.. _applying_patches:

Applying Patches To The Linux Kernel
++++++++++++++++++++++++++++++++++++

Original by:
	Jesper Juhl, August 2005

.. note::

   This document is obsolete.  In most cases, rather than using ``patch``
   manually, you'll almost certainly want to look at using Git instead.

A frequently asked question on the Linux Kernel Mailing List is how to apply
a patch to the kernel or, more specifically, what base kernel a patch for
one of the many trees/branches should be applied to. Hopefully this document
will explain this to you.

In addition to explaining how to apply and revert patches, a brief
description of the different kernel trees (and examples of how to apply
their specific patches) is also provided.


What is a patch?
================

A patch is a small text document containing a delta of changes between two
different versions of a source tree. Patches are created with the ``diff``
program.

To correctly apply a patch you need to know what base it was generated from
and what new version the patch will change the source tree into. These
should both be present in the patch file metadata or be possible to deduce
from the filename.


How do I apply or revert a patch?
=================================

You apply a patch with the ``patch`` program. The patch program reads a diff
(or patch) file and makes the changes to the source tree described in it.

Patches for the Linux kernel are generated relative to the parent directory
holding the kernel source dir.

This means that paths to files inside the patch file contain the name of the
kernel source directories it was generated against (or some other directory
names like "a/" and "b/").

Since this is unlikely to match the name of the kernel source dir on your
local machine (but is often useful info to see what version an otherwise
unlabeled patch was generated against) you should change into your kernel
source directory and then strip the first element of the path from filenames
in the patch file when applying it (the ``-p1`` argument to ``patch`` does
this).

To revert a previously applied patch, use the -R argument to patch.
So, if you applied a patch like this::

	patch -p1 < ../patch-x.y.z

You can revert (undo) it like this::

	patch -R -p1 < ../patch-x.y.z


How do I feed a patch/diff file to ``patch``?
=============================================

This (as usual with Linux and other UNIX like operating systems) can be
done in several different ways.

In all the examples below I feed the file (in uncompressed form) to patch
via stdin using the following syntax::

	patch -p1 < path/to/patch-x.y.z

If you just want to be able to follow the examples below and don't want to
know of more than one way to use patch, then you can stop reading this
section here.

Patch can also get the name of the file to use via the -i argument, like
this::

	patch -p1 -i path/to/patch-x.y.z

If your patch file is compressed with gzip or xz and you don't want to
uncompress it before applying it, then you can feed it to patch like this
instead::

	xzcat path/to/patch-x.y.z.xz | patch -p1
	bzcat path/to/patch-x.y.z.gz | patch -p1

If you wish to uncompress the patch file by hand first before applying it
(what I assume you've done in the examples below), then you simply run
gunzip or xz on the file -- like this::

	gunzip patch-x.y.z.gz
	xz -d patch-x.y.z.xz

Which will leave you with a plain text patch-x.y.z file that you can feed to
patch via stdin or the ``-i`` argument, as you prefer.

A few other nice arguments for patch are ``-s`` which causes patch to be silent
except for errors which is nice to prevent errors from scrolling out of the
screen too fast, and ``--dry-run`` which causes patch to just print a listing of
what would happen, but doesn't actually make any changes. Finally ``--verbose``
tells patch to print more information about the work being done.


Common errors when patching
===========================

When patch applies a patch file it attempts to verify the sanity of the
file in different ways.

Checking that the file looks like a valid patch file and checking the code
around the bits being modified matches the context provided in the patch are
just two of the basic sanity checks patch does.

If patch encounters something that doesn't look quite right it has two
options. It can either refuse to apply the changes and abort or it can try
to find a way to make the patch apply with a few minor changes.

One example of something that's not 'quite right' that patch will attempt to
fix up is if all the context matches, the lines being changed match, but the
line numbers are different. This can happen, for example, if the patch makes
a change in the middle of the file but for some reasons a few lines have
been added or removed near the beginning of the file. In that case
everything looks good it has just moved up or down a bit, and patch will
usually adjust the line numbers and apply the patch.

Whenever patch applies a patch that it had to modify a bit to make it fit
it'll tell you about it by saying the patch applied with **fuzz**.
You should be wary of such changes since even though patch probably got it
right it doesn't /always/ get it right, and the result will sometimes be
wrong.

When patch encounters a change that it can't fix up with fuzz it rejects it
outright and leaves a file with a ``.rej`` extension (a reject file). You can
read this file to see exactly what change couldn't be applied, so you can
go fix it up by hand if you wish.

If you don't have any third-party patches applied to your kernel source, but
only patches from kernel.org and you apply the patches in the correct order,
and have made no modifications yourself to the source files, then you should
never see a fuzz or reject message from patch. If you do see such messages
anyway, then there's a high risk that either your local source tree or the
patch file is corrupted in some way. In that case you should probably try
re-downloading the patch and if things are still not OK then you'd be advised
to start with a fresh tree downloaded in full from kernel.org.

Let's look a bit more at some of the messages patch can produce.

