Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752340AbaKDI3L (ORCPT ); Tue, 4 Nov 2014 03:29:11 -0500 Received: from casper.infradead.org ([85.118.1.10]:35042 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750959AbaKDI3H (ORCPT ); Tue, 4 Nov 2014 03:29:07 -0500 Date: Tue, 4 Nov 2014 09:29:01 +0100 From: Peter Zijlstra To: Maxime COQUELIN Cc: gong.chen@linux.intel.com, Ingo Molnar , " Paul E. McKenney" , tytso@mit.edu, linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel@stlinux.com, eric.paire@st.com Subject: Re: [PATCH] bitops: Fix shift overflow in GENMASK macros Message-ID: <20141104082901.GF10501@worktop.programming.kicks-ass.net> References: <1415036398-26984-1-git-send-email-maxime.coquelin@st.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1415036398-26984-1-git-send-email-maxime.coquelin@st.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 03, 2014 at 06:39:58PM +0100, Maxime COQUELIN wrote: > On some 32 bits architectures, including x86, GENMASK(31, 0) returns 0 > instead of the expected ~0UL. > > This is the same on some 64 bits architectures with GENMASK_ULL(63, 0). > > This is due to an overflow in the shift operand, 1 << 32 for GENMASK, > 1 << 64 for GENMASK_ULL. > > Fixes: 10ef6b0dffe404bcc54e94cb2ca1a5b18445a66b > Cc: #v3.13+ > Reported-by: Eric Paire > Signed-off-by: Maxime Coquelin > --- > include/linux/bitops.h | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index be5fd38..81f9725 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -18,8 +18,12 @@ > * position @h. For example > * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. > */ > -#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 - 1) - (h))) & ~((1UL << (l)) - 1)) > + > +#define GENMASK_ULL(h, l) \ > + ((~0ULL >> ((BITS_PER_LONG_LONG - 1) - (h))) & ~((1ULL << (l)) - 1)) > + I was not expecting the mask there, but instead something like: ((~0UL >> (BITS_PER_LONG - (h-l+1))) << l) which shifts the bits to the desired length and then back to the desired place. Would that not be more readable? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/