linux-kernel - Re: Is there any word about this bug in gcc ?

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Date:	Tue, 20 Nov 2007 14:10:52 +0800
From:	WANG Cong <xiyou.wangcong@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	zhengyi <goodmenkernel@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Is there any word about this bug in gcc ?

On Tue, Nov 20, 2007 at 02:03:12PM +0800, Li Zefan wrote:
>WANG Cong wrote:
>> On Mon, Nov 19, 2007 at 09:10:44PM -0800, H. Peter Anvin wrote:
>>> WANG Cong wrote:
>>>> On Tue, Nov 20, 2007 at 10:13:42AM +0800, zhengyi wrote:
>>>>> Is there any relevance to the kernel ?
>>>>>
>>>>> I found the folowing code here:
>>>>> http://linux.solidot.org/article.pl?sid=07/11/19/0512218&from=rss
>>>>>
>>>>> -------------------------------------------------------------------
>>>>> int main( void )
>>>>> {
>>>>> int i=2;
>>>>> if( -10*abs (i-1) == 10*abs(i-1) )
>>>>>   printf ("OMG,-10==10 in linux!\n");
>>>>> else
>>>>>   printf ("nothing special here\n") ;
>>>>>
>>>>> return 0 ;
>>>>> }
>>>> I think no. It is considered a bug in abs(), kernel, of course,
>>>> doesn't use glibc's abs().
>>>>
>>> Wrong.
>>>
>>> abs() is internal to gcc, and the above is optimized out at compile 
>>> time, so any user of abs() as a function at all is vulnerable.
>> 
>> This is an urgent bug, I think.
>> 
>> And you mean abs() is not in glibc, then where is it? Built in gcc?
>> And what's more, why not put it in glibc?
>> 
>
>Gcc optimises abs() to use gcc builtin-in abs(). So if we use -fno-builin, 
>we'll get the correct result. That is to say the bug has nothing to do with
>glibc.
>
>And this bug has been fixed just several days ago.
>
>http://www.nabble.com/-PATCH--Fix-PR34130,-extract_muldiv-broken-t4826688.html
>

Good explanation! Thank you!


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