linux-kernel - Re: [PATCH 1/2] bitops: Avoid integer overflow warning in GENMASK

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Date:   Thu, 3 Aug 2017 10:03:55 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     zijun_hu <zijun_hu@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Laura Abbott <labbott@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, Greg Hackmann <ghackmann@...gle.com>,
        Doug Anderson <dianders@...omium.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] bitops: Avoid integer overflow warning in GENMASK_ULL

El Thu, Aug 03, 2017 at 04:24:56PM +0300 Yury Norov ha dit:

> On Wed, Aug 02, 2017 at 03:51:58PM -0700, Matthias Kaehlcke wrote:
> > GENMASK_ULL performs a left-shift of (~0ULL), which technically
> > results in an integer overflow. clang raises a warning about
> > this if the overflow occurs in a preprocessor expression. To
> > avoid the overflow first perform a right-shift to clear the
> > bits that are shifted out.
> > 
> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> >  include/linux/bitops.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> > index a83c822c35c2..21dfe63001e3 100644
> > --- a/include/linux/bitops.h
> > +++ b/include/linux/bitops.h
> > @@ -22,7 +22,7 @@
> >  	(((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))
> 
> Once you touche GENMASK_ULL, why not to fix also GENMASK.

Will do, thanks.

Even though GENMASK_ULL can't be used (as is) to define the arm64
PAGE_OFFSET as initially intended it seems the change is still
worthwhile since there are a few other preprocessor expressions using
GENMASK_ULL.