On Sat, Mar 11, 2017 at 05:46:31PM +0100, wm4 wrote: > On Sat, 11 Mar 2017 14:50:42 +0100 > Michael Niedermayer <mich...@niedermayer.cc> wrote: > > > On Sat, Mar 11, 2017 at 02:04:25PM +0100, wm4 wrote: > > > On Sat, 11 Mar 2017 01:26:33 +0100 > > > Michael Niedermayer <mich...@niedermayer.cc> wrote: > > > > > > > On Fri, Mar 10, 2017 at 04:01:36PM +0100, wm4 wrote: > > > > > On Fri, 10 Mar 2017 15:24:52 +0100 > > > > > Michael Niedermayer <mich...@niedermayer.cc> wrote: > > > > > > > > > > > Fixes: 755/clusterfuzz-testcase-5369072516595712 > > > > > > > > > > > > See: [FFmpeg-devel] [PATCH 1/2] avcodec/h264_direct: Fix runtime > > > > > > error: signed integer overflow: 2147483647 - -14133 cannot be > > > > > > represented in type 'int' > > > > > > > > > > > > Found-by: continuous fuzzing process > > > > > > https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg > > > > > > Signed-off-by: Michael Niedermayer <mich...@niedermayer.cc> > > > > > > --- > > > > > > libavcodec/h264_direct.c | 7 ++++++- > > > > > > 1 file changed, 6 insertions(+), 1 deletion(-) > > > > > > > > > > > > diff --git a/libavcodec/h264_direct.c b/libavcodec/h264_direct.c > > > > > > index cbb84665b3..66e54479d1 100644 > > > > > > --- a/libavcodec/h264_direct.c > > > > > > +++ b/libavcodec/h264_direct.c > > > > > > @@ -39,7 +39,12 @@ static int get_scale_factor(H264SliceContext *sl, > > > > > > int poc, int poc1, int i) > > > > > > { > > > > > > int poc0 = sl->ref_list[0][i].poc; > > > > > > - int td = av_clip_int8(poc1 - poc0); > > > > > > + int64_t pocdiff = poc1 - (int64_t)poc0; > > > > > > + int td = av_clip_int8(pocdiff); > > > > > > + > > > > > > + if (pocdiff != (int)pocdiff) > > > > > > + avpriv_request_sample(sl->h264->avctx, "pocdiff > > > > > > overflow\n"); > > > > > > + > > > > > > if (td == 0 || sl->ref_list[0][i].parent->long_ref) { > > > > > > return 256; > > > > > > } else { > > > > > > > > > > Hard to image that these poc values aren't bounded by something else, > > > > > but I don't know. > > > > > > > > > > > > > > Also the previous patch didn't have this request_sample call, which > > > > > inflates this whole thing by 5 lines of code. > > > > > > > > yes thats why i suggested it initially. > > > > SUINT allows overflow detection simply by #define CHECKED 1 > > > > and running under ubsan > > > > > > > > otherwise an excplicit check is needed to detect such occurances > > > > > > You can either > > > 1. ignore the error in some way that doesn't cause problems > > > 2. ignore the error in some way that doesn't cause problems in debug > > > mode > > > 3. make the error explicit and log it > > > > > > Your first patch did 2 (which I find questionable, btw.), your current > > > > My first patch should have done 1, why do you think it does not? > > Well, it still allows the signed overflow, but only in release mode. Or
i think you misread the code, the signed overflow is only possible
when CHECKED is enabled, its not enabled in release mode.
It is enabled in DEBUG mode so ubsan can be used to find such overflows
easily while there is no undefined behavior normally or in any default
build.
> do you want this patch only to make ubsan happy? (If it's not UB, why
> does ubsan warn?)
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Old school: Use the lowest level language in which you can solve the problem
conveniently.
New school: Use the highest level language in which the latest supercomputer
can solve the problem without the user falling asleep waiting.
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