non-volatile register as syscall buffer?

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Lurker

Joined: 07 Feb 2012
Posts: 6

Lurker 07 Feb 2012, 20:25

Code:

push 4
pop edx
;mov ecx,esi (does not work)
mov ecx,tsz;(does work)
mov ebx,eax
push 3
pop eax
int 0x80

I'm a novice to asm and I've been trying to code a program that needs to read 4 bytes from a file to esi/edi.
If ecx holds the offset of tsz (reserved memory) everything works fine and later I can mov esi,[tsz]. Is it possible to read to esi/edi directly?

07 Feb 2012, 20:25

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20533
Location: In your JS exploiting you and your system

revolution 08 Feb 2012, 15:00

Lurker wrote:

Is it possible to read to esi/edi directly?

If I understand you properly then the answer is no. You have to supply a memory address for the destination. You can't make ecx point to esi, you can only copy the contents of esi into ecx.

08 Feb 2012, 15:00

Lurker

Joined: 07 Feb 2012
Posts: 6

Lurker 08 Feb 2012, 15:20

yes, I was asking If It was possible to make ecx point to esi.
Thanks.

08 Feb 2012, 15:20

edfed

Joined: 20 Feb 2006
Posts: 4354
Location: Now

edfed 08 Feb 2012, 17:39

only instructions points to registers, you can change some opvalues to change the register reference in an instruction, and if you use ecx to contain that value (cl is enough), you can use as a pointer... but;;; hem.

08 Feb 2012, 17:39

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