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

From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH 4/5] avcodec/mpegvideo_dec: Fix lowres=3 field select interlaced mpeg4 frame
Date: Sun, 13 Jul 2025 19:34:50 +0200
Message-ID: <GV1P250MB0737D7693FA5DD4C1533EE6F8F55A@GV1P250MB0737.EURP250.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <20250713011030.1156550-4-michael@niedermayer.cc>

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)?

- Andreas

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