Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of the algorithm

Hannes Frederic Sowa
2014-01-16T00:46:46Z

On Thu, Jan 16, 2014 at 12:17:50AM +0000, Ben Hutchings wrote:
> On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote:
> [...]
> > --- a/include/linux/reciprocal_div.h
> > +++ b/include/linux/reciprocal_div.h
> [...]
> > + * RECIPROCAL_VALUE_TO_ZERO can be used to express an element, which
> > + * used as the argument to reciprocal_divide always yields zero.
> > */
> [...]
> > +#define RECIPROCAL_VALUE_RESULT_TO_ZERO ((struct reciprocal_value){.sh1 = 32})
> [...]
> > +static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
> > {
> > - return (u32)(((u64)A * R) >> 32);
> > + u32 t = (u32)(((u64)a * R.m) >> 32);
> > + return (t + ((a - t) >> R.sh1)) >> R.sh2;
> [...]
>
> (a - t) has type u32.
> So (a - t) >> 32 has undefined behaviour.

Good catch, totally overlooked that.

Thanks,

Hannes