All of lore.kernel.org
 help / color / mirror / Atom feed
From: zhong jiang <zhongjiang@huawei.com>
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
Date: Thu, 29 Jun 2017 15:04:13 +0800	[thread overview]
Message-ID: <5954A66D.0@huawei.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1706290832140.1861@nanos>

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
>
> .
>

WARNING: multiple messages have this Message-ID (diff)
From: zhong jiang <zhongjiang@huawei.com>
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
Date: Thu, 29 Jun 2017 15:04:13 +0800	[thread overview]
Message-ID: <5954A66D.0@huawei.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1706290832140.1861@nanos>

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
>
> .
>


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

  reply	other threads:[~2017-06-29  7:05 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-21 11:43 [PATCH] futex: avoid undefined behaviour when shift exponent is negative zhong jiang
2017-06-21 16:40 ` Ingo Molnar
2017-06-21 16:40   ` Ingo Molnar
2017-06-28  4:35   ` zhong jiang
2017-06-28  4:35     ` zhong jiang
2017-06-28 21:43     ` hpa
2017-06-28 21:43       ` hpa
2017-06-29  2:12       ` zhong jiang
2017-06-29  2:12         ` zhong jiang
2017-06-29  4:29         ` hpa
2017-06-29  4:29           ` hpa
2017-06-29  5:57           ` zhong jiang
2017-06-29  5:57             ` zhong jiang
2017-06-28 22:13     ` Thomas Gleixner
2017-06-28 22:13       ` Thomas Gleixner
2017-06-29  1:54       ` zhong jiang
2017-06-29  1:54         ` zhong jiang
2017-06-29  6:33         ` Thomas Gleixner
2017-06-29  6:33           ` Thomas Gleixner
2017-06-29  7:04           ` zhong jiang [this message]
2017-06-29  7:04             ` zhong jiang
2017-08-25  5:21       ` zhong jiang
2017-08-25  5:21         ` zhong jiang
2017-08-25 21:13         ` Thomas Gleixner
2017-08-25 21:13           ` Thomas Gleixner
2017-08-26  2:51           ` zhong jiang
2017-08-26  2:51             ` zhong jiang

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5954A66D.0@huawei.com \
    --to=zhongjiang@huawei.com \
    --cc=akpm@linux-foundation.org \
    --cc=hpa@zytor.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@suse.com \
    --cc=minchan@kernel.org \
    --cc=mingo@kernel.org \
    --cc=mingo@redhat.com \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.