linux-kernel - Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative

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Message-ID: <alpine.DEB.2.20.1706290832140.1861@nanos>
Date:   Thu, 29 Jun 2017 08:33:20 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     zhong jiang <zhongjiang@...wei.com>
cc:     Ingo Molnar <mingo@...nel.org>, akpm@...ux-foundation.org,
        mingo@...hat.com, minchan@...nel.org, mhocko@...e.com,
        hpa@...or.com, x86@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] futex: avoid undefined behaviour when shift exponent is
 negative

On Thu, 29 Jun 2017, zhong jiang wrote:
> On 2017/6/29 6:13, Thomas Gleixner wrote:
> > That's simply wrong. If oparg is negative and the SHIFT bit is set then the
> > result is undefined today and there is no way that this can be used at
> > all.
> >
> > On x86:
> >
> >    1 << -1	= 0x80000000
> >    1 << -2048	= 0x00000001
> >    1 << -2047	= 0x00000002
>   but I test the cases in x86_64 all is zero.   I wonder whether it is related to gcc or not
> 
>   zj.c:15:8: warning: left shift count is negative [-Wshift-count-negative]
>   j = 1 << -2048;
>         ^
> [root@...alhost zhongjiang]# ./zj
> j = 0

Which is not a surprise because the compiler can detect it as the shift is
a constant. oparg is not so constant ...

Thanks,

	tglx