Protect getlist() walk with a critical section (free-threading, #9852) - #9854
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Protect getlist() walk with a critical section (free-threading, #9852)#9854espressolee wants to merge 1 commit into
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getlist() (used by Image.point()) walks the caller's list via PySequence_Fast and the unchecked PySequence_Fast_GET_ITEM macro over a size captured earlier. On a free-threaded build a concurrent resize of that list drives an out-of-bounds read and a segfault (python-pillow#9852). Hold a critical section on the fast sequence during the walk and clamp the loop to its current length, mirroring the approach used for FontObject in python-pillow#9498. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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getlist()(used byImage.point()) walks the caller's list withPySequence_Fastand the uncheckedPySequence_Fast_GET_ITEMmacro over a size captured earlier in the function. On a free-threaded buildPySequence_Fastreturns the list itself for a list input, so the walk aliases the caller's list; another thread resizing it drives an out-of-bounds read and a segfault. This is #9852.This holds a critical section on the fast sequence for the walk and clamps the loop to its current length, so a resize that happened between the earlier size read and the walk cannot read out of bounds. It mirrors the critical-section approach already used for
FontObjectin #9498, and uses thePy_BEGIN_CRITICAL_SECTIONshim already vendored inthirdparty/pythoncapi_compat.h(a no-op block on < 3.13, where the GIL serialises the walk anyway).Verification
python3.14.0rc1t, built from source:img.im.point(shared, "L")while others resizeshared) goes from 10/10 SIGSEGV to 0/10 with this change; the no-mutator, decoy and GIL-on controls stay clean.Tests/test_image_point.pypasses (4/4), andpoint()output is unchanged for normal look-up tables (identity and offset LUTs verified).A concurrent resize now yields a truncated read rather than a crash; if you'd prefer it to raise instead, I'm happy to adjust.
Fixes #9852.
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