Backport some C89 compatible comments from libretro-common. - #4
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Sep 3, 2026
The CHD v5 compressed map header carries three bit widths - lengthbits,
selfbits and parentbits - as raw bytes, and they become the width argument to
bitstream_read() for every map entry. Nothing validated them, so a malformed
file could make bitstream_peek() evaluate
bitstream->buffer >> (32 - numbits)
with numbits above 32, shifting by a negative amount. Found by fuzzing:
libchdr_bitstream.c:62:27: runtime error: shift exponent -36 is negative
#0 bitstream_peek
#1 bitstream_read
#2 build_v5_map_checkpoints
#3 decompress_v5_map
#4 chd_open_core_file_callbacks
chdman derives all three from hunkbytes and the hunk count, so they never
legitimately exceed 32; anything larger means the file is corrupt. Reject it as
CHDERR_INVALID_FILE at parse time, before the value reaches the bitstream.
bitstream_remove() is hardened separately: consuming all 32 bits is a legitimate
request that peek() already serves, but the matching `buffer <<= 32` on a
uint32_t was undefined too.
Well-formed files are unaffected - decoded output is byte-identical over the
CHD corpus. 3281 malformed inputs, generated from all 17 corpus seed codecs
across header, map-region, whole-file and truncation mutations, now run clean
under ASan and UBSan with no hangs, on both 64-bit and 32-bit builds.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KMYbZzB8mioFmotWGFnAXG
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The next batch of C89 compatible comments backported from libretro-common. Again there is more to come, but its easier to not do it all at once...