1.. _applying_patches:
2
3Applying Patches To The Linux Kernel
4++++++++++++++++++++++++++++++++++++
5
6Original by:
7	Jesper Juhl, August 2005
8
9.. note::
10
11   This document is obsolete.  In most cases, rather than using ``patch``
12   manually, you'll almost certainly want to look at using Git instead.
13
14A frequently asked question on the Linux Kernel Mailing List is how to apply
15a patch to the kernel or, more specifically, what base kernel a patch for
16one of the many trees/branches should be applied to. Hopefully this document
17will explain this to you.
18
19In addition to explaining how to apply and revert patches, a brief
20description of the different kernel trees (and examples of how to apply
21their specific patches) is also provided.
22
23
24What is a patch?
25================
26
27A patch is a small text document containing a delta of changes between two
28different versions of a source tree. Patches are created with the ``diff``
29program.
30
31To correctly apply a patch you need to know what base it was generated from
32and what new version the patch will change the source tree into. These
33should both be present in the patch file metadata or be possible to deduce
34from the filename.
35
36
37How do I apply or revert a patch?
38=================================
39
40You apply a patch with the ``patch`` program. The patch program reads a diff
41(or patch) file and makes the changes to the source tree described in it.
42
43Patches for the Linux kernel are generated relative to the parent directory
44holding the kernel source dir.
45
46This means that paths to files inside the patch file contain the name of the
47kernel source directories it was generated against (or some other directory
48names like "a/" and "b/").
49
50Since this is unlikely to match the name of the kernel source dir on your
51local machine (but is often useful info to see what version an otherwise
52unlabeled patch was generated against) you should change into your kernel
53source directory and then strip the first element of the path from filenames
54in the patch file when applying it (the ``-p1`` argument to ``patch`` does
55this).
56
57To revert a previously applied patch, use the -R argument to patch.
58So, if you applied a patch like this::
59
60	patch -p1 < ../patch-x.y.z
61
62You can revert (undo) it like this::
63
64	patch -R -p1 < ../patch-x.y.z
65
66
67How do I feed a patch/diff file to ``patch``?
68=============================================
69
70This (as usual with Linux and other UNIX like operating systems) can be
71done in several different ways.
72
73In all the examples below I feed the file (in uncompressed form) to patch
74via stdin using the following syntax::
75
76	patch -p1 < path/to/patch-x.y.z
77
78If you just want to be able to follow the examples below and don't want to
79know of more than one way to use patch, then you can stop reading this
80section here.
81
82Patch can also get the name of the file to use via the -i argument, like
83this::
84
85	patch -p1 -i path/to/patch-x.y.z
86
87If your patch file is compressed with gzip or xz and you don't want to
88uncompress it before applying it, then you can feed it to patch like this
89instead::
90
91	xzcat path/to/patch-x.y.z.xz | patch -p1
92	bzcat path/to/patch-x.y.z.gz | patch -p1
93
94If you wish to uncompress the patch file by hand first before applying it
95(what I assume you've done in the examples below), then you simply run
96gunzip or xz on the file -- like this::
97
98	gunzip patch-x.y.z.gz
99	xz -d patch-x.y.z.xz
100
101Which will leave you with a plain text patch-x.y.z file that you can feed to
102patch via stdin or the ``-i`` argument, as you prefer.
103
104A few other nice arguments for patch are ``-s`` which causes patch to be silent
105except for errors which is nice to prevent errors from scrolling out of the
106screen too fast, and ``--dry-run`` which causes patch to just print a listing of
107what would happen, but doesn't actually make any changes. Finally ``--verbose``
108tells patch to print more information about the work being done.
109
110
111Common errors when patching
112===========================
113
114When patch applies a patch file it attempts to verify the sanity of the
115file in different ways.
116
117Checking that the file looks like a valid patch file and checking the code
118around the bits being modified matches the context provided in the patch are
119just two of the basic sanity checks patch does.
120
121If patch encounters something that doesn't look quite right it has two
122options. It can either refuse to apply the changes and abort or it can try
123to find a way to make the patch apply with a few minor changes.
