Re: [FFmpeg-devel] [PATCH] avformat/webvttdec: improve WebVTT parsing

[Marcos Del Sol via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

A note on this change: I found some .vtt files while using yt-dlp that follow
a draft version of the WebVTT standard (probably
https://www.w3.org/2013/07/webvtt.html) that use "Region:" for regions instead
of "REGION", and that was causing the conversion to .srt to fail completely.

An example of those files is available at
https://statics.3cat.cat/multimedia/vtt/2/4/1745967620742.vtt, which belongs
to a state-owned free online streaming service in Spain.

While the diff might seem large, I basically moved the current cue parsing
to a function because it made error handling much easier. The logic is
the exact same.

-----Mensaje original-----
De: Marcos <marcos@orca.pet>
Para: ffmpeg-devel <ffmpeg-devel@ffmpeg.org>
CC: Marcos <marcos@orca.pet>
Fecha: martes, 27 de mayo de 2025 12:29 CEST
Asunto: [PATCH] avformat/webvttdec: improve WebVTT parsing


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@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

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