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

From	Thomas Gleixner <tglx@linutronix.de>
Newsgroups	linux.kernel
Subject	Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative
Date	2017-06-29 08:40 +0200
Message-ID	<tXBho-47z-13@gated-at.bofh.it> (permalink)
References	<tUQZk-aB-21@gated-at.bofh.it> <tUQZk-aB-19@gated-at.bofh.it> <tXcVI-27M-11@gated-at.bofh.it> <tXttw-ZE-11@gated-at.bofh.it> <tXwUq-1c2-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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@localhost 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

Thread

Re: [PATCH] futex: avoid undefined behaviour when shift exponent is  negative Ingo Molnar <mingo@kernel.org> - 2017-06-21 18:50 +0200
  Re: [PATCH] futex: avoid undefined behaviour when shift exponent  is negative zhong jiang <zhongjiang@huawei.com> - 2017-06-28 06:40 +0200
    Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative hpa@zytor.com - 2017-06-28 23:50 +0200
      Re: [PATCH] futex: avoid undefined behaviour when shift exponent  is negative zhong jiang <zhongjiang@huawei.com> - 2017-06-29 04:20 +0200
        Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative hpa@zytor.com - 2017-06-29 06:40 +0200
          Re: [PATCH] futex: avoid undefined behaviour when shift exponent  is negative zhong jiang <zhongjiang@huawei.com> - 2017-06-29 08:10 +0200
    Re: [PATCH] futex: avoid undefined behaviour when shift exponent is  negative Thomas Gleixner <tglx@linutronix.de> - 2017-06-29 00:20 +0200
      Re: [PATCH] futex: avoid undefined behaviour when shift exponent  is negative zhong jiang <zhongjiang@huawei.com> - 2017-06-29 04:00 +0200
        Re: [PATCH] futex: avoid undefined behaviour when shift exponent is  negative Thomas Gleixner <tglx@linutronix.de> - 2017-06-29 08:40 +0200
          Re: [PATCH] futex: avoid undefined behaviour when shift exponent  is negative zhong jiang <zhongjiang@huawei.com> - 2017-06-29 09:10 +0200

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