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

[Nuo Mi]

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

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.


> --
> Anton Khirnov
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