124
125One example of something that's not 'quite right' that patch will attempt to
126fix up is if all the context matches, the lines being changed match, but the
127line numbers are different. This can happen, for example, if the patch makes
128a change in the middle of the file but for some reasons a few lines have
129been added or removed near the beginning of the file. In that case
130everything looks good it has just moved up or down a bit, and patch will
131usually adjust the line numbers and apply the patch.
132
133Whenever patch applies a patch that it had to modify a bit to make it fit
134it'll tell you about it by saying the patch applied with **fuzz**.
135You should be wary of such changes since even though patch probably got it
136right it doesn't /always/ get it right, and the result will sometimes be
137wrong.
138
139When patch encounters a change that it can't fix up with fuzz it rejects it
140outright and leaves a file with a ``.rej`` extension (a reject file). You can
141read this file to see exactly what change couldn't be applied, so you can
142go fix it up by hand if you wish.
143
144If you don't have any third-party patches applied to your kernel source, but
145only patches from kernel.org and you apply the patches in the correct order,
146and have made no modifications yourself to the source files, then you should
147never see a fuzz or reject message from patch. If you do see such messages
148anyway, then there's a high risk that either your local source tree or the
149patch file is corrupted in some way. In that case you should probably try
150re-downloading the patch and if things are still not OK then you'd be advised
151to start with a fresh tree downloaded in full from kernel.org.
152
153Let's look a bit more at some of the messages patch can produce.
154
155If patch stops and presents a ``File to patch:`` prompt, then patch could not
156find a file to be patched. Most likely you forgot to specify -p1 or you are
157in the wrong directory. Less often, you'll find patches that need to be
158applied with ``-p0`` instead of ``-p1`` (reading the patch file should reveal if
159this is the case -- if so, then this is an error by the person who created
160the patch but is not fatal).
161
162If you get ``Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines).`` or a
163message similar to that, then it means that patch had to adjust the location
164of the change (in this example it needed to move 7 lines from where it
165expected to make the change to make it fit).
166
167The resulting file may or may not be OK, depending on the reason the file
168was different than expected.
169
170This often happens if you try to apply a patch that was generated against a
171different kernel version than the one you are trying to patch.
172
173If you get a message like ``Hunk #3 FAILED at 2387.``, then it means that the
174patch could not be applied correctly and the patch program was unable to
175fuzz its way through. This will generate a ``.rej`` file with the change that
176caused the patch to fail and also a ``.orig`` file showing you the original
177content that couldn't be changed.
178
179If you get ``Reversed (or previously applied) patch detected!  Assume -R? [n]``
180then patch detected that the change contained in the patch seems to have
181already been made.
182
183If you actually did apply this patch previously and you just re-applied it
184in error, then just say [n]o and abort this patch. If you applied this patch
185previously and actually intended to revert it, but forgot to specify -R,
186then you can say [**y**]es here to make patch revert it for you.
187
188This can also happen if the creator of the patch reversed the source and
189destination directories when creating the patch, and in that case reverting
190the patch will in fact apply it.
191
192A message similar to ``patch: **** unexpected end of file in patch`` or
193``patch unexpectedly ends in middle of line`` means that patch could make no
194sense of the file you fed to it. Either your download is broken, you tried to
195feed patch a compressed patch file without uncompressing it first, or the patch
196file that you are using has been mangled by a mail client or mail transfer
197agent along the way somewhere, e.g., by splitting a long line into two lines.
198Often these warnings can easily be fixed by joining (concatenating) the
199two lines that had been split.
200
201As I already mentioned above, these errors should never happen if you apply
202a patch from kernel.org to the correct version of an unmodified source tree.
203So if you get these errors with kernel.org patches then you should probably
204assume that either your patch file or your tree is broken and I'd advise you
205to start over with a fresh download of a full kernel tree and the patch you
206wish to apply.
207
208
209Are there any alternatives to ``patch``?
210========================================
211
212
213Yes there are alternatives.
214
215You can use the ``interdiff`` program (http://cyberelk.net/tim/patchutils/) to
216generate a patch representing the differences between two patches and then
217apply the result.
218
219This will let you move from something like 5.7.2 to 5.7.3 in a single
220step. The -z flag to interdiff will even let you feed it patches in gzip or
221bzip2 compressed form directly without the use of zcat or bzcat or manual
222decompression.
