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From: Jan Engelhardt <jengelh@linux01.gwdg.de>
To: "linux-os (Dick Johnson)" <linux-os@analogic.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Jeff Garzik <jeff@garzik.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: somebody dropped a (warning) bomb
Date: Fri, 9 Feb 2007 23:05:36 +0100 (MET)	[thread overview]
Message-ID: <Pine.LNX.4.61.0702092247430.31955@yvahk01.tjqt.qr> (raw)
In-Reply-To: <Pine.LNX.4.61.0702091509360.8711@chaos.analogic.com>


On Feb 9 2007 15:29, linux-os (Dick Johnson) wrote:
>>
>> I was referring to "absolute memory", not the offset magic that assembler
>> allows. After all, (reg+relativeOffset) will yield an absolute address.
>> What I was out at: for machines that have more than 2 GB of memory, you
>> don't call the address that is given by 0x80000000U actually "byte
>> -2147483648", but "byte 2147483648".
>
>Don't make any large bets on that!
>
>char foo()
>{
>    volatile char *p = (char *)0x80000000;
>    return *p;
>}
>Optimized....
> 	.file	"zzz.c"
> 	.text
> 	.p2align 2,,3
>.globl foo
> 	.type	foo, @function
>foo:
> 	pushl	%ebp
> 	movb	-2147483648, %al
> 	movl	%esp, %ebp
> 	movsbl	%al,%eax
> 	leave
> 	ret
> 	.size	foo, .-foo
> 	.section	.note.GNU-stack,"",@progbits
> 	.ident	"GCC: (GNU) 3.3.3 20040412 (Red Hat Linux 3.3.3-7)"

00000000 <foo>:
   0:   55                      push   %ebp
   1:   0f b6 05 00 00 00 80    movzbl 0x80000000,%eax
   8:   89 e5                   mov    %esp,%ebp
   a:   5d                      pop    %ebp
   b:   0f be c0                movsbl %al,%eax
   e:   c3                      ret    

You do know that there is a bijection between the set of signed [32bit] 
integers and unsigned [32bit] integers, don't you?
For the CPU, it's just bits. Being signed or unsigned is not important 
when just accessing memory. It will, when a comparison is involved, but 
that was not the point here. void* comparisons are unsigned. Period.
Because a compiler doing signed comparisons will "map" the memory [from 
2 GB to 4 GB] as part of the signed comparison before the memory [from 0 
GB to 2 GB], which collides with - let's call it - "the world view".


Jan
-- 
ft: http://freshmeat.net/p/chaostables/

  reply	other threads:[~2007-02-09 22:09 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-08 15:00 Jeff Garzik
2007-02-08 16:33 ` Linus Torvalds
2007-02-08 18:42   ` Jan Engelhardt
2007-02-08 19:53     ` Linus Torvalds
2007-02-08 21:10       ` Jan Engelhardt
2007-02-08 21:37         ` Linus Torvalds
2007-02-08 23:12           ` David Rientjes
2007-02-08 23:37             ` Linus Torvalds
2007-02-09  0:24               ` David Rientjes
2007-02-09  0:42                 ` Linus Torvalds
2007-02-09  0:59                   ` Linus Torvalds
2007-02-09  0:59                   ` David Rientjes
2007-02-09  1:11                     ` Linus Torvalds
2007-02-09  1:18                       ` David Rientjes
2007-02-09 15:38                         ` Linus Torvalds
2007-02-09  3:27                   ` D. Hazelton
2007-02-09 19:54                     ` Pete Zaitcev
2007-02-09 12:34                   ` Jan Engelhardt
2007-02-09 13:16                     ` linux-os (Dick Johnson)
2007-02-09 17:45                       ` Jan Engelhardt
2007-02-09 20:29                         ` linux-os (Dick Johnson)
2007-02-09 22:05                           ` Jan Engelhardt [this message]
2007-02-09 22:58                             ` Martin Mares
2007-02-12 18:50                             ` linux-os (Dick Johnson)
2007-02-13 15:14                     ` Dick Streefland
2007-02-08 21:13       ` J.A. Magallón
2007-02-08 21:42         ` Linus Torvalds
2007-02-08 22:03           ` Linus Torvalds
2007-02-08 22:19             ` Willy Tarreau
2007-02-09  0:03             ` J.A. Magallón
2007-02-09  0:22               ` Linus Torvalds
2007-02-09 12:38             ` Sergei Organov
2007-02-09 15:58               ` Linus Torvalds
2007-02-12 11:12                 ` Sergei Organov
2007-02-12 16:26                   ` Linus Torvalds
2007-02-13 18:06                     ` Sergei Organov
2007-02-13 18:26                       ` Pekka Enberg
2007-02-13 19:14                         ` Sergei Organov
2007-02-13 19:43                           ` Pekka Enberg
2007-02-13 20:29                             ` Sergei Organov
2007-02-13 21:31                               ` Jeff Garzik
2007-02-13 23:21                               ` Linus Torvalds
2007-02-15 13:20                                 ` Sergei Organov
2007-02-15 15:57                                   ` Linus Torvalds
2007-02-15 18:53                                     ` Sergei Organov
2007-02-15 19:02                                       ` Linus Torvalds
2007-02-15 20:23                                         ` me, not " Oleg Verych
2007-02-16  4:26                                         ` Rene Herman
2007-02-19 11:58                                           ` Sergei Organov
2007-02-19 13:58                                         ` Sergei Organov
2007-02-15 22:32                                     ` Lennart Sorensen
2007-02-13 19:25                       ` Linus Torvalds
2007-02-13 19:59                         ` Sergei Organov
2007-02-13 20:24                           ` Linus Torvalds
2007-02-15 15:15                             ` Sergei Organov
2007-02-13 21:13                         ` Rob Landley
2007-02-13 22:21                       ` Olivier Galibert
2007-02-14 12:52                         ` Sergei Organov
2007-02-15 20:06                         ` Sergei Organov
2007-02-09 15:10     ` Sergei Organov
2007-02-08 16:35 ` Kumar Gala
     [not found] <7Mj5f-3oz-21@gated-at.bofh.it>
     [not found] ` <7MktH-5EW-35@gated-at.bofh.it>
     [not found]   ` <7Mmvy-vj-17@gated-at.bofh.it>
     [not found]     ` <7MnBC-2fk-13@gated-at.bofh.it>
     [not found]       ` <7MoQx-4p8-11@gated-at.bofh.it>
     [not found]         ` <7MpjE-50z-7@gated-at.bofh.it>
     [not found]           ` <7MpCS-5Fe-9@gated-at.bofh.it>
     [not found]             ` <7MDd7-17w-1@gated-at.bofh.it>
     [not found]               ` <7MGkB-62k-31@gated-at.bofh.it>
     [not found]                 ` <7NHoe-2Mb-37@gated-at.bofh.it>
     [not found]                   ` <7NMe9-1ZN-7@gated-at.bofh.it>
     [not found]                     ` <7Oagl-6bO-1@gated-at.bofh.it>
     [not found]                       ` <7ObvW-89N-23@gated-at.bofh.it>
     [not found]                         ` <7Oc8t-NS-1@gated-at.bofh.it>
2007-02-15 20:08                           ` Bodo Eggert
2007-02-16 11:21                             ` Sergei Organov
2007-02-16 14:51                               ` Bodo Eggert
2007-02-19 11:56                                 ` Sergei Organov
2007-02-16 12:46                             ` Sergei Organov
2007-02-16 17:40                               ` Bodo Eggert
2007-02-19 12:17                                 ` Sergei Organov

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