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gh-127545: Specify minimum PyGC_Head alignment to fix build failure#127546

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@fthainfthain commentedDec 3, 2024•
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As documented in InternalDocs/garbage_collector.md, the garbage collectorstores flags in the least significant two bits of the _gc_prev pointerin struct PyGC_Head. Consequently, this pointer is only capable of storinga location that's aligned to a 4-byte boundary.This alignment requirement is documented but it's not actually encoded.The code works when python happens to run on a platform that has a largeminimum alignment, but fails otherwise.Since we know that 2 bits are needed, we also know the minimum alignmentthat's needed. Let's make that explicit, so the compiler can then makesure those 2 bits are available.This patch fixes a segfault in _bootstrap_python. It also clarifies thecode by making the actual requirement explicit.This bug was investigated by Adrian Glaubitz here:https://lists.debian.org/debian-68k/2024/11/msg00020.htmlhttps://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1087600Although Adrian's patch isn't really correct (because natural alignmentis not needed), he deserves full credit for finding the root cause.

#127545

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ghost commentedDec 3, 2024•
edited by ghost
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Most changes to Pythonrequire a NEWS entry. Add one using theblurb_it web app or theblurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply theskip news label instead.

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I think this needs a blurb entry. Could you add one using one of the links the bot gave?

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No need to force push--everything is squashed at the end anyway.

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Provisionally, this LGTM. (I say "provisionally" because I don't totally understand the issue--there's just nothing wrong with this PR from a triage standpoint.)

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Provisionally, this LGTM. (I say "provisionally" because I don't totally understand the issue--there's just nothing wrong with this PR from a triage standpoint.)

I think the patch can made more understandable, by making use of the existing macro definitions. I will update the branch.

@tomasr8tomasr8 removed the needs backport to 3.12only security fixes labelApr 10, 2025
@serhiy-storchakaserhiy-storchaka added the needs backport to 3.14bugs and security fixes labelMay 8, 2025
As documented in InternalDocs/garbage_collector.md, the garbage collectorstores flags in the least significant two bits of the _gc_prev pointerin struct PyGC_Head. Consequently, this pointer is only capable of storinga location that's aligned to a 4-byte boundary.This alignment requirement is documented but it's not actually encoded.The code works when python happens to run on a platform that has a largeminimum alignment, but fails otherwise.Since we know that 2 bits are needed, we also know the minimum alignmentthat's needed. Let's make that explicit, so the compiler can then makesure those 2 bits are available.This patch fixes a segfault in _bootstrap_python. It also clarifies thecode by explicitly stating the actual requirement, in terms of_PyGC_PREV_SHIFT.This bug was previously investigated by Adrian Glaubitz here:https://lists.debian.org/debian-68k/2024/11/msg00020.htmlhttps://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1087600Although Adrian's patch isn't really correct (because natural alignmentis not needed), he deserves full credit for finding the root cause.
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I just noticed that@markshannon refactored the same headers 2 months ago. So I have now updated the fix to resolve those merge conflicts. Please review. This bug is still present in 3.14.0beta1.

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We're applying this patch in Debian's python3.14 to get it to build on m68k.

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@markshannonmarkshannonAwaiting requested review from markshannon

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