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1	.. _applying_patches:
2
3	Applying Patches To The Linux Kernel
4	++++++++++++++++++++++++++++++++++++
5
6	Original by:
7	Jesper Juhl, August 2005
8
9	Last update:
10	2016-09-14
11
12
13	A frequently asked question on the Linux Kernel Mailing List is how to apply
14	a patch to the kernel or, more specifically, what base kernel a patch for
15	one of the many trees/branches should be applied to. Hopefully this document
16	will explain this to you.
17
18	In addition to explaining how to apply and revert patches, a brief
19	description of the different kernel trees (and examples of how to apply
20	their specific patches) is also provided.
21
22
23	What is a patch?
24	================
25
26	A patch is a small text document containing a delta of changes between two
27	different versions of a source tree. Patches are created with the ``diff``
28	program.
29
30	To correctly apply a patch you need to know what base it was generated from
31	and what new version the patch will change the source tree into. These
32	should both be present in the patch file metadata or be possible to deduce
33	from the filename.
34
35
36	How do I apply or revert a patch?
37	=================================
38
39	You apply a patch with the ``patch`` program. The patch program reads a diff
40	(or patch) file and makes the changes to the source tree described in it.
41
42	Patches for the Linux kernel are generated relative to the parent directory
43	holding the kernel source dir.
44
45	This means that paths to files inside the patch file contain the name of the
46	kernel source directories it was generated against (or some other directory
47	names like "a/" and "b/").
48
49	Since this is unlikely to match the name of the kernel source dir on your
50	local machine (but is often useful info to see what version an otherwise
51	unlabeled patch was generated against) you should change into your kernel
52	source directory and then strip the first element of the path from filenames
53	in the patch file when applying it (the ``-p1`` argument to ``patch`` does
54	this).
55
56	To revert a previously applied patch, use the -R argument to patch.
57	So, if you applied a patch like this::
58
59	patch -p1 < ../patch-x.y.z
60
61	You can revert (undo) it like this::
62
63	patch -R -p1 < ../patch-x.y.z
64
65
66	How do I feed a patch/diff file to ``patch``?
67	=============================================
68
69	This (as usual with Linux and other UNIX like operating systems) can be
70	done in several different ways.
71
72	In all the examples below I feed the file (in uncompressed form) to patch
73	via stdin using the following syntax::
74
75	patch -p1 < path/to/patch-x.y.z
76
77	If you just want to be able to follow the examples below and don't want to
78	know of more than one way to use patch, then you can stop reading this
79	section here.
80
81	Patch can also get the name of the file to use via the -i argument, like
82	this::
83
84	patch -p1 -i path/to/patch-x.y.z
85
86	If your patch file is compressed with gzip or xz and you don't want to
87	uncompress it before applying it, then you can feed it to patch like this
88	instead::
89
90	xzcat path/to/patch-x.y.z.xz | patch -p1
91	bzcat path/to/patch-x.y.z.gz | patch -p1
92
93	If you wish to uncompress the patch file by hand first before applying it
94	(what I assume you've done in the examples below), then you simply run
95	gunzip or xz on the file -- like this::
96
97	gunzip patch-x.y.z.gz
98	xz -d patch-x.y.z.xz
99
100	Which will leave you with a plain text patch-x.y.z file that you can feed to
101	patch via stdin or the ``-i`` argument, as you prefer.
102
103	A few other nice arguments for patch are ``-s`` which causes patch to be silent
104	except for errors which is nice to prevent errors from scrolling out of the
105	screen too fast, and ``--dry-run`` which causes patch to just print a listing of
106	what would happen, but doesn't actually make any changes. Finally ``--verbose``
107	tells patch to print more information about the work being done.
108
109
110	Common errors when patching
111	===========================
112
113	When patch applies a patch file it attempts to verify the sanity of the
114	file in different ways.
115
116	Checking that the file looks like a valid patch file and checking the code
117	around the bits being modified matches the context provided in the patch are
118	just two of the basic sanity checks patch does.
119
120	If patch encounters something that doesn't look quite right it has two
121	options. It can either refuse to apply the changes and abort or it can try
122	to find a way to make the patch apply with a few minor changes.
123
124	One example of something that's not 'quite right' that patch will attempt to
125	fix up is if all the context matches, the lines being changed match, but the
126	line numbers are different. This can happen, for example, if the patch makes
127	a change in the middle of the file but for some reasons a few lines have
128	been added or removed near the beginning of the file. In that case
129	everything looks good it has just moved up or down a bit, and patch will
130	usually adjust the line numbers and apply the patch.
131
132	Whenever patch applies a patch that it had to modify a bit to make it fit
133	it'll tell you about it by saying the patch applied with **fuzz**.
134	You should be wary of such changes since even though patch probably got it
135	right it doesn't /always/ get it right, and the result will sometimes be
136	wrong.
