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

Ben Hutchings
2014-01-16T00:18:01Z

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.

Ben.

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Ben Hutchings, Staff Engineer, Solarflare
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