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

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Message-ID: <20141104104414.GL3337@twins.programming.kicks-ass.net>
Date:	Tue, 4 Nov 2014 11:44:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Maxime COQUELIN <maxime.coquelin@...com>
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 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.

My bad for not explicitly doing that when suggesting the alternative.
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