[FFmpeg-devel] [PATCH] avformat/mp3dec: improve junk skipping heuristic

[Michael Niedermayer]

On Fri, Oct 16, 2015 at 08:04:08PM +0200, wm4 wrote:
> Commit 2b3e9bbfb529e6bde238aeb511b55ebe461664c8 caused problems for a
> certain API user:
> 
> https://code.google.com/p/chromium/issues/detail?id=537725
> https://code.google.com/p/chromium/issues/detail?id=542032
> 
> The problem seems rather arbitrary, because if there's junk, anything
> can happen. In this case, the imperfect junk skipping just caused it to
> read different junk, from what I can see.
> 
> We can improve the accuracy of junk detection by a lot by checking if 2
> consecutive frames use the same configuration. While in theory it might
> be completely fine for the 1st frame to have a different format than the
> 2nd frame, it's exceedingly unlikely, and I can't think of a legitimate
> use-case.
> 
> This is approximately the same mpg123 does for junk skipping, and the
> set of compared header bits is exactly the same.
> ---
> The reason Chromium had so much problems with this is that they don't
> allow mid-stream format changes at all, and the junk triggered format
> changes.
> 
> (Would be nice if Google paid me for this.)
> ---
>  libavformat/mp3dec.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c
> index d3080d7..e7b6234 100644
> --- a/libavformat/mp3dec.c
> +++ b/libavformat/mp3dec.c
> @@ -54,7 +54,7 @@ typedef struct {
>      int is_cbr;
>  } MP3DecContext;
>  
> -static int check(AVIOContext *pb, int64_t pos);
> +static int check(AVIOContext *pb, int64_t pos, uint32_t *header);
>  
>  /* mp3 read */
>  
> @@ -374,12 +374,21 @@ static int mp3_read_header(AVFormatContext *s)
>  
>      off = avio_tell(s->pb);
>      for (i = 0; i < 64 * 1024; i++) {
> +        uint32_t header, header2;
> +        int frame_size;
>          if (!(i&1023))
>              ffio_ensure_seekback(s->pb, i + 1024 + 4);
> -        if (check(s->pb, off + i) >= 0) {
> -            av_log(s, AV_LOG_INFO, "Skipping %d bytes of junk at %"PRId64".\n", i, off);
> -            avio_seek(s->pb, off + i, SEEK_SET);
> -            break;
> +        frame_size = check(s->pb, off + i, &header);
> +        if (frame_size > 0) {
> +            avio_seek(s->pb, off, SEEK_SET);
> +            ffio_ensure_seekback(s->pb, i + 1024 + frame_size + 4);
> +            if (check(s->pb, off + i + frame_size, &header2) >= 0 &&
> +                (header & 0xFFFEFCF0) == (header2 & 0xFFFEFCF0))

This also requires the bitrate and stereo type to match
teh bitrate can change in VBR, the stereo coding type can also change
i think

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User
questions about the command line tools should be sent to the ffmpeg-user ML.
And questions about how to use libav* should be sent to the libav-user ML.
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