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Fix mstrGetFlag out-of-bounds read without metadata - #16023
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mstrGetFlag() called mstrFlagsRef() unconditionally, which points before the allocation when no metadata is attached. Reading the flags word then reads 2 bytes to the left of the allocation (ASAN heap-buffer-overflow), and a caller that trusts a nonzero result can reach an out-of-range metadata pointer. Check mstrIsMetaAttached() first, as the header comment already tells callers to do. Adds an mstrTest case for a metadata-less string.
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Summary
mstrGetFlag()calledmstrFlagsRef()unconditionally, which points before the allocation when no metadata is attached. Reading the flags word then reads 2 bytes to the left of the allocation (ASAN heap-buffer-overflow), and a caller that trusts a nonzero result can reach an out-of-range metadata pointer.Check
mstrIsMetaAttached()first, as the header comment already tells callers to do.Test
Adds an
mstrTestcase for a metadata-less string.AI assistance
Tool(s) used: Claude Code
How you used it: the assistant read the code, wrote the fix and the test, and ran the built-in test. A human reviewed the diff and takes responsibility for the change.
Note
Low Risk
Small, localized safety fix in an inline helper plus a unit test; behavior for metadata-backed strings is unchanged.
Overview
Fixes
mstrGetFlagso it no longer reads flag bits when anmstrhas no metadata attached. The inline helper now returns 0 unlessmstrIsMetaAttached(s)is true, matching the documented expectation that callers should verify metadata before using flag/metadata APIs.Previously it always called
mstrFlagsRef()and dereferenced the flags word; without metadata that address sits before the allocation, which triggers ASAN heap-buffer-overflow and could yield bogus nonzero flags.Adds an
mstrTestcase that builds a plainmstrNewstring and assertsmstrGetFlag(s, 0) == 0.Reviewed by Cursor Bugbot for commit e61d1a8. Bugbot is set up for automated code reviews on this repo. Configure here.