[FFmpeg-devel] [PATCH] avformat/webvttdec: improve WebVTT parsing
Marcos Del Sol Vives
marcos at orca.pet
Tue May 27 13:28:11 EEST 2025
The parser will now strictly check if WebVTT files start with the correct
"WEBVTT" marker. Before, files were not checked if they truly started
with it.
It will also now ignore all non-cue blocks, instead of only a hardcoded
list. This is closer to the specification that calls for no action
if unknown blocks are encountered.
Signed-off-by: Marcos Del Sol Vives <marcos at orca.pet>
---
libavformat/webvttdec.c | 178 ++++++++++++++++++++++------------------
1 file changed, 98 insertions(+), 80 deletions(-)
diff --git a/libavformat/webvttdec.c b/libavformat/webvttdec.c
index 6feda1585e..b454b2c1cf 100644
--- a/libavformat/webvttdec.c
+++ b/libavformat/webvttdec.c
@@ -58,6 +58,79 @@ static int64_t read_ts(const char *s)
return AV_NOPTS_VALUE;
}
+static int webvtt_parse_cue(WebVTTContext *webvtt, AVBPrint *cue, int64_t pos)
+{
+ int i;
+ AVPacket *sub;
+ const char *p, *identifier, *settings;
+ size_t identifier_len, settings_len;
+ int64_t ts_start, ts_end;
+
+ p = identifier = cue->str;
+
+ /* optional cue identifier (can be a number like in SRT or some kind of
+ * chaptering id) */
+ for (i = 0; p[i] && p[i] != '\n' && p[i] != '\r'; i++) {
+ if (!strncmp(p + i, "-->", 3)) {
+ identifier = NULL;
+ break;
+ }
+ }
+ if (!identifier)
+ identifier_len = 0;
+ else {
+ identifier_len = strcspn(p, "\r\n");
+ p += identifier_len;
+ if (*p == '\r')
+ p++;
+ if (*p == '\n')
+ p++;
+ }
+
+ /* cue timestamps */
+ if ((ts_start = read_ts(p)) == AV_NOPTS_VALUE)
+ return AVERROR_INVALIDDATA;
+ if (!(p = strstr(p, "-->")))
+ return AVERROR_INVALIDDATA;
+ p += 2;
+ do p++; while (*p == ' ' || *p == '\t');
+ if ((ts_end = read_ts(p)) == AV_NOPTS_VALUE)
+ return AVERROR_INVALIDDATA;
+
+ /* optional cue settings */
+ p += strcspn(p, "\n\r\t ");
+ while (*p == '\t' || *p == ' ')
+ p++;
+ settings = p;
+ settings_len = strcspn(p, "\r\n");
+ p += settings_len;
+ if (*p == '\r')
+ p++;
+ if (*p == '\n')
+ p++;
+
+ /* create packet */
+ sub = ff_subtitles_queue_insert(&webvtt->q, p, strlen(p), 0);
+ if (!sub)
+ return AVERROR(ENOMEM);
+ sub->pos = pos;
+ sub->pts = ts_start;
+ sub->duration = ts_end - ts_start;
+
+#define SET_SIDE_DATA(name, type) do { \
+ if (name##_len) { \
+ uint8_t *buf = av_packet_new_side_data(sub, type, name##_len); \
+ if (!buf) \
+ return AVERROR(ENOMEM); \
+ memcpy(buf, name, name##_len); \
+ } \
+} while (0)
+
+ SET_SIDE_DATA(identifier, AV_PKT_DATA_WEBVTT_IDENTIFIER);
+ SET_SIDE_DATA(settings, AV_PKT_DATA_WEBVTT_SETTINGS);
+ return 0;
+}
+
static int webvtt_read_header(AVFormatContext *s)
{
WebVTTContext *webvtt = s->priv_data;
@@ -74,13 +147,27 @@ static int webvtt_read_header(AVFormatContext *s)