137
138	When patch encounters a change that it can't fix up with fuzz it rejects it
139	outright and leaves a file with a ``.rej`` extension (a reject file). You can
140	read this file to see exactly what change couldn't be applied, so you can
141	go fix it up by hand if you wish.
142
143	If you don't have any third-party patches applied to your kernel source, but
144	only patches from kernel.org and you apply the patches in the correct order,
145	and have made no modifications yourself to the source files, then you should
146	never see a fuzz or reject message from patch. If you do see such messages
147	anyway, then there's a high risk that either your local source tree or the
148	patch file is corrupted in some way. In that case you should probably try
149	re-downloading the patch and if things are still not OK then you'd be advised
150	to start with a fresh tree downloaded in full from kernel.org.
151
152	Let's look a bit more at some of the messages patch can produce.
153
154	If patch stops and presents a ``File to patch:`` prompt, then patch could not
155	find a file to be patched. Most likely you forgot to specify -p1 or you are
156	in the wrong directory. Less often, you'll find patches that need to be
157	applied with ``-p0`` instead of ``-p1`` (reading the patch file should reveal if
158	this is the case -- if so, then this is an error by the person who created
159	the patch but is not fatal).
160
161	If you get ``Hunk #2 succeeded at 1887 with fuzz 2 (offset 7 lines).`` or a
162	message similar to that, then it means that patch had to adjust the location
163	of the change (in this example it needed to move 7 lines from where it
164	expected to make the change to make it fit).
165
166	The resulting file may or may not be OK, depending on the reason the file
167	was different than expected.
168
169	This often happens if you try to apply a patch that was generated against a
170	different kernel version than the one you are trying to patch.
171
172	If you get a message like ``Hunk #3 FAILED at 2387.``, then it means that the
173	patch could not be applied correctly and the patch program was unable to
174	fuzz its way through. This will generate a ``.rej`` file with the change that
175	caused the patch to fail and also a ``.orig`` file showing you the original
176	content that couldn't be changed.
177
178	If you get ``Reversed (or previously applied) patch detected! Assume -R? [n]``
179	then patch detected that the change contained in the patch seems to have
180	already been made.
181
182	If you actually did apply this patch previously and you just re-applied it
183	in error, then just say [n]o and abort this patch. If you applied this patch
184	previously and actually intended to revert it, but forgot to specify -R,
185	then you can say [**y**]es here to make patch revert it for you.
186
187	This can also happen if the creator of the patch reversed the source and
188	destination directories when creating the patch, and in that case reverting
189	the patch will in fact apply it.
190
191	A message similar to ``patch: **** unexpected end of file in patch`` or
192	``patch unexpectedly ends in middle of line`` means that patch could make no
193	sense of the file you fed to it. Either your download is broken, you tried to
194	feed patch a compressed patch file without uncompressing it first, or the patch
195	file that you are using has been mangled by a mail client or mail transfer
196	agent along the way somewhere, e.g., by splitting a long line into two lines.
197	Often these warnings can easily be fixed by joining (concatenating) the
198	two lines that had been split.
199
200	As I already mentioned above, these errors should never happen if you apply
201	a patch from kernel.org to the correct version of an unmodified source tree.
202	So if you get these errors with kernel.org patches then you should probably
203	assume that either your patch file or your tree is broken and I'd advise you
204	to start over with a fresh download of a full kernel tree and the patch you
205	wish to apply.
206
207
208	Are there any alternatives to ``patch``?
209	========================================
210
211
212	Yes there are alternatives.
213
214	You can use the ``interdiff`` program (http://cyberelk.net/tim/patchutils/) to
215	generate a patch representing the differences between two patches and then
216	apply the result.
217
218	This will let you move from something like 4.7.2 to 4.7.3 in a single
219	step. The -z flag to interdiff will even let you feed it patches in gzip or
220	bzip2 compressed form directly without the use of zcat or bzcat or manual
221	decompression.
222
223	Here's how you'd go from 4.7.2 to 4.7.3 in a single step::
224
225	interdiff -z ../patch-4.7.2.gz ../patch-4.7.3.gz | patch -p1
226
227	Although interdiff may save you a step or two you are generally advised to
228	do the additional steps since interdiff can get things wrong in some cases.
229
230	Another alternative is ``ketchup``, which is a python script for automatic
231	downloading and applying of patches (http://www.selenic.com/ketchup/).
232
233	Other nice tools are diffstat, which shows a summary of changes made by a
234	patch; lsdiff, which displays a short listing of affected files in a patch
235	file, along with (optionally) the line numbers of the start of each patch;
236	and grepdiff, which displays a list of the files modified by a patch where
237	the patch contains a given regular expression.
