Re: [FFmpeg-devel] [PATCH 4/5] avcodec/mpegvideo_dec: Fix lowres=3 field select interlaced mpeg4 frame

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 4/5] avcodec/mpegvideo_dec: Fix lowres=3 field select interlaced mpeg4 frame
Date: Mon, 14 Jul 2025 20:52:30 +0200
Message-ID: <20250714185230.GJ29660@pb2> (raw)
In-Reply-To: <GV1P250MB0737D7693FA5DD4C1533EE6F8F55A@GV1P250MB0737.EURP250.PROD.OUTLOOK.COM>

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On Sun, Jul 13, 2025 at 07:34:50PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > Fixes: out of array read in the chroma plane
> > Fixes: 428034092/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_DEC_fuzzer-5582608941776896.test
> > 
> > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >  libavcodec/mpegvideo_dec.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/libavcodec/mpegvideo_dec.c b/libavcodec/mpegvideo_dec.c
> > index 4a54f6cd614..85e24c667e3 100644
> > --- a/libavcodec/mpegvideo_dec.c
> > +++ b/libavcodec/mpegvideo_dec.c
> > @@ -557,7 +557,7 @@ static av_always_inline void mpeg_motion_lowres(MpegEncContext *s,
> >      ptr_cr = ref_picture[2] + uvsrc_y * uvlinesize + uvsrc_x;
> >  
> >      if ((unsigned) src_x > FFMAX( h_edge_pos - (!!sx) - 2 * block_s,       0) || uvsrc_y<0 ||
> > -        (unsigned) src_y > FFMAX((v_edge_pos >> field_based) - (!!sy) - FFMAX(h, hc<<s->chroma_y_shift), 0)) {
> > +        (unsigned) src_y > FFMAX((v_edge_pos >> field_based) - (!!sy) - FFMAX(h, field_select + hc<<s->chroma_y_shift), 0)) {
> >          s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, ptr_y,
> >                                   linesize >> field_based, linesize >> field_based,
> >                                   17, 17 + field_based,
> 
> Is lowres actually used for mpegvideo-decoders at all (in 2025)?

why not ?

Example: making thumbnails of videos.
If a video can be decoded in 2min instead of 3min that still allows to scan
it sooner for potential thumbnails

thx

[...]

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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

You can kill me, but you cannot change the truth.

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