Re: Segfault on the i386 enter instruction

[linux-os (Dick Johnson)]

On Fri, 12 May 2006, Denis Vlasenko wrote:

> On Friday 12 May 2006 17:07, linux-os (Dick Johnson) wrote:
>>> 	.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!
>
> No. Try it yourself.
> --
> vda

It doesn't matter. It means that you still own the space there
(it's mapped into your process). The code is bogus, broken beyond
all repair. It has nothing to do with 'enter' it has to do with
putting the stack pointer (wrapping it) to somewhere it shouldn't
be. The stack pointer is normally around 0xafff0000. It just got
wrapped down past zero up to fafff00d, then stuff got pushed
onto it for the call.

>

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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