[FFmpeg-devel] [PATCH] probetest.c

[Michael Niedermayer]

On Wed, Sep 16, 2009 at 07:51:03AM +0300, Kostya wrote:
> On Tue, Sep 15, 2009 at 09:12:35PM +0200, Michael Niedermayer wrote:
> > On Tue, Sep 15, 2009 at 09:04:22PM +0300, Kostya wrote:
> > > On Tue, Sep 15, 2009 at 07:39:59PM +0200, Vitor Sessak wrote:
> > > > Michael Niedermayer wrote:
> > > >> On Mon, Sep 14, 2009 at 05:19:03PM +0200, Michael Niedermayer wrote:
> > > >>> Some simple probe testing code should be attached
> > > >>>
> > > >>> its output, aka formats that detect random data with a score >25 are:
> > > >>> Failure of h261 probing code with score=50 type=0 p=B53 size=2
> > > >>> Failure of vc1test probing code with score=50 type=0 p=40E size=4
> > > >>> Failure of h263 probing code with score=50 type=0 p=C1E size=4
> > > >>> Failure of idcin probing code with score=50 type=0 p=E93 size=8
> > > >>> Failure of mm probing code with score=50 type=0 p=FC0 size=128
> > > >>> Failure of dv probing code with score=75 type=1 p=E85 size=256
> > > >>> Failure of vmd probing code with score=50 type=0 p=AF4 size=512
> > > >>> Failure of mpeg probing code with score=52 type=0 p=D9C size=4096
> > > >>> Failure of dts probing code with score=51 type=1 p=B8F size=8192
> > > >>>
> > > >>> maintainers of the listed formats should look into fixing their probe
> > > >>> code!
> > > >>> ill try to look at h26*/mpeg if i find the time, that said if someone
> > > >>> else is faster thats welcome of course ...
> > > >>
> > > >> and as everyone celebrates victory, probetest returns from the depths of
> > > >> hell stronger and more evil to devour more demuxers
> > > >> heres the output up to 32k
> > > >
> > > > [...]
> > > >
> > > >> Failure of mpc8 probing code with score=100 type=3 p=85A size=64
> > > 
> > > May I say that it would fail in many cases since any string of uppercase
> > > letters starting with 'MPCK' can be parsed as valid Moosepack SV8 file
> > > with strange but valid tags?
> > 
> > is that some kind of ukrainian humor?
> > lets see
> > 
> >     if(tag != TAG_STREAMHDR){
> >         av_log(s, AV_LOG_ERROR, "Stream header not found\n");
> >         return -1;
> >     }
> >     pos = url_ftell(pb);
> >     url_fskip(pb, 4); //CRC
> >     c->ver = get_byte(pb);
> >     if(c->ver != 8){
> >         av_log(s, AV_LOG_ERROR, "Unknown stream version %d\n", c->ver);
> >         return -1;
> >     }
> 
> If you read SV8 spec or even my code, you'll see that file may contain
> any number of tags before stream header, so file like:
> MPCKMNF<67 bytes with  anything>FFM<76 other bytes with anything>SH\017...
> is valid.

ill be more clear, what i meant is that
1. you should check for the main header (MPCK or so), you do that
2. you should follow the chain of chunks, you do not do that
3. if you find a stream header you should check that its valid (tag,ver,crc)
if anything is invalid return 0, because read_header will fail anyway
else if too little data has been provided by the application so the chain
        goes beyond the buffer return a score below 25
else all is valid and you can return the MAX score

i dont know if checking unknown tags to be printable upper case letters is
safe, also your new crc!=0 check looks a little odd, crcs in general can be
0.
But if checking for printable upper case letters is safe, you surely should
check the whole chain that is within the buffer!

the case that the stream header is outside the probe buffer should be very
unlikely because, first an application is _required_ to increase the buffer
size up to some limit when probing failed and second the header really should
be close to the file start

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

Breaking DRM is a little like attempting to break through a door even
though the window is wide open and the only thing in the house is a bunch
of things you dont want and which you would get tomorrow for free anyway
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