config in place, so you can skip that step above and proceed straight to the make bzImage part of the steps above. Because we installed ccache in the first step, subsequent builds may go a lot faster as the compiler hits files that haven't changed since the last time it built them.

Cleaning up

Once you have tested the kernel, and you've booted back to one of your kernels installed from an RPM, you can clean up the files that the above procedure installed by becoming root, and calling these commands. (Be sure to get the kernel version correct!) Remember above, we changed EXTRAVERSION to add a 'tag' to the kernel ? All the files it installed will have this as part of the filename. So you should be able to use wildcards to delete them safely using commands similar to those below. (Just replace 'dave' with whatever tag you chose)

rm -f /boot/config-2.6.*dave* /boot/initrd-2.6.*dave* /boot/vmlinuz-*dave* /boot/System.map-*dave*
rm -rf /lib/modules/2.6*dave*

Finally, you will need to remove the kernel as an option to your bootloader. This will change from architecture to architecture. For x86, (as root), edit /boot/grub2/grub.cfg or /boot/efi/EFI/fedora/grub.cfg if you have EFI enabled and delete the four lines relating to your kernel (They should be easy to spot, they'll be the ones with your tag). They'll look something like this..

title Fedora Core (2.6.22-rc3-dave)
root (hd0,0)
kernel /vmlinuz-2.6.22-rc3-dave ro root=/dev/md0
initrd /initrd-2.6.22-rc3-dave.img