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Martin Vogel
d89ccabef0
The MSan lane on 9724d903 stopped in the extraction suite's spill
round-trip test: uninitialized bytes in fwrite at offset 4087 of a
6,952-byte region, the parked result's arena block (the stack overflow
that followed is MemorySanitizer's own report path failing on the nested
bug). The park writes a result as an opaque image: the record header, a
struct copy, and the compacted arena block. Both carry padding bytes no
code ever wrote, inside structs and between objects, and MSan tracks that
mark through the compaction's memcpy, so zeroing the destination block
only moved the report (offset 6714 in the local MSan container). The
image is read back whole and never interpreted byte by byte; the padding
content is irrelevant on disk.
Under MSan the header and the block are declared defined right before
the write (__msan_unpoison, compiled in only when the build is
instrumented); every other build compiles the annotation to nothing. The
exact block is also zeroed once when it is created, so the gaps between
objects are deterministic in the spill file.
Verification: extraction, arena and parallel suites under ASan on the
fixed tree; the extraction suite in the local MSan container image
passes 348 / 0 with no sanitizer report (it reproduced the CI report exactly before the fix).
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>