[FFmpeg-devel] [PATCH 2/2] avcodec/pixlet: Fixes: undefined shift in av_mod_uintp2()

Paul B Mahol onemda at gmail.com
Sun Aug 20 17:06:46 EEST 2017


On 8/19/17, Michael Niedermayer <michael at niedermayer.cc> wrote:
> On Sat, Aug 19, 2017 at 04:00:18PM +0200, Paul B Mahol wrote:
>> On 8/19/17, Michael Niedermayer <michael at niedermayer.cc> wrote:
>> > On Fri, Aug 18, 2017 at 06:21:56PM +0200, Paul B Mahol wrote:
>> >> On 8/18/17, Michael Niedermayer <michael at niedermayer.cc> wrote:
>> >> > Fixes: runtime error: shift exponent 4294967289 is too large for
>> >> > 32-bit
>> >> > type
>> >> > 'int'
>> >> > Fixes: 3030/clusterfuzz-testcase-minimized-4649809254285312
>> >> >
>> >> > Found-by: continuous fuzzing process
>> >> > https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>> >> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>> >> > ---
>> >> >  libavcodec/pixlet.c | 2 +-
>> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/libavcodec/pixlet.c b/libavcodec/pixlet.c
>> >> > index 088226bdda..a9cfe085c9 100644
>> >> > --- a/libavcodec/pixlet.c
>> >> > +++ b/libavcodec/pixlet.c
>> >> > @@ -262,7 +262,7 @@ static int read_high_coeffs(AVCodecContext
>> >> > *avctx,
>> >> > uint8_t *src, int16_t *dst, i
>> >> >
>> >> >          flag = 0;
>> >> >
>> >> > -        if (state * 4ULL > 0xFF || i >= size)
>> >> > +        if ((uint64_t)state > 0xFF / 4 || i >= size)
>> >>
>> >> This is not exact same behaviour.
>> >
>> > no, it differs for a small number of cases that have one of the 2
>> > most significant bits (of the 64 bits) set in state.
>> > I think this was a bug i introduced in a previous commit.
>> >
>> > If above change is not the correct thing to do, please elaborate.
>> > state is passed into ff_clz() later which has no deterministic
>> > behavior when these MSBs are set as ff_clz() uses unsigned and state
>> > is int64_t and unsigned can be 32 or 64bit.
>>
>> Only 32 bits matters for ff_clz() anyway.
>
> your reply is a bit terse.
> Does that mean you have no objections to the patch ?
>
> about ff_clz(), with 64bit unsigned a ff_clz(1) should return 63, the
> code looks like it expects 31

yes.

>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> If you fake or manipulate statistics in a paper in physics you will never
> get a job again.
> If you fake or manipulate statistics in a paper in medicin you will get
> a job for life at the pharma industry.
>


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