1.. _applying_patches:
2
3Applying Patches To The Linux Kernel
4++++++++++++++++++++++++++++++++++++
5
6Original by:
7	Jesper Juhl, August 2005
8
9Last update:
10	2016-09-14
11
12.. note::
13
14   This document is obsolete.  In most cases, rather than using ``patch``
15   manually, you'll almost certainly want to look at using Git instead.
16
17A frequently asked question on the Linux Kernel Mailing List is how to apply
18a patch to the kernel or, more specifically, what base kernel a patch for
19one of the many trees/branches should be applied to. Hopefully this document
20will explain this to you.
21
22In addition to explaining how to apply and revert patches, a brief
23description of the different kernel trees (and examples of how to apply
24their specific patches) is also provided.
25
26
27What is a patch?
28================
29
30A patch is a small text document containing a delta of changes between two
31different versions of a source tree. Patches are created with the ``diff``
32program.
33
34To correctly apply a patch you need to know what base it was generated from
35and what new version the patch will change the source tree into. These
36should both be present in the patch file metadata or be possible to deduce
37from the filename.
38
39
40How do I apply or revert a patch?
41=================================
42
43You apply a patch with the ``patch`` program. The patch program reads a diff
44(or patch) file and makes the changes to the source tree described in it.
45
46Patches for the Linux kernel are generated relative to the parent directory
47holding the kernel source dir.
48
49This means that paths to files inside the patch file contain the name of the
50kernel source directories it was generated against (or some other directory
51names like "a/" and "b/").
52
53Since this is unlikely to match the name of the kernel source dir on your
54local machine (but is often useful info to see what version an otherwise
55unlabeled patch was generated against) you should change into your kernel
56source directory and then strip the first element of the path from filenames
57in the patch file when applying it (the ``-p1`` argument to ``patch`` does
58this).
59
60To revert a previously applied patch, use the -R argument to patch.
61So, if you applied a patch like this::
62
63	patch -p1 < ../patch-x.y.z
64
65You can revert (undo) it like this::
66
67	patch -R -p1 < ../patch-x.y.z
68
69
70How do I feed a patch/diff file to ``patch``?
71=============================================
72
73This (as usual with Linux and other UNIX like operating systems) can be
74done in several different ways.
75
76In all the examples below I feed the file (in uncompressed form) to patch
77via stdin using the following syntax::
78
79	patch -p1 < path/to/patch-x.y.z
80
81If you just want to be able to follow the examples below and don't want to
82know of more than one way to use patch, then you can stop reading this
83section here.
84
85Patch can also get the name of the file to use via the -i argument, like
86this::
87
88	patch -p1 -i path/to/patch-x.y.z
89
90If your patch file is compressed with gzip or xz and you don't want to
91uncompress it before applying it, then you can feed it to patch like this
92instead::
93
94	xzcat path/to/patch-x.y.z.xz | patch -p1
95	bzcat path/to/patch-x.y.z.gz | patch -p1
96
97If you wish to uncompress the patch file by hand first before applying it
98(what I assume you've done in the examples below), then you simply run
99gunzip or xz on the file -- like this::
100
101	gunzip patch-x.y.z.gz
102	xz -d patch-x.y.z.xz
103
104Which will leave you with a plain text patch-x.y.z file that you can feed to
105patch via stdin or the ``-i`` argument, as you prefer.
106
107A few other nice arguments for patch are ``-s`` which causes patch to be silent
108except for errors which is nice to prevent errors from scrolling out of the
109screen too fast, and ``--dry-run`` which causes patch to just print a listing of
110what would happen, but doesn't actually make any changes. Finally ``--verbose``
111tells patch to print more information about the work being done.
112
113
114Common errors when patching
115===========================
116
117When patch applies a patch file it attempts to verify the sanity of the
118file in different ways.
119
120Checking that the file looks like a valid patch file and checking the code
121around the bits being modified matches the context provided in the patch are
122just two of the basic sanity checks patch does.
123
124If patch encounters something that doesn't look quite right it has two
125options. It can either refuse to apply the changes and abort or it can try
126to find a way to make the patch apply with a few minor changes.
127
128One example of something that's not 'quite right' that patch will attempt to
129fix up is if all the context matches, the lines being changed match, but the
130line numbers are different. This can happen, for example, if the patch makes
131a change in the middle of the file but for some reasons a few lines have
132been added or removed near the beginning of the file. In that case
133everything looks good it has just moved up or down a bit, and patch will
134usually adjust the line numbers and apply the patch.
135
136Whenever patch applies a patch that it had to modify a bit to make it fit
137it'll tell you about it by saying the patch applied with **fuzz**.
