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modules/ROOT/pages/kernel/build-custom-kernel.adoc
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modules/ROOT/pages/kernel/build-custom-kernel.adoc
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[[ch-build-custom-kernel]]
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= Building a Custom Kernel
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:toc:
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This document provides instructions for advanced users who want to rebuild the
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kernel from some source.
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[NOTE]
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====
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When building or running a custom kernel, one should *not* expect support from
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the Fedora kernel team.
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====
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Some common reasons to build a custom kernel are:
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* To apply patches for testing that they either generated or obtained from
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another source
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* To reconfigure the existing kernel
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* To learn more about the kernel and kernel development
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== Get the Dependencies
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The easiest way to install all the build dependencies for the kernel is to use
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the Fedora kernel spec file:
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[source,bash]
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----
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sudo dnf install fedpkg
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fedpkg clone -a kernel
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cd kernel
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sudo dnf builddep kernel.spec
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----
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If you want to use `make xconfig`, you'll need some additional packages:
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[source,bash]
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----
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sudo dnf install qt3-devel libXi-devel gcc-c++
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----
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Make sure you add the user doing the build to `/etc/pesign/users` and run the
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authorize user script:
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[source,bash]
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----
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sudo /usr/libexec/pesign/pesign-authorize
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----
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It's also recommended that you install `ccache`, which can help speed up
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rebuilds:
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[source,bash]
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----
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sudo dnf install ccache
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----
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== Building a Kernel from the Fedora dist-git
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The kernel, like any other Fedora package, has a branch per Fedora release.
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`master` corresponds to Rawhide and each version of Fedora has a branch called
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`f<version>`. For example, to build a Fedora 28 kernel, you would first need
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to check out that branch with:
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1. Check out the branch for which you would like to build a kernel (`master`
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corresponds to Rawhide):
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[source,bash]
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----
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git checkout origin/f28
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----
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2. To avoid conflicts with existing kernels, you can set a custom buildid by
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changing `# define buildid .local` to `%define buildid .<your_custom_id_here>`
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in `kernel.spec`.
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3. Make whatever changes or customizations you need.
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4. Build the RPMs:
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[source,bash]
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----
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fedpkg local
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----
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5. Install the new kernel:
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[source,bash]
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----
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sudo dnf install --nogpgcheck ./x86_64/kernel-$version.rpm
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----
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=== Building a non-debugging kernel
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Most Rawhide kernels are built with debugging enabled by default. To make a
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kernel with debugging options disabled, you can follow the above instructions,
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but before you run `fedpkg local`, disable the debugging options with:
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[source,bash]
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----
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make release
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----
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=== Enabling configuration options
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If there are configuration options that need to be adjusted for your build, you
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can add changes in the kernel-local file. These changes will get picked up when
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you build.
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== Building a kernel from the exploded git trees
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Fedora keeps a git tree containing Fedora patches applied on top of the vanilla sources.
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[source,bash]
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----
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git clone git://git.kernel.org/pub/scm/linux/kernel/git/jwboyer/fedora.git
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git checkout -b my_branch kernel-4.7.4-200.fc24
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----
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You can now build the kernel following regular kernel instructions. This tree
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is useful for generating patches that can be applied to the kernel.spec.
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== Building a vanilla upstream kernel
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Sometimes a Fedora developer may ask you to try building and installing an
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upstream kernel (possibly with a patch added) for testing. If there are
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multiple iterations, it may be quicker for you to do this than for the
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developer to turn around several RPMs.
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[NOTE]
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====
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There is an effort underway for packaging vanilla kernels.
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https://fedoraproject.org/wiki/Kernel_Vanilla_Repositories[See if this meets
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your needs first]
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====
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=== Getting the Sources
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Clone the kernel tree from kernel.org. If you don't know what tree you need,
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you should get Linus' tree:
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[source,bash]
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----
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git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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cd linux
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----
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You may also want the stable tree (4.y.z releases), which you can add with:
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[source,bash]
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----
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git remote add -f stable git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
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----
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=== Applying patches
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To apply patch files, you can use git-am:
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[source,bash]
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----
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git am -3 <patch file>
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----
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=== Configuring the kernel
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If the developer has pointed you at a specific config file to use, save it in
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the linux directory with the filename `.config`
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Otherwise, you'll need to pick a configuration file to start from. The Linux
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kernel has thousands of configuration options, so you don't want to start from
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scratch unless you know what you're doing.
