[FFmpeg-devel] [PATCH 1/2] lavc/aac_ac3_parser: fix the potential overflow

[Alexander Strasser]

On 2020-07-01 21:05 +0200, Alexander Strasser wrote:
> On 2020-07-01 16:23 +0200, Anton Khirnov wrote:
> > Quoting Jun Zhao (2020-06-29 15:23:10)
> > > From: Jun Zhao <barryjzhao at tencent.com>
> > >
> > > Fix the potential overflow.
> > >
> > > Suggested-by: Alexander Strasser <eclipse7 at gmx.net>
> > > Signed-off-by: Jun Zhao <barryjzhao at tencent.com>
> > > ---
> > >  libavcodec/aac_ac3_parser.c         | 9 +++++----
> > >  libavcodec/aac_ac3_parser.h         | 4 ++--
> > >  tests/ref/fate/adtstoasc_ticket3715 | 2 +-
> > >  3 files changed, 8 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/libavcodec/aac_ac3_parser.c b/libavcodec/aac_ac3_parser.c
> > > index 0746798..b26790d 100644
> > > --- a/libavcodec/aac_ac3_parser.c
> > > +++ b/libavcodec/aac_ac3_parser.c
> > > @@ -98,11 +98,12 @@ get_next:
> > >          }
> > >
> > >          /* Calculate the average bit rate */
> > > -        s->frame_number++;
> > >          if (avctx->codec_id != AV_CODEC_ID_EAC3) {
> > > -            avctx->bit_rate =
> > > -                (s->last_bit_rate * (s->frame_number -1) + s->bit_rate)/s->frame_number;
> > > -            s->last_bit_rate = avctx->bit_rate;
> > > +            if (s->frame_number < UINT64_MAX) {
> > > +                s->frame_number++;
> > > +                s->last_bit_rate += (s->bit_rate - s->last_bit_rate)/s->frame_number;
> > > +                avctx->bit_rate = (int64_t)llround(s->last_bit_rate);
> > > +            }
> > >          }
> > >      }
> > >
> > > diff --git a/libavcodec/aac_ac3_parser.h b/libavcodec/aac_ac3_parser.h
> > > index b04041f..c53d16f 100644
> > > --- a/libavcodec/aac_ac3_parser.h
> > > +++ b/libavcodec/aac_ac3_parser.h
> > > @@ -55,8 +55,8 @@ typedef struct AACAC3ParseContext {
> > >      uint64_t state;
> > >
> > >      int need_next_header;
> > > -    int frame_number;
> > > -    int last_bit_rate;
> > > +    uint64_t frame_number;
> > > +    double last_bit_rate;
> >
> > This won't give the same result on all platforms anymore.
>
> It's also a bit different from what I had in mind.
>
> I was thinking more in the line of how it's implemented in
> libavcodec/mpegaudio_parser.c .
>
> There is a bit of noise there because of data that doesn't contain audio
> and also for the CBR case I think. Wouldn't be needed here AFAICT.
>
> I may well be missing something. If so understanding more would help me
> and we could fix both places. Otherwise if it's OK like it was done in
> mpegaudio_parser, we could maybe use the same strategy here too.
>
>
> Thanks for sending the patch and sorry for the delayed response.

I meant like this:

    avctx->bit_rate += (s->bit_rate - avctx->bit_rate) / s->frame_number;

Patch attached. What do you think?

Would probably be even better to sum up in an uint64_t and divide
that sum to update the bit_rate field in AVCodecContext. Could be
implemented later for both parsers if it's considered worthwhile.


  Alexander
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