def maybe_patch_segment_data(self, f, segment_data):
        """If SHA256 digest of the ELF file needs to be inserted into this segment, do so. Returns segment data."""
        segment_len = len(segment_data)
        file_pos = f.tell()  # file_pos is position in the .bin file
        if self.elf_sha256_offset >= file_pos and self.elf_sha256_offset < file_pos + segment_len:
            # SHA256 digest needs to be patched into this binary segment,
            # calculate offset of the digest inside the binary segment.
            patch_offset = self.elf_sha256_offset - file_pos
            # Sanity checks
            if patch_offset < self.SEG_HEADER_LEN or patch_offset + self.SHA256_DIGEST_LEN > segment_len:
                raise FatalError('Cannot place SHA256 digest on segment boundary' +
                                 '(elf_sha256_offset=%d, file_pos=%d, segment_size=%d)' %
                                 (self.elf_sha256_offset, file_pos, segment_len))
            if segment_data[patch_offset:patch_offset + self.SHA256_DIGEST_LEN] != b'\x00' * self.SHA256_DIGEST_LEN:
                raise FatalError('Contents of segment at SHA256 digest offset 0x%x are not all zero. Refusing to overwrite.' %
                                 self.elf_sha256_offset)
            assert(len(self.elf_sha256) == self.SHA256_DIGEST_LEN)
            # offset relative to the data part
            patch_offset -= self.SEG_HEADER_LEN
            segment_data = segment_data[0:patch_offset] + self.elf_sha256 + \
                segment_data[patch_offset + self.SHA256_DIGEST_LEN:]
        return segment_data