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3
4Handling regressions
5++++++++++++++++++++
6
7*We don't cause regressions* -- this document describes what this "first rule of
8Linux kernel development" means in practice for developers. It complements
9Documentation/admin-guide/reporting-regressions.rst, which covers the topic from a
10user's point of view; if you never read that text, go and at least skim over it
11before continuing here.
12
13The important bits (aka "The TL;DR")
14====================================
15
16#. Ensure subscribers of the `regression mailing list <https://lore.kernel.org/regressions/>`_
17 (regressions@lists.linux.dev) quickly become aware of any new regression
18 report:
19
20 * When receiving a mailed report that did not CC the list, bring it into the
21 loop by immediately sending at least a brief "Reply-all" with the list
22 CCed.
23
24 * Forward or bounce any reports submitted in bug trackers to the list.
25
26#. Make the Linux kernel regression tracking bot "regzbot" track the issue (this
27 is optional, but recommended):
28
29 * For mailed reports, check if the reporter included a line like ``#regzbot
30 introduced: v5.13..v5.14-rc1``. If not, send a reply (with the regressions
31 list in CC) containing a paragraph like the following, which tells regzbot
32 when the issue started to happen::
33
34 #regzbot ^introduced: 1f2e3d4c5b6a
35
36 * When forwarding reports from a bug tracker to the regressions list (see
37 above), include a paragraph like the following::
38
39 #regzbot introduced: v5.13..v5.14-rc1
40 #regzbot from: Some N. Ice Human <some.human@example.com>
41 #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
42
43#. When submitting fixes for regressions, add "Closes:" tags to the patch
44 description pointing to all places where the issue was reported, as
45 mandated by Documentation/process/submitting-patches.rst and
46 :ref:`Documentation/process/5.Posting.rst <development_posting>`. If you are
47 only fixing part of the issue that caused the regression, you may use
48 "Link:" tags instead. regzbot currently makes no distinction between the
49 two.
50
51#. Try to fix regressions quickly once the culprit has been identified; fixes
52 for most regressions should be merged within two weeks, but some need to be
53 resolved within two or three days.
54
55
56All the details on Linux kernel regressions relevant for developers
57===================================================================
58
59
60The important basics in more detail
61-----------------------------------
62
63
64What to do when receiving regression reports
65~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
66
67Ensure the Linux kernel's regression tracker and others subscribers of the
68`regression mailing list <https://lore.kernel.org/regressions/>`_
69(regressions@lists.linux.dev) become aware of any newly reported regression:
70
71 * When you receive a report by mail that did not CC the list, immediately bring
72 it into the loop by sending at least a brief "Reply-all" with the list CCed;
73 try to ensure it gets CCed again in case you reply to a reply that omitted
74 the list.
75
76 * If a report submitted in a bug tracker hits your Inbox, forward or bounce it
77 to the list. Consider checking the list archives beforehand, if the reporter
78 already forwarded the report as instructed by
79 Documentation/admin-guide/reporting-issues.rst.
80
81When doing either, consider making the Linux kernel regression tracking bot
82"regzbot" immediately start tracking the issue:
83
84 * For mailed reports, check if the reporter included a "regzbot command" like
85 ``#regzbot introduced: 1f2e3d4c5b6a``. If not, send a reply (with the
86 regressions list in CC) with a paragraph like the following:::
87
88 #regzbot ^introduced: v5.13..v5.14-rc1
89
90 This tells regzbot the version range in which the issue started to happen;
91 you can specify a range using commit-ids as well or state a single commit-id
92 in case the reporter bisected the culprit.
93
94 Note the caret (^) before the "introduced": it tells regzbot to treat the
95 parent mail (the one you reply to) as the initial report for the regression
96 you want to see tracked; that's important, as regzbot will later look out
97 for patches with "Closes:" tags pointing to the report in the archives on
98 lore.kernel.org.
99
100 * When forwarding a regression reported to a bug tracker, include a paragraph
101 with these regzbot commands::
102
103 #regzbot introduced: 1f2e3d4c5b6a
104 #regzbot from: Some N. Ice Human <some.human@example.com>
105 #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
106
107 Regzbot will then automatically associate patches with the report that
108 contain "Closes:" tags pointing to your mail or the mentioned ticket.
109
110What's important when fixing regressions
111~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
112
113You don't need to do anything special when submitting fixes for regression, just
114remember to do what Documentation/process/submitting-patches.rst,
115:ref:`Documentation/process/5.Posting.rst <development_posting>`, and
116Documentation/process/stable-kernel-rules.rst already explain in more detail:
117
118 * Point to all places where the issue was reported using "Closes:" tags::
119
120 Closes: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
121 Closes: https://bugzilla.kernel.org/show_bug.cgi?id=1234567890
122
123 If you are only fixing part of the issue, you may use "Link:" instead as
124 described in the first document mentioned above. regzbot currently treats
125 both of these equivalently and considers the linked reports as resolved.
126
127 * Add a "Fixes:" tag to specify the commit causing the regression.
128
129 * If the culprit was merged in an earlier development cycle, explicitly mark
130 the fix for backporting using the ``Cc: stable@vger.kernel.org`` tag.
131
132All this is expected from you and important when it comes to regression, as
133these tags are of great value for everyone (you included) that might be looking
134into the issue weeks, months, or years later. These tags are also crucial for
135tools and scripts used by other kernel developers or Linux distributions; one of
136these tools is regzbot, which heavily relies on the "Closes:" tags to associate
137reports for regression with changes resolving them.