138You should be wary of such changes since even though patch probably got it
139right it doesn't /always/ get it right, and the result will sometimes be
140wrong.
141
142When patch encounters a change that it can't fix up with fuzz it rejects it
143outright and leaves a file with a ``.rej`` extension (a reject file). You can
144read this file to see exactly what change couldn't be applied, so you can
145go fix it up by hand if you wish.
146
147If you don't have any third-party patches applied to your kernel source, but
148only patches from kernel.org and you apply the patches in the correct order,
149and have made no modifications yourself to the source files, then you should
150never see a fuzz or reject message from patch. If you do see such messages
151anyway, then there's a high risk that either your local source tree or the
152patch file is corrupted in some way. In that case you should probably try
153re-downloading the patch and if things are still not OK then you'd be advised
154to start with a fresh tree downloaded in full from kernel.org.
155
156Let's look a bit more at some of the messages patch can produce.
157
158If patch stops and presents a ``File to patch:`` prompt, then patch could not
159find a file to be patched. Most likely you forgot to specify -p1 or you are
160in the wrong directory. Less often, you'll find patches that need to be
161applied with ``-p0`` instead of ``-p1`` (reading the patch file should reveal if
162this is the case -- if so, then this is an error by the person who created
163the patch but is not fatal).
164
165If you get ``Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines).`` or a
166message similar to that, then it means that patch had to adjust the location
167of the change (in this example it needed to move 7 lines from where it
168expected to make the change to make it fit).
169
170The resulting file may or may not be OK, depending on the reason the file
171was different than expected.
172
173This often happens if you try to apply a patch that was generated against a
174different kernel version than the one you are trying to patch.
175
176If you get a message like ``Hunk #3 FAILED at 2387.``, then it means that the
177patch could not be applied correctly and the patch program was unable to
178fuzz its way through. This will generate a ``.rej`` file with the change that
179caused the patch to fail and also a ``.orig`` file showing you the original
180content that couldn't be changed.
181
182If you get ``Reversed (or previously applied) patch detected!  Assume -R? [n]``
183then patch detected that the change contained in the patch seems to have
184already been made.
185
186If you actually did apply this patch previously and you just re-applied it
187in error, then just say [n]o and abort this patch. If you applied this patch
188previously and actually intended to revert it, but forgot to specify -R,
189then you can say [**y**]es here to make patch revert it for you.
190
191This can also happen if the creator of the patch reversed the source and
192destination directories when creating the patch, and in that case reverting
193the patch will in fact apply it.
194
195A message similar to ``patch: **** unexpected end of file in patch`` or
196``patch unexpectedly ends in middle of line`` means that patch could make no
197sense of the file you fed to it. Either your download is broken, you tried to
198feed patch a compressed patch file without uncompressing it first, or the patch
199file that you are using has been mangled by a mail client or mail transfer
200agent along the way somewhere, e.g., by splitting a long line into two lines.
201Often these warnings can easily be fixed by joining (concatenating) the
202two lines that had been split.
203
204As I already mentioned above, these errors should never happen if you apply
205a patch from kernel.org to the correct version of an unmodified source tree.
206So if you get these errors with kernel.org patches then you should probably
207assume that either your patch file or your tree is broken and I'd advise you
208to start over with a fresh download of a full kernel tree and the patch you
209wish to apply.
210
211
212Are there any alternatives to ``patch``?
213========================================
214
215
216Yes there are alternatives.
217
218You can use the ``interdiff`` program (http://cyberelk.net/tim/patchutils/) to
219generate a patch representing the differences between two patches and then
220apply the result.
221
222This will let you move from something like 4.7.2 to 4.7.3 in a single
223step. The -z flag to interdiff will even let you feed it patches in gzip or
224bzip2 compressed form directly without the use of zcat or bzcat or manual
225decompression.
226
227Here's how you'd go from 4.7.2 to 4.7.3 in a single step::
228
229	interdiff -z ../patch-4.7.2.gz ../patch-4.7.3.gz | patch -p1
230
231Although interdiff may save you a step or two you are generally advised to
232do the additional steps since interdiff can get things wrong in some cases.
233
234Another alternative is ``ketchup``, which is a python script for automatic
235downloading and applying of patches (http://www.selenic.com/ketchup/).
236
237Other nice tools are diffstat, which shows a summary of changes made by a
238patch; lsdiff, which displays a short listing of affected files in a patch
239file, along with (optionally) the line numbers of the start of each patch;
240and grepdiff, which displays a list of the files modified by a patch where
241the patch contains a given regular expression.
