Re: [FFmpeg-devel] [PATCH] avcodec/jpeg2000dsp: Use unsigned to avoid overflow

["Tomas Härdin" <git@haerdin.se>]

tis 2022-09-27 klockan 13:20 +0200 skrev Andreas Rheinhardt:
> Tomas Härdin:
> > tis 2022-09-27 klockan 03:47 +0200 skrev Andreas Rheinhardt:
> > > Affected the jpeg2000dsp checkasm test.
> > > 
> > > Signed-off-by: Andreas Rheinhardt
> > > <andreas.rheinhardt@outlook.com>
> > > ---
> > >  libavcodec/jpeg2000dsp.c | 9 ++++-----
> > >  1 file changed, 4 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/libavcodec/jpeg2000dsp.c b/libavcodec/jpeg2000dsp.c
> > > index b61be3b72f..b1bff6d5b1 100644
> > > --- a/libavcodec/jpeg2000dsp.c
> > > +++ b/libavcodec/jpeg2000dsp.c
> > > @@ -76,14 +76,13 @@ static void ict_int(void *_src0, void *_src1,
> > > void *_src2, int csize)
> > >  
> > >  static void rct_int(void *_src0, void *_src1, void *_src2, int
> > > csize)
> > >  {
> > > -    int32_t *src0 = _src0, *src1 = _src1, *src2 = _src2;
> > > -    int32_t i0, i1, i2;
> > > +    uint32_t *src0 = _src0, *src1 = _src1, *src2 = _src2;
> > >      int i;
> > >  
> > >      for (i = 0; i < csize; i++) {
> > > -        i1 = *src0 - (*src2 + *src1 >> 2);
> > > -        i0 = i1 + *src2;
> > > -        i2 = i1 + *src1;
> > > +        uint32_t i1 = *src0 - ((int32_t)(*src2 + *src1) >> 2);
> > 
> > The addition could conceivably overflow. Also could just use / 4
> > instead of >> 2.
> 
> The addition uses unsigned types, so that overflow is defined.

Wups, I was looking at the original code. You're right of course. What
about subtracting src0 (unsigned) from that result (signed)? Do they
get promoted to int64_t?


> Furthermore, the shift is performed on signed types and the rounding
> for
> negative numbers divided by four is different than what >> 2 produces
> (integer division is defined to use rounding towards zero, whereas
> right
> shifts of negative numbers are implementation defined and typically
> use
> rounding towards -inf (we require this behaviour)).

Tricky

/Tomas

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