138
139Expectations and best practices for fixing regressions
140~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
141
142As a Linux kernel developer, you are expected to give your best to prevent
143situations where a regression caused by a recent change of yours leaves users
144only these options:
145
146 * Run a kernel with a regression that impacts usage.
147
148 * Switch to an older or newer kernel series.
149
150 * Continue running an outdated and thus potentially insecure kernel for more
151 than three weeks after the regression's culprit was identified. Ideally it
152 should be less than two. And it ought to be just a few days, if the issue is
153 severe or affects many users -- either in general or in prevalent
154 environments.
155
156How to realize that in practice depends on various factors. Use the following
157rules of thumb as a guide.
158
159In general:
160
161 * Prioritize work on regressions over all other Linux kernel work, unless the
162 latter concerns a severe issue (e.g. acute security vulnerability, data loss,
163 bricked hardware, ...).
164
165 * Expedite fixing mainline regressions that recently made it into a proper
166 mainline, stable, or longterm release (either directly or via backport).
167
168 * Do not consider regressions from the current cycle as something that can wait
169 till the end of the cycle, as the issue might discourage or prevent users and
170 CI systems from testing mainline now or generally.
171
172 * Work with the required care to avoid additional or bigger damage, even if
173 resolving an issue then might take longer than outlined below.
174
175On timing once the culprit of a regression is known:
176
177 * Aim to mainline a fix within two or three days, if the issue is severe or
178 bothering many users -- either in general or in prevalent conditions like a
179 particular hardware environment, distribution, or stable/longterm series.
180
181 * Aim to mainline a fix by Sunday after the next, if the culprit made it
182 into a recent mainline, stable, or longterm release (either directly or via
183 backport); if the culprit became known early during a week and is simple to
184 resolve, try to mainline the fix within the same week.
185
186 * For other regressions, aim to mainline fixes before the hindmost Sunday
187 within the next three weeks. One or two Sundays later are acceptable, if the
188 regression is something people can live with easily for a while -- like a
189 mild performance regression.
190
191 * It's strongly discouraged to delay mainlining regression fixes till the next
192 merge window, except when the fix is extraordinarily risky or when the
193 culprit was mainlined more than a year ago.
194
195On procedure:
196
197 * Always consider reverting the culprit, as it's often the quickest and least
198 dangerous way to fix a regression. Don't worry about mainlining a fixed
199 variant later: that should be straight-forward, as most of the code went
200 through review once already.
201
202 * Try to resolve any regressions introduced in mainline during the past
203 twelve months before the current development cycle ends: Linus wants such
204 regressions to be handled like those from the current cycle, unless fixing
205 bears unusual risks.
206
207 * Consider CCing Linus on discussions or patch review, if a regression seems
208 tangly. Do the same in precarious or urgent cases -- especially if the
209 subsystem maintainer might be unavailable. Also CC the stable team, when you
210 know such a regression made it into a mainline, stable, or longterm release.
211
212 * For urgent regressions, consider asking Linus to pick up the fix straight
213 from the mailing list: he is totally fine with that for uncontroversial
214 fixes. Ideally though such requests should happen in accordance with the
215 subsystem maintainers or come directly from them.
216
217 * In case you are unsure if a fix is worth the risk applying just days before
218 a new mainline release, send Linus a mail with the usual lists and people in
219 CC; in it, summarize the situation while asking him to consider picking up
220 the fix straight from the list. He then himself can make the call and when
221 needed even postpone the release. Such requests again should ideally happen
222 in accordance with the subsystem maintainers or come directly from them.
223
224Regarding stable and longterm kernels:
225
226 * You are free to leave regressions to the stable team, if they at no point in
227 time occurred with mainline or were fixed there already.
228
229 * If a regression made it into a proper mainline release during the past
230 twelve months, ensure to tag the fix with "Cc: stable@vger.kernel.org", as a
231 "Fixes:" tag alone does not guarantee a backport. Please add the same tag,
232 in case you know the culprit was backported to stable or longterm kernels.
233
234 * When receiving reports about regressions in recent stable or longterm kernel
235 series, please evaluate at least briefly if the issue might happen in current
236 mainline as well -- and if that seems likely, take hold of the report. If in
237 doubt, ask the reporter to check mainline.
238
239 * Whenever you want to swiftly resolve a regression that recently also made it
240 into a proper mainline, stable, or longterm release, fix it quickly in
241 mainline; when appropriate thus involve Linus to fast-track the fix (see
242 above). That's because the stable team normally does neither revert nor fix
243 any changes that cause the same problems in mainline.
244
245 * In case of urgent regression fixes you might want to ensure prompt
246 backporting by dropping the stable team a note once the fix was mainlined;
247 this is especially advisable during merge windows and shortly thereafter, as
248 the fix otherwise might land at the end of a huge patch queue.
249
250On patch flow:
251
252 * Developers, when trying to reach the time periods mentioned above, remember
253 to account for the time it takes to get fixes tested, reviewed, and merged by
254 Linus, ideally with them being in linux-next at least briefly. Hence, if a
255 fix is urgent, make it obvious to ensure others handle it appropriately.
256
257 * Reviewers, you are kindly asked to assist developers in reaching the time
258 periods mentioned above by reviewing regression fixes in a timely manner.
259
260 * Subsystem maintainers, you likewise are encouraged to expedite the handling
261 of regression fixes. Thus evaluate if skipping linux-next is an option for
262 the particular fix. Also consider sending git pull requests more often than
263 usual when needed. And try to avoid holding onto regression fixes over
264 weekends -- especially when the fix is marked for backporting.
