fix: reject overlong FAST varints that silently truncate - #27
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Varint decode shifted every continuation byte into a fixed-width register with no length cap, so an 11-byte uint64/int64 (or 6-byte 32-bit) ending in the stop bit succeeded with high payload bits shifted away (e.g. [0x01, nine 0x00, 0x80] → 0). Cap at ceil(bits/7) and reject past that; FSON integers/doubles/timestamps share these decoders. Co-authored-by: Kaius Ruokonen <ruoka@users.noreply.github.com>
Co-authored-by: Kaius Ruokonen <ruoka@users.noreply.github.com>
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July 27, 2026 18:42
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Jul 28, 2026
…#28) Byte-count caps from #27 still allowed N×7 payload bits into narrower registers, so forms like uint64 [0x02, eight 0x00, 0x80] decoded as 0. Reject shifts that would drop high bits. Also fix signed size() literals that were positive (0x00000000ffffffc0ll etc.), so small negatives encoded as 5+ bytes instead of the canonical 1–4 byte forms. Co-authored-by: Cursor Agent <cursoragent@cursor.com> Co-authored-by: Kaius Ruokonen <ruoka@users.noreply.github.com>
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Bug and impact
FAST varint decode (
xson/xson-fast.c++m) accepted arbitrarily long continuation sequences as long as a stop bit eventually appeared. Extra bytes were shifted into a fixed-width register, so high payload bits were silently dropped.Trigger: an 11-byte uint64/int64 wire form such as
[0x01, nine 0x00, 0x80](or a 6-byte 32-bit form) decoded as0instead of failing.Impact: silent corruption of FSON integers, doubles (via
bit_castof uint64), and timestamps — all share these decoders on the storage path.Root cause
Decode looped
i = (i << 7) | (byte & 0x7f)with noceil(bits/7)cap (10 for 64-bit, 5 for 32-bit).Fix
Reject varints longer than the type width; throw
xson::fast overlong … varint.Validation
./tools/CB.sh debug test --tags='\[xson\]'— 425/425 passed.