If patch stops and presents a ``File to patch:`` prompt, then patch could not
find a file to be patched. Most likely you forgot to specify -p1 or you are
in the wrong directory. Less often, you'll find patches that need to be
applied with ``-p0`` instead of ``-p1`` (reading the patch file should reveal if
this is the case -- if so, then this is an error by the person who created
the patch but is not fatal).

If you get ``Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines).`` or a
message similar to that, then it means that patch had to adjust the location
of the change (in this example it needed to move 7 lines from where it
expected to make the change to make it fit).

The resulting file may or may not be OK, depending on the reason the file
was different than expected.

This often happens if you try to apply a patch that was generated against a
different kernel version than the one you are trying to patch.

If you get a message like ``Hunk #3 FAILED at 2387.``, then it means that the
patch could not be applied correctly and the patch program was unable to
fuzz its way through. This will generate a ``.rej`` file with the change that
caused the patch to fail and also a ``.orig`` file showing you the original
content that couldn't be changed.

If you get ``Reversed (or previously applied) patch detected!  Assume -R? [n]``
then patch detected that the change contained in the patch seems to have
already been made.

If you actually did apply this patch previously and you just re-applied it
in error, then just say [n]o and abort this patch. If you applied this patch
previously and actually intended to revert it, but forgot to specify -R,
then you can say [**y**]es here to make patch revert it for you.

This can also happen if the creator of the patch reversed the source and
destination directories when creating the patch, and in that case reverting
the patch will in fact apply it.

A message similar to ``patch: **** unexpected end of file in patch`` or
``patch unexpectedly ends in middle of line`` means that patch could make no
sense of the file you fed to it. Either your download is broken, you tried to
feed patch a compressed patch file without uncompressing it first, or the patch
file that you are using has been mangled by a mail client or mail transfer
agent along the way somewhere, e.g., by splitting a long line into two lines.
Often these warnings can easily be fixed by joining (concatenating) the
two lines that had been split.

As I already mentioned above, these errors should never happen if you apply
a patch from kernel.org to the correct version of an unmodified source tree.
So if you get these errors with kernel.org patches then you should probably
assume that either your patch file or your tree is broken and I'd advise you
to start over with a fresh download of a full kernel tree and the patch you
wish to apply.


Are there any alternatives to ``patch``?
========================================


Yes there are alternatives.

You can use the ``interdiff`` program (http://cyberelk.net/tim/patchutils/) to
generate a patch representing the differences between two patches and then
apply the result.

This will let you move from something like 5.7.2 to 5.7.3 in a single
step. The -z flag to interdiff will even let you feed it patches in gzip or
bzip2 compressed form directly without the use of zcat or bzcat or manual
decompression.

Here's how you'd go from 5.7.2 to 5.7.3 in a single step::

	interdiff -z ../patch-5.7.2.gz ../patch-5.7.3.gz | patch -p1

Although interdiff may save you a step or two you are generally advised to
do the additional steps since interdiff can get things wrong in some cases.

Another alternative is ``ketchup``, which is a python script for automatic
downloading and applying of patches (https://www.selenic.com/ketchup/).

Other nice tools are diffstat, which shows a summary of changes made by a
patch; lsdiff, which displays a short listing of affected files in a patch
file, along with (optionally) the line numbers of the start of each patch;
and grepdiff, which displays a list of the files modified by a patch where
the patch contains a given regular expression.


Where can I download the patches?
=================================

The patches are available at https://kernel.org/
Most recent patches are linked from the front page, but they also have
specific homes.

The 5.x.y (-stable) and 5.x patches live at

	https://www.kernel.org/pub/linux/kernel/v5.x/

The -rc patches are not stored on the webserver but are generated on
demand from git tags such as

	https://git.kernel.org/torvalds/p/v5.1-rc1/v5.0

The stable -rc patches live at

	https://www.kernel.org/pub/linux/kernel/v5.x/stable-review/


The 5.x kernels
===============

These are the base stable releases released by Linus. The highest numbered
release is the most recent.

If regressions or other serious flaws are found, then a -stable fix patch
will be released (see below) on top of this base. Once a new 5.x base
kernel is released, a patch is made available that is a delta between the
previous 5.x kernel and the new one.

To apply a patch moving from 5.6 to 5.7, you'd do the following (note
that such patches do **NOT** apply on top of 5.x.y kernels but on top of the
base 5.x kernel -- if you need to move from 5.x.y to 5.x+1 you need to
first revert the 5.x.y patch).

Here are some examples::

	# moving from 5.6 to 5.7

	$ cd ~/linux-5.6		# change to kernel source dir
	$ patch -p1 < ../patch-5.7	# apply the 5.7 patch
	$ cd ..
	$ mv linux-5.6 linux-5.7	# rename source dir

	# moving from 5.6.1 to 5.7

	$ cd ~/linux-5.6.1		# change to kernel source dir
	$ patch -p1 -R < ../patch-5.6.1	# revert the 5.6.1 patch
					# source dir is now 5.6
	$ patch -p1 < ../patch-5.7	# apply new 5.7 patch
	$ cd ..
	$ mv linux-5.6.1 linux-5.7	# rename source dir


The 5.x.y kernels
=================

Kernels with 3-digit versions are -stable kernels. They contain small(ish)
critical fixes for security problems or significant regressions discovered
in a given 5.x kernel.

