linux-kernel - Re: [PATCH v3] bitops: Fix shift overflow in GENMASK macros

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Message-ID: <20141105164454.GY3337@twins.programming.kicks-ass.net>
Date:	Wed, 5 Nov 2014 17:44:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Maxime Coquelin <maxime.coquelin@...com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	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 v3] bitops: Fix shift overflow in GENMASK macros

On Wed, Nov 05, 2014 at 04:47:14PM +0100, Maxime Coquelin wrote:
> 
> On 11/05/2014 12:10 PM, Rasmus Villemoes wrote:
> >On Tue, Nov 04 2014, Maxime COQUELIN <maxime.coquelin@...com> 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))
> >Slightly bikeshedding here, but may I suggest spelling it
> >
> >   (((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))
> >
> >? This will ensure the arguments are expanded once each, and will, IMHO,
> >DTRT if/when l > h (namely, yield 0).
> That's a good point.
> 
> Peter, what is your view?

I'm fine with that, feel free to post another version and I'll replace
v3.
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