265
266
267More aspects regarding regressions developers should be aware of
268----------------------------------------------------------------
269
270
271How to deal with changes where a risk of regression is known
272~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
273
274Evaluate how big the risk of regressions is, for example by performing a code
275search in Linux distributions and Git forges. Also consider asking other
276developers or projects likely to be affected to evaluate or even test the
277proposed change; if problems surface, maybe some solution acceptable for all
278can be found.
279
280If the risk of regressions in the end seems to be relatively small, go ahead
281with the change, but let all involved parties know about the risk. Hence, make
282sure your patch description makes this aspect obvious. Once the change is
283merged, tell the Linux kernel's regression tracker and the regressions mailing
284list about the risk, so everyone has the change on the radar in case reports
285trickle in. Depending on the risk, you also might want to ask the subsystem
286maintainer to mention the issue in his mainline pull request.
287
288What else is there to known about regressions?
289~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
290
291Check out Documentation/admin-guide/reporting-regressions.rst, it covers a lot
292of other aspects you want might want to be aware of:
293
294 * the purpose of the "no regressions" rule
295
296 * what issues actually qualify as regression
297
298 * who's in charge for finding the root cause of a regression
299
300 * how to handle tricky situations, e.g. when a regression is caused by a
301 security fix or when fixing a regression might cause another one
302
303Whom to ask for advice when it comes to regressions
304~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
305
306Send a mail to the regressions mailing list (regressions@lists.linux.dev) while
307CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
308issue might better be dealt with in private, feel free to omit the list.
309
310
311More about regression tracking and regzbot
312------------------------------------------
313
314
315Why the Linux kernel has a regression tracker, and why is regzbot used?
316~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
317
318Rules like "no regressions" need someone to ensure they are followed, otherwise
319they are broken either accidentally or on purpose. History has shown this to be
320true for the Linux kernel as well. That's why Thorsten Leemhuis volunteered to
321keep an eye on things as the Linux kernel's regression tracker, who's
322occasionally helped by other people. Neither of them are paid to do this,
323that's why regression tracking is done on a best effort basis.
324
325Earlier attempts to manually track regressions have shown it's an exhausting and
326frustrating work, which is why they were abandoned after a while. To prevent
327this from happening again, Thorsten developed regzbot to facilitate the work,
328with the long term goal to automate regression tracking as much as possible for
329everyone involved.
330
331How does regression tracking work with regzbot?
332~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
333
334The bot watches for replies to reports of tracked regressions. Additionally,
335it's looking out for posted or committed patches referencing such reports
336with "Closes:" tags; replies to such patch postings are tracked as well.
337Combined this data provides good insights into the current state of the fixing
338process.
339
340Regzbot tries to do its job with as little overhead as possible for both
341reporters and developers. In fact, only reporters are burdened with an extra
342duty: they need to tell regzbot about the regression report using the ``#regzbot
343introduced`` command outlined above; if they don't do that, someone else can
344take care of that using ``#regzbot ^introduced``.
345
346For developers there normally is no extra work involved, they just need to make
347sure to do something that was expected long before regzbot came to light: add
348links to the patch description pointing to all reports about the issue fixed.
349
350Do I have to use regzbot?
351~~~~~~~~~~~~~~~~~~~~~~~~~
352
353It's in the interest of everyone if you do, as kernel maintainers like Linus
354Torvalds partly rely on regzbot's tracking in their work -- for example when
355deciding to release a new version or extend the development phase. For this they
356need to be aware of all unfixed regression; to do that, Linus is known to look
357into the weekly reports sent by regzbot.
358
359Do I have to tell regzbot about every regression I stumble upon?
360~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
361
362Ideally yes: we are all humans and easily forget problems when something more
363important unexpectedly comes up -- for example a bigger problem in the Linux
364kernel or something in real life that's keeping us away from keyboards for a
365while. Hence, it's best to tell regzbot about every regression, except when you
366immediately write a fix and commit it to a tree regularly merged to the affected
367kernel series.
368
369How to see which regressions regzbot tracks currently?
370~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
371
372Check `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_
373for the latest info; alternatively, `search for the latest regression report
374<https://lore.kernel.org/lkml/?q=%22Linux+regressions+report%22+f%3Aregzbot>`_,
375which regzbot normally sends out once a week on Sunday evening (UTC), which is a
376few hours before Linus usually publishes new (pre-)releases.
377
378What places is regzbot monitoring?
379~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
380
381Regzbot is watching the most important Linux mailing lists as well as the git
382repositories of linux-next, mainline, and stable/longterm.
383
384What kind of issues are supposed to be tracked by regzbot?
385~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
386
387The bot is meant to track regressions, hence please don't involve regzbot for
388regular issues. But it's okay for the Linux kernel's regression tracker if you
389use regzbot to track severe issues, like reports about hangs, corrupted data,
390or internal errors (Panic, Oops, BUG(), warning, ...).
391
392Can I add regressions found by CI systems to regzbot's tracking?
393~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
394
395Feel free to do so, if the particular regression likely has impact on practical
396use cases and thus might be noticed by users; hence, please don't involve
397regzbot for theoretical regressions unlikely to show themselves in real world
398usage.
399
400How to interact with regzbot?
401~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
402
403By using a 'regzbot command' in a direct or indirect reply to the mail with the
404regression report. These commands need to be in their own paragraph (IOW: they
405need to be separated from the rest of the mail using blank lines).
