Re: Segfault on the i386 enter instruction

[linux-os (Dick Johnson)]

On Fri, 12 May 2006, Tomasz Malesinski wrote:

> The code attached below segfaults on the enter instruction. It works
> when a stack frame is created by the three commented out
> instructions and also when the first operand of the enter instruction
> is small (less than about 6500 on my system).
>
> AFAIK, the only difference between creating a stack frame with the
> enter instruction or push/mov/sub is that enter checks if the new
> value of esp is inside the stack segment limit.
>
> I tested it on a vanilla kernel 2.4.26 on Intel Celeron and also on
> probably non-vanilla 2.6.16.13 running on 3 dual core AMD Opteron,
> quite busy, server. It is working in 32-bit mode. Interestingly, on
> the second machine sometimes the program worked correctly.
>
> I am not subscribed to the list. Please cc replies to me.
>
>
> 	.file	"a.c"
> 	.version	"01.01"
> gcc2_compiled.:
> .section	.rodata
> .LC0:
> 	.string	"asdf\n"
> .text
> 	.align 4
> .globl main
> 	.type	 main,@function
> main:
> 	enter $10008, $0
> #	pushl %ebp
> #	movl %esp,%ebp
> #	subl $10008,%esp
> 	addl $-12,%esp
         ^^^^^^^^^^^^^^____________ WTF
         adding a negative number is subtracting that positive value.
         You just subtracted 0xfffffff3 (on a 32-bit machine) from
         the stack pointer. It damn-well better seg-fault!


> 	pushl $.LC0
> 	call printf
> 	addl $16,%esp
> .L2:
> 	leave
> 	ret
> .Lfe1:
> 	.size	 main,.Lfe1-main
> 	.ident	"GCC: (GNU) 2.95.4 20011002 (Debian prerelease)"
>
> --
> Tomek Malesinski
> -
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.89 BogoMips).
New book: http://www.lymanschool.com
_


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