Message-ID: <5954A66D.0@huawei.com>
Date: Thu, 29 Jun 2017 15:04:13 +0800
From: zhong jiang <zhongjiang@huawei.com>
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To: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@kernel.org>, <akpm@linux-foundation.org>,
        <mingo@redhat.com>, <minchan@kernel.org>, <mhocko@suse.com>,
        <hpa@zytor.com>, <x86@kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] futex: avoid undefined behaviour when shift exponent
 is negative
References: <1498045437-7675-1-git-send-email-zhongjiang@huawei.com> <20170621164036.4findvvz7jj4cvqo@gmail.com> <595331FE.3090700@huawei.com> <alpine.DEB.2.20.1706282353190.1890@nanos> <59545DD6.3030508@huawei.com> <alpine.DEB.2.20.1706290832140.1861@nanos>
In-Reply-To: <alpine.DEB.2.20.1706290832140.1861@nanos>
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On 2017/6/29 14:33, Thomas Gleixner wrote:
> 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 ...
  I get it. Thanks
 
  Thanks
  zhongjiang
> Thanks,
>
> 	tglx
>
> .
>