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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 regression6======================7 8This document describes how to use a ``git bisect`` to find the source code9change that broke something -- for example when some functionality stopped10working 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 the13kernel, better follow Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst14instead: it depicts everything from start to finish while covering multiple15aspects even kernel developers occasionally forget. This includes detecting16situations early where a bisection would be a waste of time, as nobody would17care about the result -- for example, because the problem happens after the18kernel marked itself as 'tainted', occurs in an abandoned version, was already19fixed, or is caused by a .config change you or your Linux distributor performed.20 21Finding the change causing a kernel issue using a bisection22===========================================================23 24*Note: the following process assumes you prepared everything for a bisection.25This includes having a Git clone with the appropriate sources, installing the26software required to build and install kernels, as well as a .config file stored27in a safe place (the following example assumes '~/prepared_kernel_.config') to28use as pristine base at each bisection step; ideally, you have also worked out29a 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 history32 you consider to be working and broken, which Git calls 'good' and 'bad'::33 34 git bisect start35 git bisect good v6.036 git bisect bad v6.137 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 the41 needs of the codebase Git checked out for testing::42 43 cp ~/prepared_kernel_.config .config44 make olddefconfig45 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 the48 bisection a later change resolves. In such cases run ``git bisect skip`` and49 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 good56 57 If it is broken, run::58 59 git bisect bad60 61 Note, getting this wrong just once will send the rest of the bisection62 totally off course. To prevent having to start anew later you thus want to63 ensure what you tell Git is correct; it is thus often wise to spend a few64 minutes more on testing in case your reproducer is unreliable.65 66 After issuing one of these two commands, Git will usually check out another67 bisection point and print something like 'Bisecting: 675 revisions left to68 test after this (roughly 10 steps)'. In that case go back to step 1.69 70 If Git instead prints something like 'cafecaca0c0dacafecaca0c0dacafecaca0c0da71 is the first bad commit', then you have finished the bisection. In that case72 move to the next point below. Note, right after displaying that line Git will73 show some details about the culprit including its patch description; this can74 easily fill your terminal, so you might need to scroll up to see the message75 mentioning the culprit's commit-id.76 77 In case you missed Git's output, you can always run ``git bisect log`` to78 print the status: it will show how many steps remain or mention the result of79 the bisection.80 81* Recommended complementary task: put the bisection log and the current .config82 file aside for the bug report; furthermore tell Git to reset the sources to83 the state before the bisection::84 85 git bisect log > ~/bisection-log86 cp .config ~/bisection-config-culprit87 git bisect reset88 89* Recommended optional task: try reverting the culprit on top of the latest90 codebase and check if that fixes your bug; if that is the case, it validates91 the bisection and enables developers to resolve the regression through a92 revert.93 94 To try this, update your clone and check out latest mainline. Then tell Git95 to revert the change by specifying its commit-id::96 97 git revert --no-edit cafec0cacaca098 99 Git might reject this, for example when the bisection landed on a merge100 commit. In that case, abandon the attempt. Do the same, if Git fails to revert101 the culprit on its own because later changes depend on it -- at least unless102 you bisected a stable or longterm kernel series, in which case you want to103 check out its latest codebase and try a revert there.104 105 If a revert succeeds, build and test another kernel to check if reverting106 resolved your regression.107 108With that the process is complete. Now report the regression as described by109Documentation/admin-guide/reporting-issues.rst.110 111 112Additional reading material113---------------------------114 115* The `man page for 'git bisect' <https://git-scm.com/docs/git-bisect>`_ and116 `fighting regressions with 'git bisect' <https://git-scm.com/docs/git-bisect-lk2009.html>`_117 in the Git documentation.118* `Working with git bisect <https://nathanchance.dev/posts/working-with-git-bisect/>`_119 from kernel developer Nathan Chancellor.120* `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_.121* `Fully automated bisecting with 'git bisect run' <https://lwn.net/Articles/317154>`_.122 123..124 end-of-content125..126 This document is maintained by Thorsten Leemhuis <linux@leemhuis.info>. If127 you spot a typo or small mistake, feel free to let him know directly and128 he'll fix it. You are free to do the same in a mostly informal way if you129 want to contribute changes to the text -- but for copyright reasons please CC130 linux-doc@vger.kernel.org and 'sign-off' your contribution as131 Documentation/process/submitting-patches.rst explains in the section 'Sign132 your work - the Developer's Certificate of Origin'.133..134 This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top135 of the file. If you want to distribute this text under CC-BY-4.0 only,136 please use 'The Linux kernel development community' for author attribution137 and link this as source:138 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/bug-bisect.rst139 140..141 Note: Only the content of this RST file as found in the Linux kernel sources142 is available under CC-BY-4.0, as versions of this text that were processed143 (for example by the kernel's build system) might contain content taken from144 files which use a more restrictive license.145