406
407One such command is ``#regzbot introduced: <version or commit>``, which makes
408regzbot consider your mail as a regressions report added to the tracking, as
409already described above; ``#regzbot ^introduced: <version or commit>`` is another
410such command, which makes regzbot consider the parent mail as a report for a
411regression which it starts to track.
412
413Once one of those two commands has been utilized, other regzbot commands can be
414used in direct or indirect replies to the report. You can write them below one
415of the `introduced` commands or in replies to the mail that used one of them
416or itself is a reply to that mail:
417
418 * Set or update the title::
419
420 #regzbot title: foo
421
422 * Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
423 the issue or a fix are discussed -- for example the posting of a patch fixing
424 the regression::
425
426 #regzbot monitor: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
427
428 Monitoring only works for lore.kernel.org and bugzilla.kernel.org; regzbot
429 will consider all messages in that thread or ticket as related to the fixing
430 process.
431
432 * Point to a place with further details of interest, like a mailing list post
433 or a ticket in a bug tracker that are slightly related, but about a different
434 topic::
435
436 #regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
437
438 * Mark a regression as fixed by a commit that is heading upstream or already
439 landed::
440
441 #regzbot fix: 1f2e3d4c5d
442
443 * Mark a regression as a duplicate of another one already tracked by regzbot::
444
445 #regzbot dup-of: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
446
447 * Mark a regression as invalid::
448
449 #regzbot invalid: wasn't a regression, problem has always existed
450
451Is there more to tell about regzbot and its commands?
452~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
453
454More detailed and up-to-date information about the Linux
455kernel's regression tracking bot can be found on its
456`project page <https://gitlab.com/knurd42/regzbot>`_, which among others
457contains a `getting started guide <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_
458and `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
459which both cover more details than the above section.
460
461Quotes from Linus about regression
462----------------------------------
463
464The following statements from Linus Torvalds provide some insight into Linux
465"no regressions" rule and how he expects regressions to be handled:
466
467On how quickly regressions should be fixed
468~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
469
470* From `2026-01-22 <https://lore.kernel.org/all/CAHk-=wheQNiW_WtHGO7bKkT7Uib-p+ai2JP9M+z+FYcZ6CAxYA@mail.gmail.com/>`_::
471
472 But a user complaining should basically result in an immediate fix -
473 possibly a "revert and rethink".
474
475 With a later clarification on `2026-01-28 <https://lore.kernel.org/all/CAHk-%3Dwi86AosXs66-yi54%2BmpQjPu0upxB8ZAfG%2BLsMyJmcuMSA@mail.gmail.com/>`_::
476
477 It's also worth noting that "immediate" obviously doesn't mean "right
478 this *second* when the problem has been reported".
479
480 But if it's a regression with a known commit that caused it, I think
481 the rule of thumb should generally be "within a week", preferably
482 before the next rc.
483
484* From `2023-04-21 <https://lore.kernel.org/all/CAHk-=wgD98pmSK3ZyHk_d9kZ2bhgN6DuNZMAJaV0WTtbkf=RDw@mail.gmail.com/>`_::
485
486 Known-broken commits either
487 (a) get a timely fix that doesn't have other questions
488 or
489 (b) get reverted
490
491* From `2021-09-20(2) <https://lore.kernel.org/all/CAHk-=wgOvmtRw1TNbMC1rn5YqyTKyn0hz+sc4k0DGNn++u9aYw@mail.gmail.com/>`_::
492
493 [...] review shouldn't hold up reported regressions of existing code. That's
494 just basic _testing_ - either the fix should be applied, or - if the fix is
495 too invasive or too ugly - the problematic source of the regression should
496 be reverted.
497
498 Review should be about new code, it shouldn't be holding up "there's a
499 bug report, here's the obvious fix".
500
501* From `2023-05-08 <https://lore.kernel.org/all/CAHk-=wgzU8_dGn0Yg+DyX7ammTkDUCyEJ4C=NvnHRhxKWC7Wpw@mail.gmail.com/>`_::
502
503 If something doesn't even build, it should damn well be fixed ASAP.
504
505On how fixing regressions with reverts can help prevent maintainer burnout
506~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
507
508* From `2026-01-28 <https://lore.kernel.org/all/CAHk-%3Dwi86AosXs66-yi54%2BmpQjPu0upxB8ZAfG%2BLsMyJmcuMSA@mail.gmail.com/>`_::
509
510 > So how can I/we make "immediate fixes" happen more often without
511 > contributing to maintainer burnout?
512
513 [...] the "revert and rethink" model [...] often a good idea in general [...]
514
515 Exactly so that maintainers don't get stressed out over having a pending
516 problem report that people keep pestering them about.
517
518 I think people are sometimes a bit too bought into whatever changes
519 they made, and reverting is seen as "too drastic", but I think it's
520 often the quick and easy solution for when there isn't some obvious
521 response to a regression report.
522
523On mainlining fixes when the last -rc or a new release is close
524~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
525
526* From `2026-02-01 <https://lore.kernel.org/all/CAHk-%3DwhXTw1oPsa%2BTLuY1Rc9D1OAiPVOdR_-R2xG45kwDObKdA@mail.gmail.com/>`_::
527
528 So I think I'd rather see them hit rc8 (later today) and have a week
529 of testing in my tree and be reverted if they cause problems, than
530 have them go in after rc8 and then cause problems in the 6.19 release
531 instead.
