[patch] Fix BZ 19165 -- overflow in fread / fwrite

[Paul Pluzhnikov]

On Mon, Oct 26, 2015 at 12:59 AM, Florian Weimer <fweimer@redhat.com> wrote:

> > +  if (count > SIZE_MAX / size)
> > +    {
> > +      __set_errno(EOVERFLOW);
> > +      return 0;
> > +    }
>
> Can you avoid the division?  Maybe it makes sense to add a separate
> abstraction for this (a saturated multiplication function).

This https://sourceware.org/bugzilla/show_bug.cgi?id=19165#c4 is how
OpenBSD avoids the division in common case.

Do we want something like:

inline int
mul_would_overflow (size_t a, size_t b)
{
  // sqrt (SIZE_MAX + 1)
  const size_t mul_no_overflow = (size_t) 1 << 4 * sizeof (size_t);

  if ((a >= mul_no_overflow || b >= mul_no_overflow)
      && b > 1 && a > SIZE_MAX / b)
    return 1;

  return 0;
}

> It could use the built-in function with GCC 5.

What's the builtin?

Thanks,
-- 
Paul Pluzhnikov