def _offset_of_next_ff_byte(self, start):
        """
        Return the offset of the next '\xFF' byte in *stream* starting with
        the byte at offset *start*. Returns *start* if the byte at that
        offset is a hex 255; it does not necessarily advance in the stream.
        """
        self._stream.seek(start)
        byte_ = self._read_byte()
        while byte_ != b'\xFF':
            byte_ = self._read_byte()
        offset_of_ff_byte = self._stream.tell() - 1
        return offset_of_ff_byte