Verifier on buffer with size prefix and file identifier reads file identifier at the wrong offset

[ovenystas-mp]

Hi,

I'm just starting to use FlatBuffers and I noticed this bug.

In verifier.c, functions flatcc_verify_buffer_header_with_size() and flatcc_verify_typed_buffer_header_with_size() doesn't add the size_prefix offset when reading the file identifier. This results in that it reads the file identifier at the wrong place and verifier falsely fails with error flatcc_verify_error_identifier_mismatch.

I suggest this code change in both functions:

From:
id = read_thash(buf, offset_size);

To:
id = read_thash(buf, 2 * offset_size);