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==== Starting from an installed kernel configuration
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If you want to tweak the configuration of a kernel you already have installed,
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you can start with its configuration which is stored in /boot/. For example,
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to start with the configuration of the currently running kernel:
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[source,bash]
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----
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cp /boot/config-`uname -r`* .config
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----
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==== Starting from dist-git
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If you want to use the configuration for a kernel you do not have installed,
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you can get the configuration from the Fedora dist-git repository. For example,
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to start with the latest Rawhide configuration:
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[source,bash]
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----
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cd <dist-git directory>
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git checkout master
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./build_configs.sh # Ensure the latest configuration files are generated
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cp kernel-<arch>.config <linux kernel directory>.config
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----
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The debug versions of the configuration files are in
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`kernel-<arch>-debug.config` if you would like to build a kernel with debugging
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options enabled.
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=== Changing the configuration
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There are several ways to change the configuration. You can run `make help` and
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look at the `Configuration targets` for the full list, but `make menuconfig`
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is a good place to start. You can also just edit the `.config` file directly.
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[NOTE]
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====
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One configuration option you may want to set is CONFIG_MODULE_COMPRESS, which
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compresses the modules (with gzip by default) when installing them. Without
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this setting, the modules can be very large.
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====
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=== Building the kernel
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Once you've configured the kernel, you're ready to build it. Before you do so,
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you'll want to change the `EXTRAVERSION` in the `Makefile` to something you'll
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recognize later. For example, if it reads `EXTRAVERSION = -rc5` change it to
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`EXTRAVERSION = -rc5-dave`:
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[source,bash]
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----
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$EDITOR Makefile
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----
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Now you're ready to build the kernel:
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[source,bash]
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----
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make oldconfig
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make bzImage
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make modules
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----
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=== Installing the kernel
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Installing the kernel is as simple as:
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[source,bash]
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----
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sudo make modules_install
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sudo make install
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----
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=== Rebuilding
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If you have been asked to try several different things, the procedure once you
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have already built the tree once is mostly the same. Running `make clean` is
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recommended between builds. This will leave the `.config` in place, so you can
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skip that step above and proceed straight to the `make bzImage` part of the steps
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above. Because we installed `ccache` in the first step, subsequent builds may go
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a lot faster as the compiler hits files that haven't changed since the last
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time it built them.
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=== Cleaning up
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Once you have tested the kernel, and you've booted back to one of your kernels
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installed from an RPM, you can clean up the files that the above procedure
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installed.
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[WARNING]
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====
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When running the following commands, be sure to get the kernel version correct!
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====
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Because you changed `EXTRAVERSION` in the `Makefile` to add a 'tag', all the
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files it installed will have this as part of the filename. So you should be
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able to use wildcards to delete them safely using commands similar to those
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below (just replace 'dave' with whatever tag you chose):
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[source,bash]
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----
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rm -f /boot/config-2.6.*dave* /boot/initrd-2.6.*dave* /boot/vmlinuz-*dave* /boot/System.map-*dave*
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rm -rf /lib/modules/2.6*dave*
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----
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Finally, you will need to remove the kernel as an option to your bootloader.
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This will change from architecture to architecture. For x86, (as root), edit
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`/boot/grub2/grub.cfg` or `/boot/efi/EFI/fedora/grub.cfg` if you have EFI enabled
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and delete the four lines relating to your kernel (They should be easy to spot,
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they'll be the ones with your tag). They'll look something like:
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----
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title Fedora Core (2.6.22-rc3-dave)
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root (hd0,0)
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kernel /vmlinuz-2.6.22-rc3-dave ro root=/dev/md0
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initrd /initrd-2.6.22-rc3-dave.img
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----
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