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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
Hello everyone. I have some little problem about mov instruction. Problem is that I cant move string to string I mean add string to string.. for example
I have defined strings..
str1 db 'ABC',0
str2 db 'DEF',0
and now I'm trying to do following:
mov [str1+4],str2
but I got error Invalid use of symbol.. whats problem here ?.. thank you.
Post 12 Oct 2010, 13:46
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
str1 is defined as a byte pointer. So internally fasm does this:
Code:
mov byte[str1+4],str2    
and since str2 is at least a word value (or bigger, you didn't show the rest of your code so we can't know for sure) and can't fit into a byte location.

However, you can't just add strings like that. Rather than copy a pointer, you have to copy the entire string and attach it to the end of the other string. str1 needs to have enough space to store the resultant string else you will overwrite whatever follows it. Read up on the rep movsb instruction to see how to copy strings.
Post 12 Oct 2010, 13:52
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
Here's code and see what's missing please.. Smile
Code:
format PE GUI 4.0
include 'WIN32AX.INC'
entry main
section '.data' data readable writeable
buffer1 rb 100
buffer2 db '\TEST',0
section '.text' code readable executable
proc main
invoke GetCurrentDirectory,100,buffer1
mov [buffer1+21],buffer2
invoke MessageBox,0,buffer1,buffer1,MB_OK
invoke ExitProcess,0
endp
section '.idata' import data readable
library user32,'user32.dll',kernel32,'kernel32.dll'
include 'API\USER32.INC'
include 'API\KERNEL32.INC'
section '.reloc' fixups data discardable
    
Post 12 Oct 2010, 14:02
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DJ Mauretto



Joined: 14 Mar 2007
Posts: 464
Location: Rome,Italy
DJ Mauretto
Code:
format PE GUI 4.0

include 'WIN32AX.INC'
entry main
section '.data' data readable writeable

buffer1 rb 100
buffer2 db '\TEST',0

section '.text' code readable executable

proc main

invoke GetCurrentDirectory,0,NULL
push eax
invoke GetCurrentDirectory,eax,buffer1
pop eax
sub eax,1
mov esi,buffer2
lea edi,[eax + buffer1]
mov ecx,6
rep movsb
invoke MessageBox,0,buffer1,buffer1,MB_OK
invoke ExitProcess,0

endp

section '.idata' import data readable

library user32,'user32.dll',kernel32,'kernel32.dll'
include 'API\USER32.INC'
include 'API\KERNEL32.INC'
section '.reloc' fixups data discardable     

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Post 12 Oct 2010, 16:16
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rugxulo



Joined: 09 Aug 2005
Posts: 2341
Location: Usono (aka, USA)
rugxulo
Code:
offset equ ; for clarity

use16      ; by default
org 100h   ; DOS .COM, for simplicity

jmp Start
str1: db 'ABC',0 ; four bytes, plus this syntax below needs a colon
str2: db 'DEF',0 ; also four bytes, aka dword

; "and now I'm trying to do following:"

Start:
;mov dword [str1+4],offset str2 ; obviously wrong, offset != contents
push dword [str2]
pop dword [str1+3] ; overwrite initial NUL
int 20h ; exit
    


This is what you were literally trying to do. But I don't think that's what you really wanted. Sure, you can easily "concat" two dwords (four bytes) thanks to 32-bit regs being that size, but anything longer needs REP MOVSB (DS:ESI -> ES:EDI, count in ECX) or manual copying or using libc's strcpy(), etc.
Post 12 Oct 2010, 22:19
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
ah thanks for info. both are epic. thanks. Smile
Post 13 Oct 2010, 06:25
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
Hey, I haven't figure out why this instruction should sub eax,1 and then lea eax+buffer1 can you comment ur code please ? thank you. Smile
Post 13 Oct 2010, 06:48
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
Overflowz,

Where does that eax come from? GetCurrentDirectory(0, NULL). What does it mean? MSDN can help. Why eax is decremented then? To account for NUL terminator. What value lea edi, [eax+buffer1] puts in edi? Naturally, eax+buffer1.
Post 13 Oct 2010, 06:57
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
I understand that but why it copies both value buffer1-1 and buffer1 to each other ? I dont understand the logic how that code works.
Post 13 Oct 2010, 11:37
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
figured out. this works bit fine for me.
Code:
format PE GUI 4.0
include 'WIN32AX.INC'
entry main
section '.data' data readable writeable
itdb db ?
buffer1 rb 100
sizeof.buffer1 = $ - buffer1
buffer2 db '\TEST',0
sizeof.buffer2 = $ - buffer2
section '.text' code readable executable
proc main
invoke GetCurrentDirectory,100,buffer1
invoke lstrlen,buffer1
mov esi,buffer2
lea edi,[buffer1+eax]
mov ecx,eax
rep movsb
invoke MessageBox,0,buffer1,buffer1,MB_OK
invoke ExitProcess,0
endp
section '.idata' import data readable
library user32,'user32.dll',kernel32,'kernel32.dll'
include 'API\USER32.INC'
include 'API\KERNEL32.INC'
section '.reloc' fixups data discardable
    
Post 13 Oct 2010, 21:23
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
I think you mean:
Code:
;...
mov ecx,sizeof.buffer2
;...    
Post 14 Oct 2010, 00:26
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
lol yes.. I forgot sizeof struct.. thanks. I'll learn more about movs movsb movsw scas and etc.. thanks for replies.
Post 14 Oct 2010, 08:54
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