linux-kernel - Re: [PATCH v2] bitops: Fix shift overflow in GENMASK macros

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Message-ID: <5458AEC5.5020606@st.com>
Date:	Tue, 4 Nov 2014 11:47:33 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	<gong.chen@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, <tytso@....edu>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
	<kernel@...inux.com>, <eric.paire@...com>
Subject: Re: [PATCH v2] bitops: Fix shift overflow in GENMASK macros


On 11/04/2014 11:44 AM, Peter Zijlstra wrote:
> On Tue, Nov 04, 2014 at 11:03:57AM +0100, Maxime COQUELIN wrote:
>
>> -#define GENMASK(h, l)		(((U32_C(1) << ((h) - (l) + 1)) - 1) << (l))
>> -#define GENMASK_ULL(h, l)	(((U64_C(1) << ((h) - (l) + 1)) - 1) << (l))
>> +#define GENMASK(h, l)     ((~0UL >> (BITS_PER_LONG - (h - l + 1))) << l)
>> +#define GENMASK_ULL(h, l) ((~0ULL >> (BITS_PER_LONG_LONG - (h - l + 1))) << l)
> OK, so you need to keep the (h) and (l) bits, macro arguments should be
> wrapped in seemingly superfluous braces in order to preserve precedence
> on expansion.
You're right, I should have  seen this..
>
> My bad for not explicitly doing that when suggesting the alternative.
Not a problem, v3 is coming.

Maxime
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