Re: [FFmpeg-devel] [PATCH v2] avformat/imfdec: check track valid before use it

[Pierre-Anthony Lemieux <pal@sandflow.com>]

On Sat, Aug 27, 2022 at 5:25 AM Steven Liu <lingjiujianke@gmail.com> wrote:
>
> Pierre-Anthony Lemieux <pal@sandflow.com> 于2022年8月27日周六 00:06写道：
> >
> > On Fri, Aug 26, 2022 at 9:01 AM Andreas Rheinhardt
> > <andreas.rheinhardt@outlook.com> wrote:
> > >
> > > Pierre-Anthony Lemieux:
> > > > On Fri, Aug 26, 2022 at 1:37 AM Andreas Rheinhardt
> > > > <andreas.rheinhardt@outlook.com> wrote:
> > > >>
> > > >> Steven Liu:
> > > >>> fix CID: 1512414
> > > >>> And return AVERROR_INVALIDDATA when get_next_track_with_minimum_timestamp
> > > >>> incorrect in imf_read_packet;
> > > >>>
> > > >>> Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
> > > >>> ---
> > > >>>  libavformat/imfdec.c | 7 +++++--
> > > >>>  1 file changed, 5 insertions(+), 2 deletions(-)
> > > >>>
> > > >>> diff --git a/libavformat/imfdec.c b/libavformat/imfdec.c
> > > >>> index 5bbe7a53f8..08f342bc1a 100644
> > > >>> --- a/libavformat/imfdec.c
> > > >>> +++ b/libavformat/imfdec.c
> > > >>> @@ -697,8 +697,9 @@ static IMFVirtualTrackPlaybackCtx *get_next_track_with_minimum_timestamp(AVForma
> > > >>>          }
> > > >>>      }
> > > >>>
> > > >>> -    av_log(s, AV_LOG_DEBUG, "Found next track to read: %d (timestamp: %lf / %lf)\n",
> > > >>> -           track->index, av_q2d(track->current_timestamp), av_q2d(minimum_timestamp));
> > > >>> +    if (track)
> > > >>> +        av_log(s, AV_LOG_DEBUG, "Found next track to read: %d (timestamp: %lf / %lf)\n",
> > > >>> +               track->index, av_q2d(track->current_timestamp), av_q2d(minimum_timestamp));
> > > >>
> > > >> Coverity actually complained about track being uninitialized, which this
> > > >> patch does not address. And the reason it does this is that it doesn't
> > > >> understand the algorithm: track will always be initialized in the first
> > > >> iteration of the loop.
> > > >
> > > > Is it possible to tell coverity that  c->track_count > 0 is a
> > > > pre-condition, or should we modify the loop/algorithm?
> > > >
> > >
> > > The typical way to do this is to add an av_assert1 or av_assert2;
> > > but this must only be done if it is indeed ensured that the assert will
> > > not be triggered.
> > >
> > > >> (If there is a first iteration of the loop -- is
> > > >> this actually guaranteed? A file without tracks seems to be pretty useless.)
> > > >
> > > > imfdec currently assumes that (a) imf_read_packet() is not called if
> > > > there are no streams/tracks and (b) a track will always be found.
> > > >
> > > > (b) will be true for a conformant IMF Composition, but I am not sure
> > > > it can always be true for a malformed one.
> > > >
> > >
> > > Can't we make it true by adding the relevant checks to read_header?
> >
> > Yes.
> Can imf add or remove track when processing? Looks like the live
> streaming change resolution or bitrate when playing.

The number of tracks is fixed and determined when the Composition
Playlist (CPL) is parsed.

> >
> > >
> > > > I think imf_read_packet() can probably be hardened. Perhaps do this as
> > > > a patch separately from addressing the coverity issue?
> > > >
> > > >> FYI: In Coverity's analysis there are loop iterations, but it just
> > > >> assumed that track is not initialized in the loop (which boils down to
> > > >> saying that it presumed the tracks' current_timestamp to be invalid
> > > >> (denominator 0). I hope this can't happen.
> > > >> (There is btw another issue: The initialization of minimum_timestamp
> > > >> presumes that int are 32bit which need not be true.)
> > > >
> > > > INT32_MAX -> INT_MAX should fix this right?
> > > >
> > >
> > > Yes.
> > >
> > > >>
> > > >>>      return track;
> > > >>>  }
> > > >>>
> > > >>> @@ -760,6 +761,8 @@ static int imf_read_packet(AVFormatContext *s, AVPacket *pkt)
> > > >>>      AVRational next_timestamp;
> > > >>>
> > > >>>      track = get_next_track_with_minimum_timestamp(s);
> > > >>> +    if (!track)
> > > >>> +        return AVERROR_INVALIDDATA;
> > > >>>
> > > >>>      ret = get_resource_context_for_timestamp(s, track, &resource);
> > > >>>      if (ret)
> > > >>
> > > >> _______________________________________________
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> > > >>
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