[FFmpeg-devel] [PATCH 1/2] avcodec/rangecoder: Do not increase the pointer beyond the buffer

[Michael Niedermayer]

On Sun, Aug 13, 2017 at 07:18:11PM -0300, James Almer wrote:
> On 8/13/2017 7:15 PM, Michael Niedermayer wrote:
> > Fixes: undefined behavior
> > 
> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > ---
> >  libavcodec/rangecoder.c | 1 +
> >  libavcodec/rangecoder.h | 8 ++++++--
> >  2 files changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/libavcodec/rangecoder.c b/libavcodec/rangecoder.c
> > index 0bb79c880e..0d53bef076 100644
> > --- a/libavcodec/rangecoder.c
> > +++ b/libavcodec/rangecoder.c
> > @@ -58,6 +58,7 @@ av_cold void ff_init_range_decoder(RangeCoder *c, const uint8_t *buf,
> >  
> >      c->low         = AV_RB16(c->bytestream);
> >      c->bytestream += 2;
> > +    c->overread    = 0;
> >      if (c->low >= 0xFF00) {
> >          c->low = 0xFF00;
> >          c->bytestream_end = c->bytestream;
> > diff --git a/libavcodec/rangecoder.h b/libavcodec/rangecoder.h
> > index c3e81d0dcb..44af88b8f5 100644
> > --- a/libavcodec/rangecoder.h
> > +++ b/libavcodec/rangecoder.h
> > @@ -42,6 +42,8 @@ typedef struct RangeCoder {
> >      uint8_t *bytestream_start;
> >      uint8_t *bytestream;
> >      uint8_t *bytestream_end;
> > +    int overread;
> > +#define MAX_OVERREAD 2
> >  } RangeCoder;
> >  
> >  void ff_init_range_encoder(RangeCoder *c, uint8_t *buf, int buf_size);
> > @@ -106,9 +108,11 @@ static inline void refill(RangeCoder *c)
> >      if (c->range < 0x100) {
> >          c->range <<= 8;
> >          c->low   <<= 8;
> > -        if (c->bytestream < c->bytestream_end)
> > +        if (c->bytestream < c->bytestream_end) {
> >              c->low += c->bytestream[0];
> > -        c->bytestream++;
> > +            c->bytestream++;
> > +        } else
> > +            c->overread ++;
> >      }
> >  }
> 
> Wouldn't it be better to port this to the bytestream2 reading api?

this is speed relevant code, i am not sure bytestream2 wouldnt add
overhead. In fact i wasnt entirely sure keeping track of the overread
bytes is a great idea. But i didnt see an easy way to avoid that.


[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Take away the freedom of one citizen and you will be jailed, take away
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