Re: [FFmpeg-devel] [PATCH 4/4] avcodec/vp3: Check width to avoid assertion failure

[Michael Niedermayer <michael@niedermayer.cc>]

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On Mon, Mar 27, 2023 at 01:45:01AM +0100, Kieran Kunhya wrote:
> On Sun, 26 Mar 2023, 23:27 Michael Niedermayer, <michael@niedermayer.cc>
> wrote:
> 
> > Fixes: Assertion failure on x86-32
> > Fixes:
> > 39641/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THEORA_fuzzer-5925660741206016
> >
> > Found-by: continuous fuzzing process
> > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by
> > <https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by>:
> > Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >  libavcodec/vp3.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/libavcodec/vp3.c b/libavcodec/vp3.c
> > index 9660def675f..22348559461 100644
> > --- a/libavcodec/vp3.c
> > +++ b/libavcodec/vp3.c
> > @@ -2353,6 +2353,8 @@ static av_cold int vp3_decode_init(AVCodecContext
> > *avctx)
> >      s->avctx  = avctx;
> >      s->width  = FFALIGN(avctx->coded_width, 16);
> >      s->height = FFALIGN(avctx->coded_height, 16);
> > +    if (s->width < 18)
> > +        return AVERROR_PATCHWELCOME;
> >      if (avctx->codec_id != AV_CODEC_ID_THEORA)
> >          avctx->pix_fmt = AV_PIX_FMT_YUV420P;
> >      avctx->chroma_sample_location = AVCHROMA_LOC_CENTER;
> > @@ -2919,7 +2921,9 @@ static int theora_decode_header(AVCodecContext
> > *avctx, GetBitContext *gb)
> >      /* sanity check */
> >      if (av_image_check_size(visible_width, visible_height, 0, avctx) < 0
> > ||
> >          visible_width  + offset_x > s->width ||
> > -        visible_height + offset_y > s->height) {
> > +        visible_height + offset_y > s->height ||
> > +        visible_width < 18
> > +    ) {
> >          av_log(avctx, AV_LOG_ERROR,
> >                 "Invalid frame dimensions - w:%d h:%d x:%d y:%d
> > (%dx%d).\n",
> >                 visible_width, visible_height, offset_x, offset_y,
> > @@ -2965,6 +2969,8 @@ static int theora_decode_header(AVCodecContext
> > *avctx, GetBitContext *gb)
> >      } else
> >          avctx->pix_fmt = AV_PIX_FMT_YUV420P;
> >
> > +    if (s->width < 18)
> > +        return AVERROR_PATCHWELCOME;
> >      ret = ff_set_dimensions(avctx, s->width, s->height);
> >      if (ret < 0)
> >          return ret;
> > --
> > 2.17.1
> >
> 
> Please provide some explanation around the number "18"

Its because the source needed for MC of a block is 9x9 and chroma is 1/2
so a width of 18 is ok but 16 could result in a 8 linesize into which a 9x9
block would not fit.
This is x86-32 specific probably as the linesize is rouded up more on 64bit

If theres some value in such small sizes i could look into adjusting the
linesize up in these cases?

ill add this to the commit message:
Fixes: Assertion failure on x86-32
       av_assert2(block_w * sizeof(pixel) <= FFABS(buf_linesize)); in ff_emulated_edge_mc()


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

Complexity theory is the science of finding the exact solution to an
approximation. Benchmarking OTOH is finding an approximation of the exact

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