242
243
244Where can I download the patches?
245=================================
246
247The patches are available at http://kernel.org/
248Most recent patches are linked from the front page, but they also have
249specific homes.
250
251The 4.x.y (-stable) and 4.x patches live at
252
253	https://www.kernel.org/pub/linux/kernel/v4.x/
254
255The -rc patches live at
256
257	https://www.kernel.org/pub/linux/kernel/v4.x/testing/
258
259
260The 4.x kernels
261===============
262
263These are the base stable releases released by Linus. The highest numbered
264release is the most recent.
265
266If regressions or other serious flaws are found, then a -stable fix patch
267will be released (see below) on top of this base. Once a new 4.x base
268kernel is released, a patch is made available that is a delta between the
269previous 4.x kernel and the new one.
270
271To apply a patch moving from 4.6 to 4.7, you'd do the following (note
272that such patches do **NOT** apply on top of 4.x.y kernels but on top of the
273base 4.x kernel -- if you need to move from 4.x.y to 4.x+1 you need to
274first revert the 4.x.y patch).
275
276Here are some examples::
277
278	# moving from 4.6 to 4.7
279
280	$ cd ~/linux-4.6		# change to kernel source dir
281	$ patch -p1 < ../patch-4.7	# apply the 4.7 patch
282	$ cd ..
283	$ mv linux-4.6 linux-4.7	# rename source dir
284
285	# moving from 4.6.1 to 4.7
286
287	$ cd ~/linux-4.6.1		# change to kernel source dir
288	$ patch -p1 -R < ../patch-4.6.1	# revert the 4.6.1 patch
289					# source dir is now 4.6
290	$ patch -p1 < ../patch-4.7	# apply new 4.7 patch
291	$ cd ..
292	$ mv linux-4.6.1 linux-4.7	# rename source dir
293
294
295The 4.x.y kernels
296=================
297
298Kernels with 3-digit versions are -stable kernels. They contain small(ish)
299critical fixes for security problems or significant regressions discovered
300in a given 4.x kernel.
301
302This is the recommended branch for users who want the most recent stable
303kernel and are not interested in helping test development/experimental
304versions.
305
306If no 4.x.y kernel is available, then the highest numbered 4.x kernel is
307the current stable kernel.
308
309.. note::
310
311 The -stable team usually do make incremental patches available as well
312 as patches against the latest mainline release, but I only cover the
313 non-incremental ones below. The incremental ones can be found at
314 https://www.kernel.org/pub/linux/kernel/v4.x/incr/
315
316These patches are not incremental, meaning that for example the 4.7.3
317patch does not apply on top of the 4.7.2 kernel source, but rather on top
318of the base 4.7 kernel source.
319
320So, in order to apply the 4.7.3 patch to your existing 4.7.2 kernel
321source you have to first back out the 4.7.2 patch (so you are left with a
322base 4.7 kernel source) and then apply the new 4.7.3 patch.
323
324Here's a small example::
325
326	$ cd ~/linux-4.7.2		# change to the kernel source dir
327	$ patch -p1 -R < ../patch-4.7.2	# revert the 4.7.2 patch
328	$ patch -p1 < ../patch-4.7.3	# apply the new 4.7.3 patch
329	$ cd ..
330	$ mv linux-4.7.2 linux-4.7.3	# rename the kernel source dir
331
332The -rc kernels
333===============
334
335These are release-candidate kernels. These are development kernels released
336by Linus whenever he deems the current git (the kernel's source management
337tool) tree to be in a reasonably sane state adequate for testing.
338
339These kernels are not stable and you should expect occasional breakage if
340you intend to run them. This is however the most stable of the main
341development branches and is also what will eventually turn into the next
342stable kernel, so it is important that it be tested by as many people as
343possible.
344
345This is a good branch to run for people who want to help out testing
346development kernels but do not want to run some of the really experimental
347stuff (such people should see the sections about -git and -mm kernels below).
348
349The -rc patches are not incremental, they apply to a base 4.x kernel, just
350like the 4.x.y patches described above. The kernel version before the -rcN
351suffix denotes the version of the kernel that this -rc kernel will eventually
352turn into.
353
354So, 4.8-rc5 means that this is the fifth release candidate for the 4.8
355kernel and the patch should be applied on top of the 4.7 kernel source.
356
357Here are 3 examples of how to apply these patches::
358
359	# first an example of moving from 4.7 to 4.8-rc3
360
361	$ cd ~/linux-4.7			# change to the 4.7 source dir
362	$ patch -p1 < ../patch-4.8-rc3		# apply the 4.8-rc3 patch
363	$ cd ..
