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

[Florian Weimer]

On 10/26/2015 04:59 PM, Paul Pluzhnikov wrote:

> 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;
> }

I think saturating multiplication would be the more useful abstraction:
return the product if it is exact, or (size_t)-1 if it overflows.

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

The variants available are documented here:

<https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/Integer-Overflow-Builtins.html>

Florian