[FFmpeg-cvslog] [FFmpeg-devel] vp8: change mv_{min, max}.{x, y} type to int

[Ronald S. Bultje]

On Mon, Jun 8, 2015 at 5:33 PM, Clément Bœsch <u at pkh.me> wrote:

> On Mon, Jun 08, 2015 at 11:31:15PM +0200, Andreas Cadhalpun wrote:
> > ffmpeg | branch: master | Andreas Cadhalpun <
> Andreas.Cadhalpun at googlemail.com> | Mon Jun  8 22:38:29 2015 +0200|
> [6fdbaa2b7fb56623ab2163f861952bc1408c39b3] | committer: Andreas Cadhalpun
> >
> > vp8: change mv_{min,max}.{x,y} type to int
> >
> > If one of the dimensions is larger than 8176, s->mb_width or
> > s->mb_height is larger than 511, leading to an int16_t overflow of
> > s->mv_max.{x,y}. This then causes av_clip to be called with amin > amax.
> >
> > Changing the type to int avoids the overflow and has no negative
> > effect, because s->mv_max is only used in clamp_mv for clipping.
> > Since mv_max.{x,y} is positive and mv_min.{x,y} negative, av_clip can't
> > increase the absolute value. The input to av_clip is an int16_t, and
> > thus the output fits into int16_t as well.
> >
> > For additional safety, s->mv_{min,max}.{x,y} are clipped to int16_t range
> > before use.
> >
> > Reviewed-by: Ronald S. Bultje <rsbultje at gmail.com>
> > Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> >
> > >
> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=6fdbaa2b7fb56623ab2163f861952bc1408c39b3
> > ---
> >
> >  libavcodec/vp8.c |    6 ++++--
> >  libavcodec/vp8.h |    9 +++++++--
> >  2 files changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/libavcodec/vp8.c b/libavcodec/vp8.c
> > index dbba568..becbb2c 100644
> > --- a/libavcodec/vp8.c
> > +++ b/libavcodec/vp8.c
> > @@ -757,8 +757,10 @@ static int vp8_decode_frame_header(VP8Context *s,
> const uint8_t *buf, int buf_si
> >  static av_always_inline
> >  void clamp_mv(VP8Context *s, VP56mv *dst, const VP56mv *src)
> >  {
> > -    dst->x = av_clip(src->x, s->mv_min.x, s->mv_max.x);
> > -    dst->y = av_clip(src->y, s->mv_min.y, s->mv_max.y);
> > +    dst->x = av_clip(src->x, av_clip(s->mv_min.x, INT16_MIN, INT16_MAX),
> > +                             av_clip(s->mv_max.x, INT16_MIN,
> INT16_MAX));
> > +    dst->y = av_clip(src->y, av_clip(s->mv_min.y, INT16_MIN, INT16_MAX),
> > +                             av_clip(s->mv_max.y, INT16_MIN,
> INT16_MAX));
>
> Sorry I might have missed something, but why not use av_clip_int16()?
>

Hm yeah I forgot about that, sorry. You could also consider clipping (into
a second variable, since this one is used to increment in the main block
decode loop) so that you only clip once per mb (horizontal) + once per row
(vertical), instead of twice per mv, which might be 32x for a 4x4 block.

(But I can submit a patch for that later.)

Ronald