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1.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
2.. [see the bottom of this file for redistribution information]
3
4======================
5Bisecting a regression
6======================
7
8This document describes how to use a ``git bisect`` to find the source code
9change that broke something -- for example when some functionality stopped
10working after upgrading from Linux 6.0 to 6.1.
11
12The text focuses on the gist of the process. If you are new to bisecting the
13kernel, better follow Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst
14instead: it depicts everything from start to finish while covering multiple
15aspects even kernel developers occasionally forget. This includes detecting
16situations early where a bisection would be a waste of time, as nobody would
17care about the result -- for example, because the problem happens after the
18kernel marked itself as 'tainted', occurs in an abandoned version, was already
19fixed, or is caused by a .config change you or your Linux distributor performed.
20
21Finding the change causing a kernel issue using a bisection
22===========================================================
23
24*Note: the following process assumes you prepared everything for a bisection.
25This includes having a Git clone with the appropriate sources, installing the
26software required to build and install kernels, as well as a .config file stored
27in a safe place (the following example assumes '~/prepared_kernel_.config') to
28use as pristine base at each bisection step; ideally, you have also worked out
29a fully reliable and straight-forward way to reproduce the regression, too.*
30
31* Preparation: start the bisection and tell Git about the points in the history
32 you consider to be working and broken, which Git calls 'good' and 'bad'::
33
34 git bisect start
35 git bisect good v6.0
36 git bisect bad v6.1
37
38 Instead of Git tags like 'v6.0' and 'v6.1' you can specify commit-ids, too.
39
401. Copy your prepared .config into the build directory and adjust it to the
41 needs of the codebase Git checked out for testing::
42
43 cp ~/prepared_kernel_.config .config
44 make olddefconfig
45
462. Now build, install, and boot a kernel. This might fail for unrelated reasons,
47 for example, when a compile error happens at the current stage of the
48 bisection a later change resolves. In such cases run ``git bisect skip`` and
49 go back to step 1.
50
513. Check if the functionality that regressed works in the kernel you just built.
52
53 If it works, execute::
54
55 git bisect good
56
57 If it is broken, run::
58
59 git bisect bad
60
61 Note, getting this wrong just once will send the rest of the bisection
62 totally off course. To prevent having to start anew later you thus want to
63 ensure what you tell Git is correct; it is thus often wise to spend a few
64 minutes more on testing in case your reproducer is unreliable.
65
66 After issuing one of these two commands, Git will usually check out another
67 bisection point and print something like 'Bisecting: 675 revisions left to
68 test after this (roughly 10 steps)'. In that case go back to step 1.
69
70 If Git instead prints something like 'cafecaca0c0dacafecaca0c0dacafecaca0c0da
71 is the first bad commit', then you have finished the bisection. In that case
72 move to the next point below. Note, right after displaying that line Git will
73 show some details about the culprit including its patch description; this can
74 easily fill your terminal, so you might need to scroll up to see the message
75 mentioning the culprit's commit-id.
76
77 In case you missed Git's output, you can always run ``git bisect log`` to
78 print the status: it will show how many steps remain or mention the result of
79 the bisection.
80
81* Recommended complementary task: put the bisection log and the current .config
82 file aside for the bug report; furthermore tell Git to reset the sources to
83 the state before the bisection::
84
85 git bisect log > ~/bisection-log
86 cp .config ~/bisection-config-culprit
87 git bisect reset
88
89* Recommended optional task: try reverting the culprit on top of the latest
90 codebase and check if that fixes your bug; if that is the case, it validates
91 the bisection and enables developers to resolve the regression through a
92 revert.
93
94 To try this, update your clone and check out latest mainline. Then tell Git
95 to revert the change by specifying its commit-id::
96
97 git revert --no-edit cafec0cacaca0
98
99 Git might reject this, for example when the bisection landed on a merge
100 commit. In that case, abandon the attempt. Do the same, if Git fails to revert
101 the culprit on its own because later changes depend on it -- at least unless
102 you bisected a stable or longterm kernel series, in which case you want to
103 check out its latest codebase and try a revert there.
104
105 If a revert succeeds, build and test another kernel to check if reverting
106 resolved your regression.
107
108With that the process is complete. Now report the regression as described by
109Documentation/admin-guide/reporting-issues.rst.
110
111Bisecting linux-next
112--------------------
113
114If you face a problem only happening in linux-next, bisect between the
115linux-next branches 'stable' and 'master'. The following commands will start
116the process for a linux-next tree you added as a remote called 'next'::
117
118 git bisect start
119 git bisect good next/stable
120 git bisect bad next/master
121
122The 'stable' branch refers to the state of linux-mainline that the current
123linux-next release (found in the 'master' branch) is based on -- the former
124thus should be free of any problems that show up in -next, but not in Linus'
125tree.
126
127This will bisect across a wide range of changes, some of which you might have
128used in earlier linux-next releases without problems. Sadly there is no simple
129way to avoid checking them: bisecting from one linux-next release to a later
130one (say between 'next-20241020' and 'next-20241021') is impossible, as they
131share no common history.
132
133Additional reading material
134---------------------------
135
136* The `man page for 'git bisect' <https://git-scm.com/docs/git-bisect>`_ and
137 `fighting regressions with 'git bisect' <https://git-scm.com/docs/git-bisect-lk2009.html>`_
138 in the Git documentation.
139* `Working with git bisect <https://nathanchance.dev/posts/working-with-git-bisect/>`_
140 from kernel developer Nathan Chancellor.
141* `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_.
142* `Fully automated bisecting with 'git bisect run' <https://lwn.net/Articles/317154>`_.
143
144..
145 end-of-content
146..
147 This document is maintained by Thorsten Leemhuis <[email protected]>. If
148 you spot a typo or small mistake, feel free to let him know directly and
149 he'll fix it. You are free to do the same in a mostly informal way if you
150 want to contribute changes to the text -- but for copyright reasons please CC
151 [email protected] and 'sign-off' your contribution as
152 Documentation/process/submitting-patches.rst explains in the section 'Sign
153 your work - the Developer's Certificate of Origin'.
154..
155 This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
156 of the file. If you want to distribute this text under CC-BY-4.0 only,
157 please use 'The Linux kernel development community' for author attribution
158 and link this as source:
159 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/bug-bisect.rst
160
161..
162 Note: Only the content of this RST file as found in the Linux kernel sources
163 is available under CC-BY-4.0, as versions of this text that were processed
164 (for example by the kernel's build system) might contain content taken from
165 files which use a more restrictive license.