av_bprint_init(&cue, 0, AV_BPRINT_SIZE_UNLIMITED);
+ res = ff_subtitles_read_chunk(s->pb, &cue);
+ if (res < 0) {
+ av_log(s, AV_LOG_ERROR, "Unable to read file header\n");
+ goto end;
+ }
+
+ if (!cue.len) {
+ av_log(s, AV_LOG_ERROR, "Unable to read file header\n");
+ res = AVERROR_EOF;
+ goto end;
+ }
+
+ if (!strncmp(cue.str, "\xEF\xBB\xBFWEBVTT", 9) &&
+ !strncmp(cue.str, "WEBVTT", 6)) {
+ av_log(s, AV_LOG_ERROR, "Invalid file header\n");
+ res = AVERROR_INVALIDDATA;
+ goto end;
+ }
+
for (;;) {
- int i;
- int64_t pos;
- AVPacket *sub;
- const char *p, *identifier, *settings;
- size_t identifier_len, settings_len;
- int64_t ts_start, ts_end;
+ int64_t pos = avio_tell(s->pb);
res = ff_subtitles_read_chunk(s->pb, &cue);
if (res < 0)
@@ -89,81 +176,12 @@ static int webvtt_read_header(AVFormatContext *s)
if (!cue.len)
break;
- p = identifier = cue.str;
- pos = avio_tell(s->pb);
-
- /* ignore header chunk */
- if (!strncmp(p, "\xEF\xBB\xBFWEBVTT", 9) ||
- !strncmp(p, "WEBVTT", 6) ||
- !strncmp(p, "STYLE", 5) ||
- !strncmp(p, "REGION", 6) ||
- !strncmp(p, "NOTE", 4))
- continue;
-
- /* optional cue identifier (can be a number like in SRT or some kind of
- * chaptering id) */
- for (i = 0; p[i] && p[i] != '\n' && p[i] != '\r'; i++) {
- if (!strncmp(p + i, "-->", 3)) {
- identifier = NULL;
- break;
- }
- }
- if (!identifier)
- identifier_len = 0;
- else {
- identifier_len = strcspn(p, "\r\n");
- p += identifier_len;
- if (*p == '\r')
- p++;
- if (*p == '\n')
- p++;
+ res = webvtt_parse_cue(webvtt, &cue, pos);
+ if (res < 0) {
+ if (res != AVERROR_INVALIDDATA)
+ goto end;
+ av_log(s, AV_LOG_DEBUG, "Ignoring non-cue block at 0x%"PRIx64"\n", pos);
}
-
- /* cue timestamps */
- if ((ts_start = read_ts(p)) == AV_NOPTS_VALUE)
- break;
- if (!(p = strstr(p, "-->")))
- break;
- p += 2;
- do p++; while (*p == ' ' || *p == '\t');
- if ((ts_end = read_ts(p)) == AV_NOPTS_VALUE)
- break;
-
- /* optional cue settings */
- p += strcspn(p, "\n\r\t ");
- while (*p == '\t' || *p == ' ')
- p++;
- settings = p;
- settings_len = strcspn(p, "\r\n");
- p += settings_len;
- if (*p == '\r')
- p++;
- if (*p == '\n')
- p++;
-
- /* create packet */
- sub = ff_subtitles_queue_insert(&webvtt->q, p, strlen(p), 0);
- if (!sub) {
- res = AVERROR(ENOMEM);
- goto end;
- }
- sub->pos = pos;
- sub->pts = ts_start;
- sub->duration = ts_end - ts_start;
-
-#define SET_SIDE_DATA(name, type) do { \
- if (name##_len) { \
- uint8_t *buf = av_packet_new_side_data(sub, type, name##_len); \
- if (!buf) { \
- res = AVERROR(ENOMEM); \
- goto end; \
- } \
- memcpy(buf, name, name##_len); \
- } \
-} while (0)
-
- SET_SIDE_DATA(identifier, AV_PKT_DATA_WEBVTT_IDENTIFIER);
- SET_SIDE_DATA(settings, AV_PKT_DATA_WEBVTT_SETTINGS);
}
ff_subtitles_queue_finalize(s, &webvtt->q);
--
2.34.1
More information about the ffmpeg-devel
mailing list