[FFmpeg-devel] Bug in libavformat/mov.c?

Ivan Zezyulya ivan
Wed Jul 9 13:56:44 CEST 2008

Hi all,

please anyone can tell me the meaning of the following lines in 
libavformat/mov.c:1043 in function mov_read_stsz:

  if(entries >= UINT_MAX / sizeof(int))
        return -1;

here is more context, this fucntion reads the 'stsz' atom in a quicktime 
movie file:

static int mov_read_stsz(MOVContext *c, ByteIOContext *pb, MOV_atom_t atom)
    AVStream *st = c->fc->streams[c->fc->nb_streams-1];
    MOVStreamContext *sc = st->priv_data;
    unsigned int i, entries, sample_size;

    get_byte(pb); /* version */
    get_be24(pb); /* flags */

    sample_size = get_be32(pb);
    if (!sc->sample_size) /* do not overwrite value computed in stsd */
        sc->sample_size = sample_size;
    entries = get_be32(pb);
    if(entries >= UINT_MAX / sizeof(int))
        return -1;

    sc->sample_count = entries;
    if (sample_size)
        return 0;

    dprintf(c->fc, "sample_size = %d sample_count = %d\n", 
sc->sample_size, sc->sample_count);

    sc->sample_sizes = av_malloc(entries * sizeof(int));
    if (!sc->sample_sizes)
        return -1;
    for(i=0; i<entries; i++)
        sc->sample_sizes[i] = get_be32(pb);
    return 0;

The problem is that I have a .mov file, VERY large file (101GB), and it 
has more than UINT_MAX / sizeof(int) entries in one of its sample size 
tables. After reaching the above lines with check of "entries" variable, 
ffmpeg recursively quits reading .mov file and reports "error reading 
header" (in mov_read_header function in libavormat/mov.c:1743) and then 
quits. The file itself is correct, the QuickTime player can successfully 
play it.

According to the QuickTime specification 
(http://developer.apple.com/documentation/QuickTime/QTFF/qtff.pdf), the 
"Number of entries" field is "A 32-bit integer containing the count of 
entries in the sample size table.", without any upper limits.

I've just commented out this check and after it ffmpeg was able to read 
all the tracks and the whole .mov file and successfully process it 
without errors. (Of course, it was not so fast, it took for me not an 
one hour to identify this problem in deep debugging ;)
But this line of code looks a bit strange for me, and I'll be happy if 
someone can tell me why it is needed so I can comment it out without 
suspicion that I'm breaking something in ffmpeg.


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