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Differential D57955

posix_spawn(): create a guard page below the stack for rfork_thread
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Authored by kib on Jun 29 2026, 10:10 PM.
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rG2767a1f3686e: posix_spawn(3): create a guard page below the stack for rfork_thread on x86

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kib created this revision.Jun 29 2026, 10:10 PM
Herald added a subscriber: imp. · View Herald TranscriptJun 29 2026, 10:10 PM
kib requested review of this revision.Jun 29 2026, 10:10 PM
kib mentioned this in D57954: libc: gen: add a test for rtld underflowing our posix_spawn thread.
kevans added inline comments.Jun 29 2026, 10:22 PM
lib/libc/gen/posix_spawn.c
306

What would prevent us from being able to map over the guard here? I wonder if this should be an assert instead.

kib marked an inline comment as done.Jun 29 2026, 10:48 PM

The cost is +3 syscalls per spawn, but so be it.

lib/libc/gen/posix_spawn.c
306

I dislike asserts in libraries. It is not the library authors to establish the assert policy for the caller.

The check is mostly for correctness. If I somehow missed the correct args to mmap, I can see it immediately.

kevans accepted this revision.Jun 29 2026, 11:01 PM
In D57955#1328646, @kib wrote:

The cost is +3 syscalls per spawn, but so be it.

I think it's defensible for the benefit of the guard page. Worst case scenario, we add a small pool of free stacks to choose from if we get complaints? A little bit of additional complexity, but not terrible.

lib/libc/gen/posix_spawn.c
306

Right, but in this case we're presumably hiding a kernel bug if it does actually fail at this particular point, which feels like a reasonable exception to make for assertions in libraries. I don't insist either way, though.

This revision is now accepted and ready to land.Jun 29 2026, 11:01 PM
Closed by commit rG2767a1f3686e: posix_spawn(3): create a guard page below the stack for rfork_thread on x86 (authored by kib). · Explain WhyJun 30 2026, 1:10 AM
This revision was automatically updated to reflect the committed changes.
kib marked an inline comment as done.
kib added a commit: rG2767a1f3686e: posix_spawn(3): create a guard page below the stack for rfork_thread on x86.

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lib/
libc/
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lib/libc/gen/posix_spawn.c

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