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

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Date:   Thu, 29 Jun 2017 15:04:13 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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@...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 ...
  I get it. Thanks
 
  Thanks
  zhongjiang
> Thanks,
>
> 	tglx
>
> .
>