avformat/oggparsevorbis: Avoid tmp bufs when parsing VorbisComment

A single VorbisComment consists of a length field and a
non-NUL-terminated string of the form "key=value". Up until now,
when parsing such a VorbisComment, zero-terminated duplicates of
key and value would be created. This is wasteful if these duplicates
are freed shortly afterwards, as happens in particular in case of
attached pictures: In this case value is base64 encoded and only
needed to decode the actual data.

Therefore this commit changes this: The buffer is temporarily modified
so that both key and value are zero-terminated. Then the data is used
in-place and restored to its original state afterwards.

This requires that the buffer has at least one byte of padding. All
buffers currently have AV_INPUT_BUFFER_PADDING_SIZE bytes padding,
so this is ok.

Finally, this also fixes weird behaviour from ogm_chapter():
It sometimes freed given to it, leaving the caller with dangling
pointers.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/libavformat/oggdec.h b/libavformat/oggdec.h
index 4cce53de41..1d28e50aa8 100644
--- a/libavformat/oggdec.h
+++ b/libavformat/oggdec.h
@@ -130,9 +130,25 @@ extern const struct ogg_codec ff_theora_codec;
 extern const struct ogg_codec ff_vorbis_codec;
 extern const struct ogg_codec ff_vp8_codec;
 
+/**
+ * Parse Vorbis comments
+ *
+ * @note  The buffer will be temporarily modifed by this function,
+ *        so it needs to be writable. Furthermore it must be padded
+ *        by a single byte (not counted in size).
+ *        All changes will have been reverted upon return.
+ */
 int ff_vorbis_comment(AVFormatContext *ms, AVDictionary **m,
                       const uint8_t *buf, int size, int parse_picture);
 
+/**
+ * Parse Vorbis comments and add metadata to an AVStream
+ *
+ * @note  The buffer will be temporarily modifed by this function,
+ *        so it needs to be writable. Furthermore it must be padded
+ *        by a single byte (not counted in size).
+ *        All changes will have been reverted upon return.
+ */
 int ff_vorbis_stream_comment(AVFormatContext *as, AVStream *st,
                              const uint8_t *buf, int size);