223
224Here's how you'd go from 5.7.2 to 5.7.3 in a single step::
225
226	interdiff -z ../patch-5.7.2.gz ../patch-5.7.3.gz | patch -p1
227
228Although interdiff may save you a step or two you are generally advised to
229do the additional steps since interdiff can get things wrong in some cases.
230
231Another alternative is ``ketchup``, which is a python script for automatic
232downloading and applying of patches (https://www.selenic.com/ketchup/).
233
234Other nice tools are diffstat, which shows a summary of changes made by a
235patch; lsdiff, which displays a short listing of affected files in a patch
236file, along with (optionally) the line numbers of the start of each patch;
237and grepdiff, which displays a list of the files modified by a patch where
238the patch contains a given regular expression.
239
240
241Where can I download the patches?
242=================================
243
244The patches are available at https://kernel.org/
245Most recent patches are linked from the front page, but they also have
246specific homes.
247
248The 5.x.y (-stable) and 5.x patches live at
249
250	https://www.kernel.org/pub/linux/kernel/v5.x/
251
252The -rc patches are not stored on the webserver but are generated on
253demand from git tags such as
254
255	https://git.kernel.org/torvalds/p/v5.1-rc1/v5.0
256
257The stable -rc patches live at
258
259	https://www.kernel.org/pub/linux/kernel/v5.x/stable-review/
260
261
262The 5.x kernels
263===============
264
265These are the base stable releases released by Linus. The highest numbered
266release is the most recent.
267
268If regressions or other serious flaws are found, then a -stable fix patch
269will be released (see below) on top of this base. Once a new 5.x base
270kernel is released, a patch is made available that is a delta between the
271previous 5.x kernel and the new one.
272
273To apply a patch moving from 5.6 to 5.7, you'd do the following (note
274that such patches do **NOT** apply on top of 5.x.y kernels but on top of the
275base 5.x kernel -- if you need to move from 5.x.y to 5.x+1 you need to
276first revert the 5.x.y patch).
277
278Here are some examples::
279
280	# moving from 5.6 to 5.7
281
282	$ cd ~/linux-5.6		# change to kernel source dir
283	$ patch -p1 < ../patch-5.7	# apply the 5.7 patch
284	$ cd ..
285	$ mv linux-5.6 linux-5.7	# rename source dir
286
287	# moving from 5.6.1 to 5.7
288
289	$ cd ~/linux-5.6.1		# change to kernel source dir
290	$ patch -p1 -R < ../patch-5.6.1	# revert the 5.6.1 patch
291					# source dir is now 5.6
292	$ patch -p1 < ../patch-5.7	# apply new 5.7 patch
293	$ cd ..
294	$ mv linux-5.6.1 linux-5.7	# rename source dir
295
296
297The 5.x.y kernels
298=================
299
300Kernels with 3-digit versions are -stable kernels. They contain small(ish)
301critical fixes for security problems or significant regressions discovered
302in a given 5.x kernel.
303
304This is the recommended branch for users who want the most recent stable
305kernel and are not interested in helping test development/experimental
306versions.
307
308If no 5.x.y kernel is available, then the highest numbered 5.x kernel is
309the current stable kernel.
310
311.. note::
312
313 The -stable team usually do make incremental patches available as well
314 as patches against the latest mainline release, but I only cover the
315 non-incremental ones below. The incremental ones can be found at
316 https://www.kernel.org/pub/linux/kernel/v5.x/incr/
317
318These patches are not incremental, meaning that for example the 5.7.3
319patch does not apply on top of the 5.7.2 kernel source, but rather on top
320of the base 5.7 kernel source.
321
322So, in order to apply the 5.7.3 patch to your existing 5.7.2 kernel
323source you have to first back out the 5.7.2 patch (so you are left with a
324base 5.7 kernel source) and then apply the new 5.7.3 patch.
325
326Here's a small example::
327
328	$ cd ~/linux-5.7.2		# change to the kernel source dir
329	$ patch -p1 -R < ../patch-5.7.2	# revert the 5.7.2 patch
330	$ patch -p1 < ../patch-5.7.3	# apply the new 5.7.3 patch
331	$ cd ..
