[FFmpeg-devel] [PATCH v3] libavformat/flacdec: Workaround for truncated metadata picture size

[Mattias Wadman]

lavf flacenc could previously write truncated metadata picture size if
the picture data did not fit in 24 bits. Detect this by truncting the
size found inside the picture block and if it matches the block size
use it and read rest of picture data.

Also only enable this workaround flac files and not ogg files with flac
METADATA_BLOCK_PICTURE comment.

flacenc was fixed in e447a4d112bcfee10126c54eb4481fa8712957c8
before the fix a broken flac for reproduction could be generated with:
ffmpeg -f lavfi -i sine -f lavfi -i color=red:size=2400x2400 -map 0:0 -map 1:0 -c:v:0 bmp -disposition:1 attached_pic -t 1 test.flac

Fixes ticket 6333
---
 libavformat/flac_picture.c   | 35 +++++++++++++++++++++++++++--------
 libavformat/flac_picture.h   |  2 +-
 libavformat/flacdec.c        |  2 +-
 libavformat/oggparsevorbis.c |  2 +-
 4 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/libavformat/flac_picture.c b/libavformat/flac_picture.c
index 81ddf80465..61277e9dee 100644
--- a/libavformat/flac_picture.c
+++ b/libavformat/flac_picture.c
@@ -27,7 +27,7 @@
 #include "id3v2.h"
 #include "internal.h"
 
-int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size)
+int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size, int truncate_workaround)
 {
     const CodecMime *mime = ff_id3v2_mime_tags;
     enum AVCodecID id = AV_CODEC_ID_NONE;
@@ -36,7 +36,8 @@ int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size)
     GetByteContext g;
     AVStream *st;
     int width, height, ret = 0;
-    unsigned int len, type;
+    unsigned int type;
+    uint32_t len, left, trunclen = 0;
 
     if (buf_size < 34) {
         av_log(s, AV_LOG_ERROR, "Attached picture metadata block too short\n");
@@ -114,16 +115,34 @@ int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size)
 
     /* picture data */
     len = bytestream2_get_be32u(&g);
-    if (len <= 0 || len > bytestream2_get_bytes_left(&g)) {
-        av_log(s, AV_LOG_ERROR, "Attached picture metadata block too short\n");
-        if (s->error_recognition & AV_EF_EXPLODE)
-            ret = AVERROR_INVALIDDATA;
-        goto fail;
+
+    left = bytestream2_get_bytes_left(&g);
+    if (len <= 0 || len > left) {
+        // Workaround lavf flacenc bug that allowed writing truncated metadata picture block size if
+        // picture size did not fit in 24 bits
+        if (truncate_workaround && len > left && (len & 0xffffff) == left) {
+            av_log(s, AV_LOG_INFO, "Correcting truncated metadata picture size from %d to %d\n", left, len);
+            trunclen = len - left;
+        } else {
+            av_log(s, AV_LOG_ERROR, "Attached picture metadata block too short\n");
+            if (s->error_recognition & AV_EF_EXPLODE)
+                ret = AVERROR_INVALIDDATA;
+            goto fail;
+        }
     }
     if (!(data = av_buffer_alloc(len + AV_INPUT_BUFFER_PADDING_SIZE))) {
         RETURN_ERROR(AVERROR(ENOMEM));
     }
-    bytestream2_get_bufferu(&g, data->data, len);
+
+    if (trunclen == 0) {
+        bytestream2_get_bufferu(&g, data->data, len);
+    } else {
+        // If truncation was detect copy all data from block and read missing bytes
+        // not included in the block size
+        bytestream2_get_bufferu(&g, data->data, left);
+        if (avio_read(s->pb, data->data + len - trunclen, trunclen) < trunclen)
+            RETURN_ERROR(AVERROR_INVALIDDATA);
+    }
     memset(data->data + len, 0, AV_INPUT_BUFFER_PADDING_SIZE);
 
     if (AV_RB64(data->data) == PNGSIG)
diff --git a/libavformat/flac_picture.h b/libavformat/flac_picture.h
index 4374b6f4f6..61fd0c8806 100644
--- a/libavformat/flac_picture.h
+++ b/libavformat/flac_picture.h
@@ -26,6 +26,6 @@
 
 #define RETURN_ERROR(code) do { ret = (code); goto fail; } while (0)
 
-int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size);
+int ff_flac_parse_picture(AVFormatContext *s, uint8_t *buf, int buf_size, int truncate_workaround);
 
 #endif /* AVFORMAT_FLAC_PICTURE_H */
diff --git a/libavformat/flacdec.c b/libavformat/flacdec.c
index cb516fb1f3..79c05f14bf 100644
--- a/libavformat/flacdec.c
+++ b/libavformat/flacdec.c
@@ -146,7 +146,7 @@ static int flac_read_header(AVFormatContext *s)
             }
             av_freep(&buffer);
         } else if (metadata_type == FLAC_METADATA_TYPE_PICTURE) {
-            ret = ff_flac_parse_picture(s, buffer, metadata_size);
+            ret = ff_flac_parse_picture(s, buffer, metadata_size, 1);
             av_freep(&buffer);
             if (ret < 0) {
                 av_log(s, AV_LOG_ERROR, "Error parsing attached picture.\n");
diff --git a/libavformat/oggparsevorbis.c b/libavformat/oggparsevorbis.c
index 27d2c686b6..6f15204ada 100644
--- a/libavformat/oggparsevorbis.c
+++ b/libavformat/oggparsevorbis.c
@@ -165,7 +165,7 @@ int ff_vorbis_comment(AVFormatContext *as, AVDictionary **m,
                 av_freep(&tt);
                 av_freep(&ct);
                 if (ret > 0)
-                    ret = ff_flac_parse_picture(as, pict, ret);
+                    ret = ff_flac_parse_picture(as, pict, ret, 0);
                 av_freep(&pict);
                 if (ret < 0) {
                     av_log(as, AV_LOG_WARNING, "Failed to parse cover art block.\n");
-- 
2.18.0