[FFmpeg-devel] [PATCH 2/2] avcodec/mjpegdec: silently ignore APPx stubs

[Ramiro Polla]

On Tue, Nov 26, 2024 at 11:56 AM Anton Khirnov <anton at khirnov.net> wrote:
> Quoting Ramiro Polla (2024-10-22 11:25:58)
> > On Mon, Oct 21, 2024 at 1:41 AM Michael Niedermayer
> > <michael at niedermayer.cc> wrote:
> > > On Thu, Oct 17, 2024 at 01:00:12PM +0200, Ramiro Polla wrote:
> > > > Consider APPx fields that are too short to contain an id field (32-bit)
> > > > as stubs, and silently ignore them.
> > > >
> > > > This has been seen in the MJPEG output from some webcams (such as the
> > > > Logitech C270 and C920) and the JPEG images embedded in DNG images
> > > > from the Pentax K-1 camera.
> > > > ---
> > > >  libavcodec/mjpegdec.c | 20 +++++++++++---------
> > > >  1 file changed, 11 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
> > > > index a775fdca30..3cd9904595 100644
> > > > --- a/libavcodec/mjpegdec.c
> > > > +++ b/libavcodec/mjpegdec.c
> > > > @@ -1856,20 +1856,22 @@ static int mjpeg_decode_app(MJpegDecodeContext *s)
> > > >      int len, id, i;
> > > >
> > > >      len = get_bits(&s->gb, 16);
> > > > -    if (len < 6) {
> > > > -        if (s->bayer) {
> > > > -            // Pentax K-1 (digital camera) JPEG images embedded in DNG images contain unknown APP0 markers
> > > > -            av_log(s->avctx, AV_LOG_WARNING, "skipping APPx (len=%"PRId32") for bayer-encoded image\n", len);
> > > > -            skip_bits(&s->gb, len);
> > > > -            return 0;
> > > > -        } else
> > > > -            return AVERROR_INVALIDDATA;
> > > > +    if (len < 2)
> > > > +        return AVERROR_INVALIDDATA;
> > > > +    len -= 2;
> > > > +
> > > > +    if (len < 4) {
> > > > +        /* Silently ignore APPx stubs */
> > > > +        if (show_bits(&s->gb, 8 * len) == 0)
> > > > +            goto out;
> > > > +        return AVERROR_INVALIDDATA;
> > >
> > > this silently errors on the ones it doesnt ignore, it was more informative
> > > before
> >
> > It still prints the error message after returning AVERROR_INVALIDDATA
> > from mjpeg_decode_app():
> > [mjpeg @ 0x7fc4a0002dc0] unable to decode APP fields: Invalid data
> > found when processing input
> >
> > I could also add another verbose log before "goto out" to
> > not-so-silently ignore the APPx stubs if you want.
>
> The best approach IMO is to check for AV_EF_EXPLODE and error out if
> it's set.

New patch attached. It errors out if AV_EF_EXPLODE is set, otherwise
it does an av_log(AV_LOG_VERBOSE) and skips the stub.
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