238
239
240	Where can I download the patches?
241	=================================
242
243	The patches are available at http://kernel.org/
244	Most recent patches are linked from the front page, but they also have
245	specific homes.
246
247	The 4.x.y (-stable) and 4.x patches live at
248
249	ftp://ftp.kernel.org/pub/linux/kernel/v4.x/
250
251	The -rc patches live at
252
253	ftp://ftp.kernel.org/pub/linux/kernel/v4.x/testing/
254
255	In place of ``ftp.kernel.org`` you can use ``ftp.cc.kernel.org``, where cc is a
256	country code. This way you'll be downloading from a mirror site that's most
257	likely geographically closer to you, resulting in faster downloads for you,
258	less bandwidth used globally and less load on the main kernel.org servers --
259	these are good things, so do use mirrors when possible.
260
261
262	The 4.x kernels
263	===============
264
265	These are the base stable releases released by Linus. The highest numbered
266	release is the most recent.
267
268	If regressions or other serious flaws are found, then a -stable fix patch
269	will be released (see below) on top of this base. Once a new 4.x base
270	kernel is released, a patch is made available that is a delta between the
271	previous 4.x kernel and the new one.
272
273	To apply a patch moving from 4.6 to 4.7, you'd do the following (note
274	that such patches do **NOT** apply on top of 4.x.y kernels but on top of the
275	base 4.x kernel -- if you need to move from 4.x.y to 4.x+1 you need to
276	first revert the 4.x.y patch).
277
278	Here are some examples::
279
280	# moving from 4.6 to 4.7
281
282	$ cd ~/linux-4.6 # change to kernel source dir
283	$ patch -p1 < ../patch-4.7 # apply the 4.7 patch
284	$ cd ..
285	$ mv linux-4.6 linux-4.7 # rename source dir
286
287	# moving from 4.6.1 to 4.7
288
289	$ cd ~/linux-4.6.1 # change to kernel source dir
290	$ patch -p1 -R < ../patch-4.6.1 # revert the 4.6.1 patch
291	# source dir is now 4.6
292	$ patch -p1 < ../patch-4.7 # apply new 4.7 patch
293	$ cd ..
294	$ mv linux-4.6.1 linux-4.7 # rename source dir
295
296
297	The 4.x.y kernels
298	=================
299
300	Kernels with 3-digit versions are -stable kernels. They contain small(ish)
301	critical fixes for security problems or significant regressions discovered
302	in a given 4.x kernel.
303
304	This is the recommended branch for users who want the most recent stable
305	kernel and are not interested in helping test development/experimental
306	versions.
307
308	If no 4.x.y kernel is available, then the highest numbered 4.x kernel is
309	the 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	ftp://ftp.kernel.org/pub/linux/kernel/v4.x/incr/
317
318	These patches are not incremental, meaning that for example the 4.7.3
319	patch does not apply on top of the 4.7.2 kernel source, but rather on top
320	of the base 4.7 kernel source.
321
322	So, in order to apply the 4.7.3 patch to your existing 4.7.2 kernel
323	source you have to first back out the 4.7.2 patch (so you are left with a
324	base 4.7 kernel source) and then apply the new 4.7.3 patch.
325
326	Here's a small example::
327
328	$ cd ~/linux-4.7.2 # change to the kernel source dir
329	$ patch -p1 -R < ../patch-4.7.2 # revert the 4.7.2 patch
330	$ patch -p1 < ../patch-4.7.3 # apply the new 4.7.3 patch
331	$ cd ..
332	$ mv linux-4.7.2 linux-4.7.3 # rename the kernel source dir
333
334	The -rc kernels
335	===============
336
337	These are release-candidate kernels. These are development kernels released
338	by Linus whenever he deems the current git (the kernel's source management
339	tool) tree to be in a reasonably sane state adequate for testing.
340
341	These kernels are not stable and you should expect occasional breakage if
342	you intend to run them. This is however the most stable of the main
343	development branches and is also what will eventually turn into the next
344	stable kernel, so it is important that it be tested by as many people as
345	possible.
346
347	This is a good branch to run for people who want to help out testing
348	development kernels but do not want to run some of the really experimental
349	stuff (such people should see the sections about -git and -mm kernels below).
350
351	The -rc patches are not incremental, they apply to a base 4.x kernel, just
352	like the 4.x.y patches described above. The kernel version before the -rcN
353	suffix denotes the version of the kernel that this -rc kernel will eventually
354	turn into.
355
356	So, 4.8-rc5 means that this is the fifth release candidate for the 4.8
357	kernel and the patch should be applied on top of the 4.7 kernel source.
358
359	Here are 3 examples of how to apply these patches::
360
361	# first an example of moving from 4.7 to 4.8-rc3
362
363	$ cd ~/linux-4.7 # change to the 4.7 source dir
364	$ patch -p1 < ../patch-4.8-rc3 # apply the 4.8-rc3 patch
365	$ cd ..
