From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ben Hutchings <bhutchings@solarflare.com>
Subject: Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of
 the algorithm
Date: Thu, 16 Jan 2014 00:17:50 +0000
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Cc: <davem@davemloft.net>, <netdev@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>,
	Hannes Frederic Sowa <hannes@stressinduktion.org>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	"Austin S Hemmelgarn" <ahferroin7@gmail.com>,
	Jesse Gross <jesse@nicira.com>,
	"Jamal Hadi Salim" <jhs@mojatatu.com>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Matt Mackall <mpm@selenic.com>,
	Pekka Enberg <penberg@kernel.org>,
	Christoph Lameter <cl@linux-foundation.org>,
	"Andy Gospodarek" <andy@greyhouse.net>,
	Veaceslav Falico <vfalico@redhat.com>,
	"Jay Vosburgh" <fubar@us.ibm.com>,
	Jakub Zawadzki <darkjames-ws@darkjames.pl>
To: Daniel Borkmann <dborkman@redhat.com>
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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.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.