Function ParseOptionalChunks

src/codec/utils/WebpDecoder.cpp:120–183

Skips to the next VP8/VP8L chunk header in the data given the size of the RIFF chunk 'riff_size'. Returns VP8_STATUS_BITSTREAM_ERROR if any invalid chunk size is encountered, VP8_STATUS_NOT_ENOUGH_DATA in case of insufficient data, and VP8_STATUS_OK otherwise. If an alpha chunk is found, *alpha_data and *alpha_size are set appropriately.

// If an alpha chunk is found, *alpha_data and *alpha_size are set
// appropriately.
static VP8StatusCode ParseOptionalChunks(const uint8_t** const data, size_t* const data_size,
                                         size_t const riff_size, const uint8_t** const alpha_data,
                                         size_t* const alpha_size) {
  const uint8_t* buf;
  size_t buf_size;
  uint32_t total_size = TAG_SIZE +           // "WEBP".
                        CHUNK_HEADER_SIZE +  // "VP8Xnnnn".
                        VP8X_CHUNK_SIZE;     // data.
  assert(data != nullptr);
  assert(data_size != nullptr);
  buf = *data;
  buf_size = *data_size;

  assert(alpha_data != nullptr);
  assert(alpha_size != nullptr);
  *alpha_data = nullptr;
  *alpha_size = 0;

  while (1) {
    uint32_t chunk_size;
    uint32_t disk_chunk_size;  // chunk_size with padding

    *data = buf;
    *data_size = buf_size;

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

    chunk_size = GetLE32(buf + TAG_SIZE);
    if (chunk_size > MAX_CHUNK_PAYLOAD) {
      return VP8_STATUS_BITSTREAM_ERROR;  // Not a valid chunk size.
    }
    // For odd-sized chunk-payload, there's one byte padding at the end.
    disk_chunk_size = (CHUNK_HEADER_SIZE + chunk_size + 1) & ~1;
    total_size += disk_chunk_size;

    // Check that total bytes skipped so far does not exceed riff_size.
    if (riff_size > 0 && (total_size > riff_size)) {
      return VP8_STATUS_BITSTREAM_ERROR;  // Not a valid chunk size.
    }

    // Start of a (possibly incomplete) VP8/VP8L chunk implies that we have
    // parsed all the optional chunks.
    // Note: This check must occur before the check 'buf_size < disk_chunk_size'
    // below to allow incomplete VP8/VP8L chunks.
    if (!memcmp(buf, "VP8 ", TAG_SIZE) || !memcmp(buf, "VP8L", TAG_SIZE)) {
      return VP8_STATUS_OK;
    }

    if (buf_size < disk_chunk_size) {  // Insufficient data.
      return VP8_STATUS_NOT_ENOUGH_DATA;
    }

    if (!memcmp(buf, "ALPH", TAG_SIZE)) {  // A valid ALPH header.
      *alpha_data = buf + CHUNK_HEADER_SIZE;
      *alpha_size = chunk_size;
    }