532
533* From `2023-04-20 <https://lore.kernel.org/all/CAHk-=wis_qQy4oDNynNKi5b7Qhosmxtoj1jxo5wmB6SRUwQUBQ@mail.gmail.com/>`_::
534
535 But something like this, where the regression was in the previous release
536 and it's just a clear fix with no semantic subtlety, I consider to be just a
537 regular regression that should be expedited - partly to make it into stable,
538 and partly to avoid having to put the fix into _another_ stable kernel.
539
540On sending merge requests with just one fix
541~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
542
543* From `2024-04-24 <https://lore.kernel.org/all/CAHk-=wjy_ph9URuFt-pq+2AJ__p7gFDx=yzVSCsx16xAYvNw9g@mail.gmail.com/>`_::
544
545 If the issue is just that there's nothing else happening, I think people
546 should just point me to the patch and say "can you apply this single fix?"
547
548* From `2023-04-20 <https://lore.kernel.org/all/CAHk-=wis_qQy4oDNynNKi5b7Qhosmxtoj1jxo5wmB6SRUwQUBQ@mail.gmail.com/>`_::
549
550 I'm always open to direct fixes when there is no controversy about the fix.
551 No problem. I still happily deal with individual patches.
552
553On the importance of pointing to bug reports using Link:/Closes: tags
554~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
555
556* From `2025-07-29(1) <https://lore.kernel.org/all/CAHk-=wj2kJRPWx8B09AAtzj+_g+T6UBX11TP0ebs1WJdTtv=WQ@mail.gmail.com/>`_::
557
558 [...] revert like this, it really would be good to link to the problems, so
559 that when people try to re-enable it, they have the history for why it
560 didn't work the first time.
561
562* From `2022-05-08 <https://lore.kernel.org/all/CAHk-=wjMmSZzMJ3Xnskdg4+GGz=5p5p+GSYyFBTh0f-DgvdBWg@mail.gmail.com/>`_::
563
564 So I have to once more complain [...]
565
566 [...] There's no link to the actual problem the patch fixes.
567
568* From `2022-06-22 <https://lore.kernel.org/all/CAHk-=wjxzafG-=J8oT30s7upn4RhBs6TX-uVFZ5rME+L5_DoJA@mail.gmail.com/>`_::
569
570 See, *that* link [to the report] would have been useful in the commit.
571
572On why the "no regressions" rule exists
573~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
574
575* From `2026-01-22 <https://lore.kernel.org/all/CAHk-=wheQNiW_WtHGO7bKkT7Uib-p+ai2JP9M+z+FYcZ6CAxYA@mail.gmail.com/>`_::
576
577 But the basic rule is: be so good about backwards compatibility that
578 users never have to worry about upgrading. They should absolutely feel
579 confident that any kernel-reported problem will either be solved, or
580 have an easy solution that is appropriate for *them* (ie a
581 non-technical user shouldn't be expected to be able to do a lot).
582
583 Because the last thing we want is people holding back from trying new
584 kernels.
585
586* From `2024-05-28 <https://lore.kernel.org/all/CAHk-=wgtb7y-bEh7tPDvDWru7ZKQ8-KMjZ53Tsk37zsPPdwXbA@mail.gmail.com/>`_::
587
588 I introduced that "no regressions" rule something like two decades
589 ago, because people need to be able to update their kernel without
590 fear of something they relied on suddenly stopping to work.
591
592* From `2018-08-03 <https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
593
594 The whole point of "we do not regress" is so that people can upgrade
595 the kernel and never have to worry about it.
596
597 [...]
598
599 Because the only thing that matters IS THE USER.
600
601* From `2017-10-26(1) <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
602
603 If the kernel used to work for you, the rule is that it continues to work
604 for you.
605
606 [...]
607
608 People should basically always feel like they can update their kernel
609 and simply not have to worry about it.
610
611 I refuse to introduce "you can only update the kernel if you also
612 update that other program" kind of limitations. If the kernel used to
613 work for you, the rule is that it continues to work for you.
614
615On exceptions to the "no regressions" rule
616~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
617
618* From `2026-01-22 <https://lore.kernel.org/all/CAHk-=wheQNiW_WtHGO7bKkT7Uib-p+ai2JP9M+z+FYcZ6CAxYA@mail.gmail.com/>`_::
619
620 There are _very_ few exceptions to that rule, the main one being "the
621 problem was a fundamental huge and gaping security issue and we *had* to
622 make that change, and we couldn't even make your limited use-case just
623 continue to work".
624
625 The other exception is "the problem was reported years after it was
626 introduced, and now most people rely on the new behavior".
627
628 [...]
629
630 Now, if it's one or two users and you can just get them to recompile,
631 that's one thing. Niche hardware and odd use-cases can sometimes be
632 solved that way, and regressions can sometimes be fixed by handholding
633 every single reporter if the reporter is willing and able to change
634 his or her workflow.
635
636* From `2023-04-20 <https://lore.kernel.org/all/CAHk-=wis_qQy4oDNynNKi5b7Qhosmxtoj1jxo5wmB6SRUwQUBQ@mail.gmail.com/>`_::
637
638 And yes, I do consider "regression in an earlier release" to be a
639 regression that needs fixing.
640
641 There's obviously a time limit: if that "regression in an earlier
642 release" was a year or more ago, and just took forever for people to
643 notice, and it had semantic changes that now mean that fixing the
644 regression could cause a _new_ regression, then that can cause me to
645 go "Oh, now the new semantics are what we have to live with".
