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seekable_format: Reject offsets past the end in decompress - #4902
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ZSTD_seekable_decompress() limited len with offset + len > eos. An offset well past the end made eos - offset underflow into a huge byte count that ZSTD_isError() did not flag. An offset + len that wrapped around skipped the limit, and the loop never exited. Return parameter_outOfBound for an offset past the end, and compare len against eos - offset. Reading at the end still returns 0.
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…e data The return value is smaller than dstSize when the data ends first, 0 when offset is the end of the data, and an error past it.
Cyan4973
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Thanks for the fix!
Just added a commit documenting the return value in these limit scenarios.
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What and why
ZSTD_seekable_decompress()limitslenwithif (offset + len > eos) len = eos - offset;, and two kinds of input get past it.When
offsetis past the end of the data,eos - offsetunderflows. On a 1000-byte stream, reading 16 bytes at offset 5000 returns 18446744073709547616, andZSTD_isError()reports it as a success. Whenoffset + lenwraps around, the limit is skipped and the decompress loop never exits.The fix returns
parameter_outOfBoundfor an offset past the end and compareslenagainsteos - offsetinstead. Reading at exactly the end still returns 0.The new Test 6 in
seekable_tests.cfails without the fix.Checklist
A gain in one configuration is not a general gain: see the Performance section of CONTRIBUTING.md.
Tools used
Claude Code found the bug and wrote the fix and the test.