avformat/mp3dec: Require probing data to be 50% mp3 frames for low score probing to succeed

This massively reduces the detection of random data as low score mp3
It may improve security by making it harder to read non multimedia data
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mp3dec.c

@@ -71,6 +71,7 @@ static int mp3_read_probe(AVProbeData *p)
     int max_frames, first_frames = 0;
     int whole_used = 0;
     int frames, ret;
+    int framesizes, max_framesizes;
     uint32_t header;
     const uint8_t *buf, *buf0, *buf2, *end;
 
@@ -80,11 +81,12 @@ static int mp3_read_probe(AVProbeData *p)
         buf0++;
 
     max_frames = 0;
+    max_framesizes = 0;
     buf = buf0;
 
     for(; buf < end; buf= buf2+1) {
         buf2 = buf;
-        for(frames = 0; buf2 < end; frames++) {
+        for(framesizes = frames = 0; buf2 < end; frames++) {
             MPADecodeHeader h;
 
             header = AV_RB32(buf2);
@@ -92,8 +94,10 @@ static int mp3_read_probe(AVProbeData *p)
             if (ret != 0)
                 break;
             buf2 += h.frame_size;
+            framesizes += h.frame_size;
         }
         max_frames = FFMAX(max_frames, frames);
+        max_framesizes = FFMAX(max_framesizes, framesizes);
         if(buf == buf0) {
             first_frames= frames;
             if (buf2 == end + sizeof(uint32_t))
@@ -104,11 +108,11 @@ static int mp3_read_probe(AVProbeData *p)
     // issues with MPEG-files!
     if   (first_frames>=7) return AVPROBE_SCORE_EXTENSION + 1;
     else if(max_frames>200)return AVPROBE_SCORE_EXTENSION;
-    else if(max_frames>=4 && max_frames >= p->buf_size/10000) return AVPROBE_SCORE_EXTENSION / 2;
+    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)
                            return p->buf_size < PROBE_BUF_MAX ? AVPROBE_SCORE_EXTENSION / 4 : AVPROBE_SCORE_EXTENSION - 2;
     else if(first_frames > 1 && whole_used) return 5;
-    else if(max_frames>=1 && max_frames >= p->buf_size/10000) return 1;
+    else if(max_frames>=1 && p->buf_size < 2*max_framesizes) return 1;
     else                   return 0;
 //mpegps_mp3_unrecognized_format.mpg has max_frames=3
 }