[FFmpeg-devel] [PATCH] avcodec/movtextenc: fix compile warning for type-limits

[Carl Eugen Hoyos]

Am So., 14. Feb. 2021 um 17:30 Uhr schrieb Nuo Mi <nuomi2021 at gmail.com>:
>
> On Sun, Feb 14, 2021 at 6:35 PM Anton Khirnov <anton at khirnov.net> wrote:
>
> > Quoting Nuo Mi (2021-02-14 07:27:39)
> > > CC      libavcodec/mpegaudiodec_common.o
> > > libavcodec/movtextenc.c: In function ‘mov_text_style_start’:
> > > libavcodec/movtextenc.c:358:26: warning: comparison is always false due
> > to limited range of data type [-Wtype-limits]
> > >   358 |         if (s->count + 1 > SIZE_MAX /
> > sizeof(*s->style_attributes) ||
> > > ---
> > >  libavcodec/movtextenc.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/libavcodec/movtextenc.c b/libavcodec/movtextenc.c
> > > index 1bef21e0b9..cd0e43a79b 100644
> > > --- a/libavcodec/movtextenc.c
> > > +++ b/libavcodec/movtextenc.c
> > > @@ -355,7 +355,7 @@ static int mov_text_style_start(MovTextContext *s)
> > >          StyleBox *tmp;
> > >
> > >          // last style != defaults, end the style entry and start a new
> > one
> > > -        if (s->count + 1 > SIZE_MAX / sizeof(*s->style_attributes) ||
> > > +        if ((s->count + 1) *  sizeof(*s->style_attributes) > SIZE_MAX ||
> >
> > What guarantees the multiplication does not overflow?
> >
> Hi Anton,
> Thanks for the review.
> It never overflows on 64bits system.

But not every system is a 64bit system

> This why the compiler has warning on
> my 64 bits system.

Yes, we also see the warning on 64bit systems.

> If you check
> https://github.com/FFmpeg/FFmpeg/blob/21346672270ae723aa774a9c8b0749954a75b3df/libavcodec/movtextenc.c#L110
> s->count * sizeof(*s->style_attributes) never > 32 bits.

> I can add some check for s->count check based on the code if you
> are preferred.

Isn't this exactly the check that is already there?

Carl Eugen