This is the recommended branch for users who want the most recent stable
kernel and are not interested in helping test development/experimental
versions.

If no 5.x.y kernel is available, then the highest numbered 5.x kernel is
the current stable kernel.

.. note::

 The -stable team usually do make incremental patches available as well
 as patches against the latest mainline release, but I only cover the
 non-incremental ones below. The incremental ones can be found at
 https://www.kernel.org/pub/linux/kernel/v5.x/incr/

These patches are not incremental, meaning that for example the 5.7.3
patch does not apply on top of the 5.7.2 kernel source, but rather on top
of the base 5.7 kernel source.

So, in order to apply the 5.7.3 patch to your existing 5.7.2 kernel
source you have to first back out the 5.7.2 patch (so you are left with a
base 5.7 kernel source) and then apply the new 5.7.3 patch.

Here's a small example::

	$ cd ~/linux-5.7.2		# change to the kernel source dir
	$ patch -p1 -R < ../patch-5.7.2	# revert the 5.7.2 patch
	$ patch -p1 < ../patch-5.7.3	# apply the new 5.7.3 patch
	$ cd ..
	$ mv linux-5.7.2 linux-5.7.3	# rename the kernel source dir

The -rc kernels
===============

These are release-candidate kernels. These are development kernels released
by Linus whenever he deems the current git (the kernel's source management
tool) tree to be in a reasonably sane state adequate for testing.

These kernels are not stable and you should expect occasional breakage if
you intend to run them. This is however the most stable of the main
development branches and is also what will eventually turn into the next
stable kernel, so it is important that it be tested by as many people as
possible.

This is a good branch to run for people who want to help out testing
development kernels but do not want to run some of the really experimental
stuff (such people should see the sections about -next and -mm kernels below).

The -rc patches are not incremental, they apply to a base 5.x kernel, just
like the 5.x.y patches described above. The kernel version before the -rcN
suffix denotes the version of the kernel that this -rc kernel will eventually
turn into.

So, 5.8-rc5 means that this is the fifth release candidate for the 5.8
kernel and the patch should be applied on top of the 5.7 kernel source.

Here are 3 examples of how to apply these patches::

	# first an example of moving from 5.7 to 5.8-rc3

	$ cd ~/linux-5.7			# change to the 5.7 source dir
	$ patch -p1 < ../patch-5.8-rc3		# apply the 5.8-rc3 patch
	$ cd ..
	$ mv linux-5.7 linux-5.8-rc3		# rename the source dir

	# now let's move from 5.8-rc3 to 5.8-rc5

	$ cd ~/linux-5.8-rc3			# change to the 5.8-rc3 dir
	$ patch -p1 -R < ../patch-5.8-rc3	# revert the 5.8-rc3 patch
	$ patch -p1 < ../patch-5.8-rc5		# apply the new 5.8-rc5 patch
	$ cd ..
	$ mv linux-5.8-rc3 linux-5.8-rc5	# rename the source dir

	# finally let's try and move from 5.7.3 to 5.8-rc5

	$ cd ~/linux-5.7.3			# change to the kernel source dir
	$ patch -p1 -R < ../patch-5.7.3		# revert the 5.7.3 patch
	$ patch -p1 < ../patch-5.8-rc5		# apply new 5.8-rc5 patch
	$ cd ..
	$ mv linux-5.7.3 linux-5.8-rc5		# rename the kernel source dir


The -mm patches and the linux-next tree
=======================================

The -mm patches are experimental patches released by Andrew Morton.

In the past, -mm tree were used to also test subsystem patches, but this
function is now done via the
`linux-next <https://www.kernel.org/doc/man-pages/linux-next.html>`
tree. The Subsystem maintainers push their patches first to linux-next,
and, during the merge window, sends them directly to Linus.

The -mm patches serve as a sort of proving ground for new features and other
experimental patches that aren't merged via a subsystem tree.
Once such patches has proved its worth in -mm for a while Andrew pushes
it on to Linus for inclusion in mainline.

The linux-next tree is daily updated, and includes the -mm patches.
Both are in constant flux and contains many experimental features, a
lot of debugging patches not appropriate for mainline etc., and is the most
experimental of the branches described in this document.

These patches are not appropriate for use on systems that are supposed to be
stable and they are more risky to run than any of the other branches (make
sure you have up-to-date backups -- that goes for any experimental kernel but
even more so for -mm patches or using a Kernel from the linux-next tree).

Testing of -mm patches and linux-next is greatly appreciated since the whole
point of those are to weed out regressions, crashes, data corruption bugs,
build breakage (and any other bug in general) before changes are merged into
the more stable mainline Linus tree.

But testers of -mm and linux-next should be aware that breakages are
more common than in any other tree.


This concludes this list of explanations of the various kernel trees.
I hope you are now clear on how to apply the various patches and help testing
the kernel.

Thank you's to Randy Dunlap, Rolf Eike Beer, Linus Torvalds, Bodo Eggert,
Johannes Stezenbach, Grant Coady, Pavel Machek and others that I may have
forgotten for their reviews and contributions to this document.