Function ParseVP8Header

src/codec/utils/WebpDecoder.cpp:198–236

Validates the VP8/VP8L Header ("VP8 nnnn" or "VP8L nnnn") and skips over it. Returns VP8_STATUS_BITSTREAM_ERROR for invalid (chunk larger than riff_size) VP8/VP8L header, VP8_STATUS_NOT_ENOUGH_DATA in case of insufficient data, and VP8_STATUS_OK otherwise. If a VP8/VP8L chunk is found, *chunk_size is set to the total number of bytes extracted from the VP8/VP8L chunk header. The flag '*is_lossless'

// extracted from the VP8/VP8L chunk header.
// The flag '*is_lossless' is set to 1 in case of VP8L chunk / raw VP8L data.
static VP8StatusCode ParseVP8Header(const uint8_t** const data_ptr, size_t* const data_size,
                                    int have_all_data, size_t riff_size, size_t* const chunk_size,
                                    int* const is_lossless) {
  const uint8_t* const data = *data_ptr;
  const int is_vp8 = !memcmp(data, "VP8 ", TAG_SIZE);
  const int is_vp8l = !memcmp(data, "VP8L", TAG_SIZE);
  const uint32_t minimal_size = TAG_SIZE + CHUNK_HEADER_SIZE;  // "WEBP" + "VP8 nnnn" OR
  // "WEBP" + "VP8Lnnnn"
  assert(data != nullptr);
  assert(data_size != nullptr);
  assert(chunk_size != nullptr);
  assert(is_lossless != nullptr);

  if (*data_size < CHUNK_HEADER_SIZE) {
    return VP8_STATUS_NOT_ENOUGH_DATA;  // Insufficient data.
  }

  if (is_vp8 || is_vp8l) {
    // Bitstream contains VP8/VP8L header.
    const uint32_t size = GetLE32(data + TAG_SIZE);
    if ((riff_size >= minimal_size) && (size > riff_size - minimal_size)) {
      return VP8_STATUS_BITSTREAM_ERROR;  // Inconsistent size information.
    }
    if (have_all_data && (size > *data_size - CHUNK_HEADER_SIZE)) {
      return VP8_STATUS_NOT_ENOUGH_DATA;  // Truncated bitstream.
    }
    // Skip over CHUNK_HEADER_SIZE bytes from VP8/VP8L Header.
    *chunk_size = size;
    *data_ptr += CHUNK_HEADER_SIZE;
    *data_size -= CHUNK_HEADER_SIZE;
    *is_lossless = is_vp8l;
  } else {
    // Raw VP8/VP8L bitstream (no header).
    *is_lossless = VP8LCheckSignature(data, *data_size);
    *chunk_size = *data_size;
  }

  return VP8_STATUS_OK;
}

int VP8CheckSignature(const uint8_t* const data, size_t data_size) {
  return (data_size >= 3 && data[0] == 0x9d && data[1] == 0x01 && data[2] == 0x2a);