[FFmpeg-devel] [PATCH 04/11] avformat/utils: Move +1 to avoid overflow

[Michael Niedermayer]

On Wed, Oct 21, 2020 at 08:47:22AM +0200, Andreas Rheinhardt wrote:
> Andreas Rheinhardt:
> > Michael Niedermayer:
> >> Fixes: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long'
> >> Fixes: Timeout
> >> Fixes: 26434/clusterfuzz-testcase-minimized-ffmpeg_dem_MV_fuzzer-5752845451919360
> >> Fixes: 26444/clusterfuzz-testcase-minimized-ffmpeg_dem_BINK_fuzzer-4697773380993024
> >>
> >> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> >> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> >> ---
> >>  libavformat/utils.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/libavformat/utils.c b/libavformat/utils.c
> >> index e8335a601f..59d65a8092 100644
> >> --- a/libavformat/utils.c
> >> +++ b/libavformat/utils.c
> >> @@ -253,7 +253,7 @@ int ffio_limit(AVIOContext *s, int size)
> >>              remaining= FFMAX(remaining, 0);
> >>          }
> >>  
> >> -        if (s->maxsize>= 0 && remaining+1 < size) {
> >> +        if (s->maxsize>= 0 && remaining < size - (int64_t)1) {
> >>              av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG, "Truncating packet of size %d to %"PRId64"\n", size, remaining+1);
> >>              size = remaining+1;
> >>          }
> >>
> > The +1 here seems very weird. If remaining + 1 == size, the packet will
> > not be truncated, despite there not being enough data left.
> > 
> > The only reason for its existence I can think of is that it is designed
> > to counter the potential -1 from s->maxsize = newsize - !newsize a few
> > lines above. Yet this makes no sense: If newsize == 0, s->maxsize has
> > just been set to -1 (indicating that future calls to ffio_limit() should
> > not try again to get the filesize) and the second check above will not
> > be reached at all.
> > 
> > So how about just removing the +1 in all three lines above?
> > 
> > - Andreas
> > 
> > PS: And while just at it: You can also add a space to "s->maxsize>= 0".
> > 
> On second look (seeing "remaining ?") this seems to be done to make sure
>  not to truncate to zero in order to always read something at all. So
> how about this here (untested):
> 
> diff --git a/libavformat/utils.c b/libavformat/utils.c
> index e8335a601f..dd1b9e41c1 100644
> --- a/libavformat/utils.c
> +++ b/libavformat/utils.c
> @@ -253,9 +253,11 @@ int ffio_limit(AVIOContext *s, int size)
>              remaining= FFMAX(remaining, 0);
>          }
> 
> -        if (s->maxsize>= 0 && remaining+1 < size) {
> -            av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG,
> "Truncating packet of size %d to %"PRId64"\n", size, remaining+1);
> -            size = remaining+1;
> +        if (s->maxsize >= 0 && remaining < size && size > 1) {
> +            av_log(NULL, remaining ? AV_LOG_ERROR : AV_LOG_DEBUG,
> +                   "Truncating packet of size %d to %"PRId64"\n",
> +                   size, remaining + !remaining);
> +            size = remaining + !remaining;
>          }
>      }
>      return size;

LGTM, please apply

thx

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

It is what and why we do it that matters, not just one of them.
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