646
647* From `2017-10-26(2) <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
648
649 There have been exceptions, but they are few and far between, and they
650 generally have some major and fundamental reasons for having happened,
651 that were basically entirely unavoidable, and people _tried_hard_ to
652 avoid them. Maybe we can't practically support the hardware any more
653 after it is decades old and nobody uses it with modern kernels any
654 more. Maybe there's a serious security issue with how we did things,
655 and people actually depended on that fundamentally broken model. Maybe
656 there was some fundamental other breakage that just _had_ to have a
657 flag day for very core and fundamental reasons.
658
659On situations where updating something in userspace can resolve regressions
660~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
661
662* From `2018-08-03 <https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
663
664 And dammit, we upgrade the kernel ALL THE TIME without upgrading any
665 other programs at all. It is absolutely required, because flag-days
666 and dependencies are horribly bad.
667
668 And it is also required simply because I as a kernel developer do not
669 upgrade random other tools that I don't even care about as I develop the
670 kernel, and I want any of my users to feel safe doing the same time.
671
672* From `2017-10-26(3) <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
673
674 But if something actually breaks, then the change must get fixed or
675 reverted. And it gets fixed in the *kernel*. Not by saying "well, fix your
676 user space then". It was a kernel change that exposed the problem, it needs
677 to be the kernel that corrects for it, because we have a "upgrade in place"
678 model. We don't have a "upgrade with new user space".
679
680 And I seriously will refuse to take code from people who do not understand
681 and honor this very simple rule.
682
683 This rule is also not going to change.
684
685 And yes, I realize that the kernel is "special" in this respect. I'm proud
686 of it.
687
688* From `2017-10-26(4) <https://lore.kernel.org/all/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/>`_::
689
690 If you break existing user space setups THAT IS A REGRESSION.
691
692 It's not ok to say "but we'll fix the user space setup".
693
694 Really. NOT OK.
695
696On what qualifies as userspace interface, ABI, API, documented interfaces, etc.
697~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
698
699* From `2026-01-20 <https://lore.kernel.org/all/CAHk-=wga8Qu0-OSE9VZbviq9GuqwhPhLUXeAt-S7_9+fMCLkKg@mail.gmail.com/>`_::
700
701 So I absolutely detest the whole notion of "ABI changes". It's a
702 meaningless concept, and I hate it with a passion, [...]
703
704 The Linux rule for regressions is basically based on the philosophical
705 question of "If a tree falls in the forest, and nobody is around to
706 hear it, does it make a sound?".
707
708 So the only thing that matters is if something breaks user-*conscious*
709 behavior.
710
711 And when that happens, the distinction between "bug fix" and "new
712 feature" and "ABI change" matters not one whit, and the change needs
713 to be done differently.
714
715 [...]
716
717 I just wanted to point out that the argument about whether it's an ABI
718 change or not is irrelevant. If it turns out that some program - not a test
719 script, but something with relevance to conscious user expectations ~
720 depended on the old broken behavior, then it needs to be done some other
721 way.
722
723* From `2026-02-13 <https://lore.kernel.org/all/CAHk-=whY-N8kjm8kiFUV5Ei-8AuYw--EPGD-AR3Pd+5GTx2sAQ@mail.gmail.com/>`_::
724
725 > [...] this should not fall under the don't break user space rule [...]
726
727 Note that the rule is about breaking *users*, not breaking user space per
728 se. [...]
729
730 If some user setup breaks, things need fixing.
731
732 [...] but I want to make it very clear that there are no excuses about "user
733 space applications".
734
735* From `2021-09-20(4) <https://lore.kernel.org/all/CAHk-=wi7DB2SJ-wngVvsJ7Ak2cM556Q8437sOXo4EJt2BWPdEg@mail.gmail.com/>`_::
736
737 [...] a regression is a bit like Schrödinger's cat - if nobody is around
738 to notice it and it doesn't actually affect any real workload, then you
739 can treat the regression as if it doesn't exist.
740
741* From `2020-05-21 <https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
742
743 The rules about regressions have never been about any kind of documented
744 behavior, or where the code lives.
745
746 The rules about regressions are always about "breaks user workflow".
747
748 Users are literally the _only_ thing that matters.
749
750* From `2019-09-15 <https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA@mail.gmail.com/>`_::
751
752 One _particularly_ last-minute revert is the top-most commit (ignoring
753 the version change itself) done just before the release, and while
754 it's very annoying, it's perhaps also instructive.
755
756 What's instructive about it is that I reverted a commit that wasn't
757 actually buggy. In fact, it was doing exactly what it set out to do,
758 and did it very well. In fact it did it _so_ well that the much
759 improved IO patterns it caused then ended up revealing a user-visible
760 regression due to a real bug in a completely unrelated area.
761
762 The actual details of that regression are not the reason I point that
763 revert out as instructive, though. It's more that it's an instructive
764 example of what counts as a regression, and what the whole "no
765 regressions" kernel rule means.
766
767 [...] The reverted commit didn't change any API's, and it didn't introduce
768 any new bugs. But it ended up exposing another problem, and as such caused
769 a kernel upgrade to fail for a user. So it got reverted.
770
771 The point here being that we revert based on user-reported _behavior_, not
772 based on some "it changes the ABI" or "it caused a bug" concept. The problem
773 was really pre-existing, and it just didn't happen to trigger before. [...]
774
775 Take-away from the whole thing: it's not about whether you change the
776 kernel-userspace ABI, or fix a bug, or about whether the old code
777 "should never have worked in the first place". It's about whether
778 something breaks existing users' workflow.
