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The ext packing paths took the body pointer from the bytes object but its length from len(), which a bytes subclass can override, so msgpack_pack_raw_body copied past the buffer. Size the body with Py_SIZE of the same object in _pack_inner and pack_ext_type.
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Packertakes the ext body pointer from the bytes object's buffer but its length fromlen(). Abytessubclass can override__len__, so the length no longer matches the pointer andmsgpack_pack_raw_bodycopies past the end of the buffer. Both theExtTypebranch of_pack_inner(reached bypackb(ExtType(code, data))) andPacker.pack_ext_type(code, data)are affected. PackingExtType(0x42, data)wheredatais abytessubclass whose__len__returns 4096 but whose real length is 2 reads 4094 bytes beyond the payload into the output.Size the body with
Py_SIZEof the same object instead oflen(), matching thebytesbranch just above it. The pure Python fallback is not affected: it writes the real bytes and cannot read out of bounds.test_pack_ext_type_len_overrideis skipped under the pure-Python fallback like the other C-extension-only tests. It fails on the unmodified extension (emits anext 16header claiming 4096 bytes) and passes after the change. Fulltest/passes under both the C extension andMSGPACK_PUREPYTHON=1; ruff check and format pass.