Re: [FFmpeg-devel] [PATCH] avformat/mp3dec: Workaround mp3 detection failure (PR #20212)

[Zhao Zhili <quinkblack@foxmail.com>]

> On Aug 11, 2025, at 18:51, Michael Niedermayer <michael@niedermayer.cc> wrote:
> 
> On Mon, Aug 11, 2025 at 11:37:55AM +0300, quink wrote:
>> PR #20212 opened by quink
>> URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20212
>> Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20212.patch
>> 
>> A set of files begins with the following byte sequence.
>> 
>> 00000000  4c 41 4d 45 33 2e 31 30  30 aa aa aa aa aa aa aa  |LAME3.100.......|
>> 00000010  aa aa aa aa aa aa aa aa  aa aa aa aa aa aa aa aa  |................|
>> *
>> 
>> It seems they are encoded with lame, but missing header bytes at the
>> beginning.
>> 
>> Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
>> 
>> 
>> From e24f7b8d0fbc4295b3dfc9832d7b552086f45ff0 Mon Sep 17 00:00:00 2001
>> From: Zhao Zhili <zhilizhao@tencent.com>
>> Date: Mon, 11 Aug 2025 15:23:36 +0800
>> Subject: [PATCH] avformat/mp3dec: Workaround mp3 detection failure
>> 
>> A set of files begins with the following byte sequence.
>> 
>> 00000000  4c 41 4d 45 33 2e 31 30  30 aa aa aa aa aa aa aa  |LAME3.100.......|
>> 00000010  aa aa aa aa aa aa aa aa  aa aa aa aa aa aa aa aa  |................|
>> *
>> 
>> It seems they are encoded with lame, but missing header bytes at the
>> beginning.
>> 
>> Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
>> ---
>> libavformat/mp3dec.c | 7 +++++--
>> 1 file changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c
>> index 31eeb68ebb..fd692a9c76 100644
>> --- a/libavformat/mp3dec.c
>> +++ b/libavformat/mp3dec.c
>> @@ -121,8 +121,11 @@ static int mp3_read_probe(const AVProbeData *p)
>>     // issues with MPEG-files!
>>     if   (first_frames>=7) return AVPROBE_SCORE_EXTENSION + 1;
>>     else if (max_frames>200 && p->buf_size < 2*max_framesizes)return AVPROBE_SCORE_EXTENSION;
>> -    else if (max_frames>=4 && p->buf_size < 2*max_framesizes) return AVPROBE_SCORE_EXTENSION / 2;
>> -    else if (ff_id3v2_match(buf0, ID3v2_DEFAULT_MAGIC) && 2*ff_id3v2_tag_len(buf0) >= p->buf_size)
>> +    else if (max_frames>=4 && p->buf_size < 2*max_framesizes) {
>> +        if (!memcmp(p->buf, "LAME3.100", sizeof("LAME3.100") - 1))
>> +            return AVPROBE_SCORE_EXTENSION - 2;
>> +        return AVPROBE_SCORE_EXTENSION / 2;
>> +    } else if (ff_id3v2_match(buf0, ID3v2_DEFAULT_MAGIC) && 2*ff_id3v2_tag_len(buf0) >= p->buf_size)
> 
> why is this needed ?
> 
> Naively i would expect that the probe code would detect a mp3 stream like this
> based on max_frames
> why does that fail ?
> as what is teh stream detected instead ?
> maybe whatever detects it, is wrong instead and can be improved by not missdetecting it

probe just report invalid data:

Probing mp3 score:25 size:2048
Probing h263 score:25 size:4096
Probing mp3 score:25 size:4096
Probing h263 score:25 size:8192
Probing mp3 score:25 size:8192
Probing h263 score:25 size:16384
Probing mp3 score:25 size:16384
Probing h263 score:25 size:20160
Probing mp3 score:25 size:20160
Probing h263 score:25 size:20160
Probing mp3 score:25 size:20160
[AVIOContext @ 0x127f04ac0] Statistics: 20160 bytes read, 0 seeks

3ac558a6ebf467b42e4a42be3014c6c5.mp3: Invalid data found when processing input

These files are only 1 to 2 seconds contains a single English word. I think the size of file
is one of the reason of failed to detect.

> 
> thx
> 
> [...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
> If the United States is serious about tackling the national security threats 
> related to an insecure 5G network, it needs to rethink the extent to which it
> values corporate profits and government espionage over security.-Bruce Schneier
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