779
780* From `2017-11-05 <https://lore.kernel.org/all/CA+55aFzUvbGjD8nQ-+3oiMBx14c_6zOj2n7KLN3UsJ-qsd4Dcw@mail.gmail.com/>`_::
781
782 And our regression rule has never been "behavior doesn't change".
783 That would mean that we could never make any changes at all.
784
785* From `2020-05-21 <https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
786
787 No amount of "you shouldn't have used this" or "that behavior was
788 undefined, it's your own fault your app broke" or "that used to work
789 simply because of a kernel bug" is at all relevant.
790
791* From `2021-05-21 <https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
792
793 But no, "that was documented to be broken" (whether it's because the code
794 was in staging or because the man-page said something else) is irrelevant.
795 If staging code is so useful that people end up using it, that means that
796 it's basically regular kernel code with a flag saying "please clean this
797 up".
798
799 [...]
800
801 The other side of the coin is that people who talk about "API stability" are
802 entirely wrong. API's don't matter either. You can make any changes to an
803 API you like - as long as nobody notices.
804
805 Again, the regression rule is not about documentation, not about API's, and
806 not about the phase of the moon.
807
808* From `2012-07-06 <https://lore.kernel.org/all/CA+55aFwnLJ+0sjx92EGREGTWOx84wwKaraSzpTNJwPVV8edw8g@mail.gmail.com/>`_::
809
810 > Now this got me wondering if Debian _unstable_ actually qualifies as a
811 > standard distro userspace.
812
813 Oh, if the kernel breaks some standard user space, that counts. Tons
814 of people run Debian unstable
815
816* From `2011-05-06 <https://lore.kernel.org/all/BANLkTi=KVXjKR82sqsz4gwjr+E0vtqCmvA@mail.gmail.com/>`_::
817
818 It's clearly NOT an internal tracepoint. By definition. It's being
819 used by powertop.
820
821On regressions noticed by users or test-suites/CIs
822~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
823
824* From `2026-01-22 <https://lore.kernel.org/all/CAHk-=wheQNiW_WtHGO7bKkT7Uib-p+ai2JP9M+z+FYcZ6CAxYA@mail.gmail.com/>`_::
825
826 Users complaining is the only real line in the end.
827
828 [...] a test-suite complaining is then often a *very* good indication that
829 maybe users will hit some problem, and test suite issues should be taken
830 very seriously [...]
831
832 But a test-suite error isn't necessarily where you have to draw the
833 line - it's a big red flag [...]
834
835* From `2024-29-01 <https://lore.kernel.org/all/CAHk-=wg8BrZEzjJ5kUyZzHPZmFqH6ooMN1gRBCofxxCfucgjaw@mail.gmail.com/>`_::
836
837 The "no regressions" rule is not about made-up "if I do this, behavior
838 changes".
839
840 The "no regressions" rule is about *users*.
841
842 If you have an actual user that has been doing insane things, and we
843 change something, and now the insane thing no longer works, at that
844 point it's a regression, and we'll sigh, and go "Users are insane" and
845 have to fix it.
846
847 But if you have some random test that now behaves differently, it's
848 not a regression. It's a *warning* sign, sure: tests are useful.
849
850On accepting when a regression occurred
851~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
852
853* From `2026-01-22 <https://lore.kernel.org/all/CAHk-=wheQNiW_WtHGO7bKkT7Uib-p+ai2JP9M+z+FYcZ6CAxYA@mail.gmail.com/>`_::
854
855 But starting to argue about users reporting breaking changes is
856 basically the final line for me. I have a couple of people that I have
857 in my spam block-list and refuse to have anything to do with, and they
858 have generally been about exactly that.
859
860 Note how it's not about making mistakes and _causing_ the regression.
861 That's normal. That's development. But then arguing about it is a
862 no-no.
863
864* From `2024-06-23 <https://lore.kernel.org/all/CAHk-=wi_KMO_rJ6OCr8mAWBRg-irziM=T9wxGC+J1VVoQb39gw@mail.gmail.com/>`_::
865
866 We don't introduce regressions and then blame others.
867
868 There's a very clear rule in kernel development: things that break
869 other things ARE NOT FIXES.
870
871 EVER.
872
873 They get reverted, or the thing they broke gets fixed.
874
875* From `2021-06-05 <https://lore.kernel.org/all/CAHk-=wiUVqHN76YUwhkjZzwTdjMMJf_zN4+u7vEJjmEGh3recw@mail.gmail.com/>`_::
876
877 THERE ARE NO VALID ARGUMENTS FOR REGRESSIONS.
878
879 Honestly, security people need to understand that "not working" is not
880 a success case of security. It's a failure case.
881
882 Yes, "not working" may be secure. But security in that case is *pointless*.
883
884* From `2017-10-26(5) <https://lore.kernel.org/lkml/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/>`_::
885
886 [...] when regressions *do* occur, we admit to them and fix them, instead of
887 blaming user space.
888
889 The fact that you have apparently been denying the regression now for
890 three weeks means that I will revert, and I will stop pulling apparmor
891 requests until the people involved understand how kernel development
892 is done.
893
894On back-and-forth
895~~~~~~~~~~~~~~~~~
896
897* From `2024-05-28 <https://lore.kernel.org/all/CAHk-=wgtb7y-bEh7tPDvDWru7ZKQ8-KMjZ53Tsk37zsPPdwXbA@mail.gmail.com/>`_::
898
899 The "no regressions" rule is that we do not introduce NEW bugs.
900
901 It *literally* came about because we had an endless dance of "fix two
902 bugs, introduce one new one", and that then resulted in a system that
903 you cannot TRUST.
