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

[Michael Niedermayer]

On Mon, Aug 14, 2017 at 11:30:06PM -0300, James Almer wrote:
> On 8/14/2017 8:07 PM, Michael Niedermayer wrote:
> > 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.
> 
> Probably no overhead, since renorm_encoder() can use the unchecked
> reader much like it's doing it now.
> 
> In any case, the buffer may be both read and written to, which means
> you'd have to split things in two to use both the put and get
> bytestream2 APIs, so it may not be worth doing (and probably the reason
> why it's not already been done in the first place).

yes, so unless there are objections i will apply this in a day or 2

thx

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

Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User
questions about the command line tools should be sent to the ffmpeg-user ML.
And questions about how to use libav* should be sent to the libav-user ML.
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