332	$ mv linux-5.7.2 linux-5.7.3	# rename the kernel source dir
333
334The -rc kernels
335===============
336
337These are release-candidate kernels. These are development kernels released
338by Linus whenever he deems the current git (the kernel's source management
339tool) tree to be in a reasonably sane state adequate for testing.
340
341These kernels are not stable and you should expect occasional breakage if
342you intend to run them. This is however the most stable of the main
343development branches and is also what will eventually turn into the next
344stable kernel, so it is important that it be tested by as many people as
345possible.
346
347This is a good branch to run for people who want to help out testing
348development kernels but do not want to run some of the really experimental
349stuff (such people should see the sections about -next and -mm kernels below).
350
351The -rc patches are not incremental, they apply to a base 5.x kernel, just
352like the 5.x.y patches described above. The kernel version before the -rcN
353suffix denotes the version of the kernel that this -rc kernel will eventually
354turn into.
355
356So, 5.8-rc5 means that this is the fifth release candidate for the 5.8
357kernel and the patch should be applied on top of the 5.7 kernel source.
358
359Here are 3 examples of how to apply these patches::
360
361	# first an example of moving from 5.7 to 5.8-rc3
362
363	$ cd ~/linux-5.7			# change to the 5.7 source dir
364	$ patch -p1 < ../patch-5.8-rc3		# apply the 5.8-rc3 patch
365	$ cd ..
366	$ mv linux-5.7 linux-5.8-rc3		# rename the source dir
367
368	# now let's move from 5.8-rc3 to 5.8-rc5
369
370	$ cd ~/linux-5.8-rc3			# change to the 5.8-rc3 dir
371	$ patch -p1 -R < ../patch-5.8-rc3	# revert the 5.8-rc3 patch
372	$ patch -p1 < ../patch-5.8-rc5		# apply the new 5.8-rc5 patch
373	$ cd ..
374	$ mv linux-5.8-rc3 linux-5.8-rc5	# rename the source dir
375
376	# finally let's try and move from 5.7.3 to 5.8-rc5
377
378	$ cd ~/linux-5.7.3			# change to the kernel source dir
379	$ patch -p1 -R < ../patch-5.7.3		# revert the 5.7.3 patch
380	$ patch -p1 < ../patch-5.8-rc5		# apply new 5.8-rc5 patch
381	$ cd ..
382	$ mv linux-5.7.3 linux-5.8-rc5		# rename the kernel source dir
383
384
385The -mm patches and the linux-next tree
386=======================================
387
388The -mm patches are experimental patches released by Andrew Morton.
389
390In the past, -mm tree were used to also test subsystem patches, but this
391function is now done via the
392`linux-next` (https://www.kernel.org/doc/man-pages/linux-next.html)
393tree. The Subsystem maintainers push their patches first to linux-next,
394and, during the merge window, sends them directly to Linus.
395
396The -mm patches serve as a sort of proving ground for new features and other
397experimental patches that aren't merged via a subsystem tree.
398Once such patches has proved its worth in -mm for a while Andrew pushes
399it on to Linus for inclusion in mainline.
400
401The linux-next tree is daily updated, and includes the -mm patches.
402Both are in constant flux and contains many experimental features, a
403lot of debugging patches not appropriate for mainline etc., and is the most
404experimental of the branches described in this document.
405
406These patches are not appropriate for use on systems that are supposed to be
407stable and they are more risky to run than any of the other branches (make
408sure you have up-to-date backups -- that goes for any experimental kernel but
409even more so for -mm patches or using a Kernel from the linux-next tree).
410
411Testing of -mm patches and linux-next is greatly appreciated since the whole
412point of those are to weed out regressions, crashes, data corruption bugs,
413build breakage (and any other bug in general) before changes are merged into
414the more stable mainline Linus tree.
415
416But testers of -mm and linux-next should be aware that breakages are
417more common than in any other tree.
418
419
420This concludes this list of explanations of the various kernel trees.
421I hope you are now clear on how to apply the various patches and help testing
422the kernel.
423
424Thank you's to Randy Dunlap, Rolf Eike Beer, Linus Torvalds, Bodo Eggert,
425Johannes Stezenbach, Grant Coady, Pavel Machek and others that I may have
426forgotten for their reviews and contributions to this document.
427