904
905* From `2021-09-20(1) <https://lore.kernel.org/all/CAHk-=wi7DB2SJ-wngVvsJ7Ak2cM556Q8437sOXo4EJt2BWPdEg@mail.gmail.com/>`_::
906
907 And the thing that makes regressions special is that back when I
908 wasn't so strict about these things, we'd end up in endless "seesaw
909 situations" where somebody would fix something, it would break
910 something else, then that something else would break, and it would
911 never actually converge on anything reliable at all.
912
913* From `2015-08-13 <https://lore.kernel.org/all/CA+55aFxk8-BsiKwr_S-c+4G6wihKPQVMLE34H9wOZpeua6W9+Q@mail.gmail.com/>`_::
914
915 The strict policy of no regressions actually originally started mainly wrt
916 suspend/resume issues, where the "fix one machine, break another" kind of
917 back-and-forth caused endless problems, and meant that we didn't actually
918 necessarily make any forward progress, just moving a problem around.
919
920On changes with a risk of causing regressions
921~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
922
923* From `2023-06-02 <https://lore.kernel.org/all/CAHk-=wgyAGUMHmQM-5Eb556z5xiHZB7cF05qjrtUH4F7P-1rSA@mail.gmail.com/>`_::
924
925 So what I think you should do is to fix the bug right, with a clean
926 patch, and no crazy hacks. That is something we can then apply and
927 test. All the while knowing full well that "uhhuh, this is a visible
928 change, we may have to revert it".
929
930 If then some *real* load ends up showing a regression, we may just be
931 screwed. Our current behavior may be buggy, but we have the rule that
932 once user space depends on kernel bugs, they become features pretty
933 much by definition, however much we might dislike it.
934
935On in-kernel workarounds to avoid regressions
936~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
937
938* From `2017-10-26(6) <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
939
940 Behavioral changes happen, and maybe we don't even support some
941 feature any more. There's a number of fields in /proc/<pid>/stat that
942 are printed out as zeroes, simply because they don't even *exist* in
943 the kernel any more, or because showing them was a mistake (typically
944 an information leak). But the numbers got replaced by zeroes, so that
945 the code that used to parse the fields still works. The user might not
946 see everything they used to see, and so behavior is clearly different,
947 but things still _work_, even if they might no longer show sensitive
948 (or no longer relevant) information.
949
950On regressions caused by bugfixes
951~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
952
953* From `2018-08-03 <https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
954
955 > Kernel had a bug which has been fixed
956
957 That is *ENTIRELY* immaterial.
958
959 Guys, whether something was buggy or not DOES NOT MATTER.
960
961 [...]
962
963 It's basically saying "I took something that worked, and I broke it,
964 but now it's better". Do you not see how f*cking insane that statement
965 is?
966
967On internal API changes
968~~~~~~~~~~~~~~~~~~~~~~~
969
970* From `2017-10-26(7) <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
971
972 We do API breakage _inside_ the kernel all the time. We will fix
973 internal problems by saying "you now need to do XYZ", but then it's
974 about internal kernel API's, and the people who do that then also
975 obviously have to fix up all the in-kernel users of that API. Nobody
976 can say "I now broke the API you used, and now _you_ need to fix it
977 up". Whoever broke something gets to fix it too.
978
979On regressions only found after a long time
980~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
981
982* From `2024-03-28 <https://lore.kernel.org/all/CAHk-=wgFuoHpMk_Z_R3qMXVDgq0N1592+bABkyGjwwSL4zBtHA@mail.gmail.com/>`_::
983
984 I'm definitely not reverting a patch from almost a decade ago as a
985 regression.
986
987 If it took that long to find, it can't be that critical of a regression.
988
989 So yes, let's treat it as a regular bug.
990
991On testing regressions fixes in linux-next
992~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
993
994* On `maintainers summit 2024 <https://lwn.net/Articles/990599/>`_::
995
996 So running fixes though linux-next is just a waste of time.
997
998On a few other aspects related to regressions
999~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1000
1001* From `2025-07-29(2) <https://lore.kernel.org/all/CAHk-=wjj9DvOZtmTkoLtyfHmy5mNKy6q_96d9=4FUEDXre=cww@mail.gmail.com/>`_
1002 [which `is not quite a regression, but a huge inconvenience <https://lore.kernel.org/all/CAHk-=wgO0Rx2LcYT4f75Xs46orbJ4JxO2jbAFQnVKDYAjV5HeQ@mail.gmail.com/>`_]::
1003
1004 I no longer have sound.
1005
1006 I also suspect that it's purely because "make oldconfig" doesn't work,
1007 and probably turned off my old Intel HDA settings. Or something.
1008
1009 Renaming config parameters is *bad*. I've harped on the Kconfig phase
1010 of the kernel build probably being our nastiest point, and a real pain
1011 point to people getting involved with development simply because
1012 building your own kernel can be so daunting with hundreds of fairly
1013 esoteric questions.
1014
1015..
1016 end-of-content
1017..
1018 This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
1019 of the file. If you want to distribute this text under CC-BY-4.0 only,
1020 please use "The Linux kernel developers" for author attribution and link
1021 this as source:
1022 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/process/handling-regressions.rst
1023..
1024 Note: Only the content of this RST file as found in the Linux kernel sources
1025 is available under CC-BY-4.0, as versions of this text that were processed
1026 (for example by the kernel's build system) might contain content taken from
1027 files which use a more restrictive license.