366	$ mv linux-4.7 linux-4.8-rc3 # rename the source dir
367
368	# now let's move from 4.8-rc3 to 4.8-rc5
369
370	$ cd ~/linux-4.8-rc3 # change to the 4.8-rc3 dir
371	$ patch -p1 -R < ../patch-4.8-rc3 # revert the 4.8-rc3 patch
372	$ patch -p1 < ../patch-4.8-rc5 # apply the new 4.8-rc5 patch
373	$ cd ..
374	$ mv linux-4.8-rc3 linux-4.8-rc5 # rename the source dir
375
376	# finally let's try and move from 4.7.3 to 4.8-rc5
377
378	$ cd ~/linux-4.7.3 # change to the kernel source dir
379	$ patch -p1 -R < ../patch-4.7.3 # revert the 4.7.3 patch
380	$ patch -p1 < ../patch-4.8-rc5 # apply new 4.8-rc5 patch
381	$ cd ..
382	$ mv linux-4.7.3 linux-4.8-rc5 # rename the kernel source dir
383
384
385	The -git kernels
386	================
387
388	These are daily snapshots of Linus' kernel tree (managed in a git
389	repository, hence the name).
390
391	These patches are usually released daily and represent the current state of
392	Linus's tree. They are more experimental than -rc kernels since they are
393	generated automatically without even a cursory glance to see if they are
394	sane.
395
396	-git patches are not incremental and apply either to a base 4.x kernel or
397	a base 4.x-rc kernel -- you can see which from their name.
398	A patch named 4.7-git1 applies to the 4.7 kernel source and a patch
399	named 4.8-rc3-git2 applies to the source of the 4.8-rc3 kernel.
400
401	Here are some examples of how to apply these patches::
402
403	# moving from 4.7 to 4.7-git1
404
405	$ cd ~/linux-4.7 # change to the kernel source dir
406	$ patch -p1 < ../patch-4.7-git1 # apply the 4.7-git1 patch
407	$ cd ..
408	$ mv linux-4.7 linux-4.7-git1 # rename the kernel source dir
409
410	# moving from 4.7-git1 to 4.8-rc2-git3
411
412	$ cd ~/linux-4.7-git1 # change to the kernel source dir
413	$ patch -p1 -R < ../patch-4.7-git1 # revert the 4.7-git1 patch
414	# we now have a 4.7 kernel
415	$ patch -p1 < ../patch-4.8-rc2 # apply the 4.8-rc2 patch
416	# the kernel is now 4.8-rc2
417	$ patch -p1 < ../patch-4.8-rc2-git3 # apply the 4.8-rc2-git3 patch
418	# the kernel is now 4.8-rc2-git3
419	$ cd ..
420	$ mv linux-4.7-git1 linux-4.8-rc2-git3 # rename source dir
421
422
423	The -mm patches and the linux-next tree
424	=======================================
425
426	The -mm patches are experimental patches released by Andrew Morton.
427
428	In the past, -mm tree were used to also test subsystem patches, but this
429	function is now done via the
430	:ref:`linux-next <https://www.kernel.org/doc/man-pages/linux-next.html>`
431	tree. The Subsystem maintainers push their patches first to linux-next,
432	and, during the merge window, sends them directly to Linus.
433
434	The -mm patches serve as a sort of proving ground for new features and other
435	experimental patches that aren't merged via a subsystem tree.
436	Once such patches has proved its worth in -mm for a while Andrew pushes
437	it on to Linus for inclusion in mainline.
438
439	The linux-next tree is daily updated, and includes the -mm patches.
440	Both are in constant flux and contains many experimental features, a
441	lot of debugging patches not appropriate for mainline etc., and is the most
442	experimental of the branches described in this document.
443
444	These patches are not appropriate for use on systems that are supposed to be
445	stable and they are more risky to run than any of the other branches (make
446	sure you have up-to-date backups -- that goes for any experimental kernel but
447	even more so for -mm patches or using a Kernel from the linux-next tree).
448
449	Testing of -mm patches and linux-next is greatly appreciated since the whole
450	point of those are to weed out regressions, crashes, data corruption bugs,
451	build breakage (and any other bug in general) before changes are merged into
452	the more stable mainline Linus tree.
453
454	But testers of -mm and linux-next should be aware that breakages are
455	more common than in any other tree.
456
457
458	This concludes this list of explanations of the various kernel trees.
459	I hope you are now clear on how to apply the various patches and help testing
460	the kernel.
461
462	Thank you's to Randy Dunlap, Rolf Eike Beer, Linus Torvalds, Bodo Eggert,
463	Johannes Stezenbach, Grant Coady, Pavel Machek and others that I may have
464	forgotten for their reviews and contributions to this document.
465
466