quick-docs/en-US/build-custom-kernel.adoc

= 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

[[dependencies-for-building-kernels]]
Dependencies for building kernels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

`# sudo dnf install fedpkg fedora-packager rpmdevtools ncurses-devel pesign`

Give the following command from the top directory of the kernel source
tree once you have checked it out

`# sudo dnf builddep kernel.spec`

if you plan to run 'make xconfig'

`# sudo dnf install qt3-devel libXi-devel gcc-c++`

Also make sure you add the user doing the build to /etc/pesign/users and
run the authorize user script:

`# sudo /usr/libexec/pesign/pesign-authorize-users`

It should be noted that pesign pesign-rh-test-certs gets pulled in
automatically for some, but not for everyone, it depends on how you
installed pesign. It is best to make sure that you have it installed.

[[building-a-kernel-from-the-fedora-source-tree]]
Building a Kernel from the Fedora source tree
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Make sure you have installed all dependencies

`$ fedpkg clone kernel`

you will likely need to checkout the source anonymously unless you have
an Fedora developer account

`$ fedpkg clone -a kernel`

As of the time of this wiki writing, the kernel is 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 from the top
of your newly created source tree.

e.g. for fedora 23,

`$ git checkout origin/f23`

You can now make whatever changes / customizations you need before
generating the rpms and installing them. You may want to consider
uncommenting

`# define buildid .local`

to avoid conflicts, e.g.

`%define buildid .local`

When finished, generate the appropriate rpms with

`$ fedpkg local`

The rpms will be generated in a subdirectory $ARCH which can then be
installed:

`$ dnf install --nogpgcheck ./x86_64/kernel-$version.rpm`

[[building-a-non-debugging-kernel]]
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`

[[enabling-config-options]]
Enabling config options
^^^^^^^^^^^^^^^^^^^^^^^

If there are configuration options that need to be adjusted for your
build, you can add changes in the kernel-local file. These changes will
get picked up when you build.

[[updating]]
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-exploded-git-trees]]
Building a kernel from the exploded git trees
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fedora keeps a git tree containing Fedora patches applied on top of the
vanilla sources.

`$ git clone `git://git.kernel.org/pub/scm/linux/kernel/git/jwboyer/fedora.git[`git://git.kernel.org/pub/scm/linux/kernel/git/jwboyer/fedora.git`]

`$ git checkout -b my_branch kernel-4.7.4-200.fc24`

You can now build the kernel following regular kernel instructions. This
tree is useful for generating patches that can be applied to the
kernel.spec.

[[building-a-kernel-from-the-source-rpm]]
Building a Kernel from the source RPM
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Instructions for this are on a link:Building_a_custom_kernel/Source_RPM[
separate page]. In general, you should use one of the other methods for
building the kernel which are much easier.

[[building-only-kernel-modules-out-of-tree-modules]]
Building Only Kernel Modules (Out Of Tree Modules)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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 `dnf`.

....
su -c 'dnf install kernel-devel'
....

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.

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]]
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]]
Existing Fedora Vanilla packages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

There is an effort underway for packaging vanilla kernels.
link:Kernel_Vanilla_Repositories[ See if this meets your needs first]

[[getting-the-sources]]
Getting the sources
^^^^^^^^^^^^^^^^^^^

Clone a kernel tree from kernel.org

`$ git clone `git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git[`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 v4.5.2`

[[applying-patches]]
Applying patches
^^^^^^^^^^^^^^^^

[[the-patch-method]]
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]]
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 `

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

[[configuring-the-kernel]]
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-\`uname -r\`* .config`

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]]
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]]
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]]
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
....

Category:Documentation Category:Tutorials
'''

See a typo, something missing or out of date, or anything else which can be
improved? Edit this document at https://pagure.io/fedora-docs/fedora-howto.