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

# sudo dnf install fedpkg fedora-packager rpmdevtools ncurses-devel pesign pesign-rh-test-certs

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.

Note.png
dnf versus yum
As of Fedora 22[1] dnf has replaced yum as the default package manager. In the event you are building a kernel for an older system you will either have to install dnf or substitute yum for dnf and/or yumdownloader from the yum-utils package.

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

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

If there are configuration options that need to be adjusted for your build, you can add changes in the config-local file. This will override anything set in the rest of the defconfigs.

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

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 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

Instructions for this are on a 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)

Warning.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 dnf.

su -c 'dnf 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.

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

Note.png
Tips for developing with kernel.org sources
The README file in the top-level directory provides very good build instructions, including how to direct the build to a particular directory, the necessary make targets, and the order in which to issue them. Save time by reviewing the Documentation directory and its sub-directories containing informative ".txt" files. View the top level Makefile for kernel version information.

$ 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 v4.5.2

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

Note.png
Potential options to the instructions below
Consider using the make targets "mrproper" and "menuconfig" as specified in the top-level README file. The "menuconfig" target is an alternative to "oldconfig" and pops up a menu that allows you to easily set virtually all build options. Accepting the defaults is generally fine, although you can use the "General Setup->Local version" option to append a label to your build instead of tweaking "EXTRAVERSION" in the .config file.

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

$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