364	$ mv linux-4.7 linux-4.8-rc3		# rename the source dir
365
366	# now let's move from 4.8-rc3 to 4.8-rc5
367
368	$ cd ~/linux-4.8-rc3			# change to the 4.8-rc3 dir
369	$ patch -p1 -R < ../patch-4.8-rc3	# revert the 4.8-rc3 patch
370	$ patch -p1 < ../patch-4.8-rc5		# apply the new 4.8-rc5 patch
371	$ cd ..
372	$ mv linux-4.8-rc3 linux-4.8-rc5	# rename the source dir
373
374	# finally let's try and move from 4.7.3 to 4.8-rc5
375
376	$ cd ~/linux-4.7.3			# change to the kernel source dir
377	$ patch -p1 -R < ../patch-4.7.3		# revert the 4.7.3 patch
378	$ patch -p1 < ../patch-4.8-rc5		# apply new 4.8-rc5 patch
379	$ cd ..
380	$ mv linux-4.7.3 linux-4.8-rc5		# rename the kernel source dir
381
382
383The -git kernels
384================
385
386These are daily snapshots of Linus' kernel tree (managed in a git
387repository, hence the name).
388
389These patches are usually released daily and represent the current state of
390Linus's tree. They are more experimental than -rc kernels since they are
391generated automatically without even a cursory glance to see if they are
392sane.
393
394-git patches are not incremental and apply either to a base 4.x kernel or
395a base 4.x-rc kernel -- you can see which from their name.
396A patch named 4.7-git1 applies to the 4.7 kernel source and a patch
397named 4.8-rc3-git2 applies to the source of the 4.8-rc3 kernel.
398
399Here are some examples of how to apply these patches::
400
401	# moving from 4.7 to 4.7-git1
402
403	$ cd ~/linux-4.7			# change to the kernel source dir
404	$ patch -p1 < ../patch-4.7-git1		# apply the 4.7-git1 patch
405	$ cd ..
406	$ mv linux-4.7 linux-4.7-git1		# rename the kernel source dir
407
408	# moving from 4.7-git1 to 4.8-rc2-git3
409
410	$ cd ~/linux-4.7-git1			# change to the kernel source dir
411	$ patch -p1 -R < ../patch-4.7-git1	# revert the 4.7-git1 patch
412						# we now have a 4.7 kernel
413	$ patch -p1 < ../patch-4.8-rc2		# apply the 4.8-rc2 patch
414						# the kernel is now 4.8-rc2
415	$ patch -p1 < ../patch-4.8-rc2-git3	# apply the 4.8-rc2-git3 patch
416						# the kernel is now 4.8-rc2-git3
417	$ cd ..
418	$ mv linux-4.7-git1 linux-4.8-rc2-git3	# rename source dir
419
420
421The -mm patches and the linux-next tree
422=======================================
423
424The -mm patches are experimental patches released by Andrew Morton.
425
426In the past, -mm tree were used to also test subsystem patches, but this
427function is now done via the
428`linux-next <https://www.kernel.org/doc/man-pages/linux-next.html>`
429tree. The Subsystem maintainers push their patches first to linux-next,
430and, during the merge window, sends them directly to Linus.
431
432The -mm patches serve as a sort of proving ground for new features and other
433experimental patches that aren't merged via a subsystem tree.
434Once such patches has proved its worth in -mm for a while Andrew pushes
435it on to Linus for inclusion in mainline.
436
437The linux-next tree is daily updated, and includes the -mm patches.
438Both are in constant flux and contains many experimental features, a
439lot of debugging patches not appropriate for mainline etc., and is the most
440experimental of the branches described in this document.
441
442These patches are not appropriate for use on systems that are supposed to be
443stable and they are more risky to run than any of the other branches (make
444sure you have up-to-date backups -- that goes for any experimental kernel but
445even more so for -mm patches or using a Kernel from the linux-next tree).
446
447Testing of -mm patches and linux-next is greatly appreciated since the whole
448point of those are to weed out regressions, crashes, data corruption bugs,
449build breakage (and any other bug in general) before changes are merged into
450the more stable mainline Linus tree.
451
452But testers of -mm and linux-next should be aware that breakages are
453more common than in any other tree.
454
455
456This concludes this list of explanations of the various kernel trees.
457I hope you are now clear on how to apply the various patches and help testing
458the kernel.
459
460Thank you's to Randy Dunlap, Rolf Eike Beer, Linus Torvalds, Bodo Eggert,
461Johannes Stezenbach, Grant Coady, Pavel Machek and others that I may have
462forgotten for their reviews and contributions to this document.
463