How to pass a local variable into procedure

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rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

Code:

format pe console
entry start

include 'win32a.inc'


section '.text' code executable
  start:
    call main
    invoke  ExitProcess,0


  proc main
    local Text2 db "01234"

    push Text
    push sFormat
    call [_printf]
    add esp, 8
    call [_getchar]

    sub esp, 12
    movsx eax, [Text]
    mov [ebp -  4], eax
    movsx eax, [Text2]
    mov [ebp -  8], eax
    mov dword[ebp - 12], 9

    ;push 4
    ;movsx eax, [!Text2]
    ;push eax
    ;movsx eax, [!Text]
    ;push eax
    call _memcpy
    add esp, 12

    push Text
    push sFormat
    call [_printf]
    add esp, 8
    call [_getchar]
  ret
  endp

  ;; INTERNAL
  proc _memcpy src:DWORD, dst:DWORD, siz:DWORD
    ;cld
    mov esi, [ebp+8];mov esi, [src]
    mov edi, [ebp+12];mov edi, [dst]
    mov ecx, [ebp+16];mov ecx, [siz]
    rep movsb
  ret
  endp


section '.rdata' data readable writeable
 sFormat db "%s",10,0
 Text db "9876543210",0


section '.idata' data readable import
 library kernel32, 'kernel32.dll',\
         msvcrt,   'msvcrt.dll'
 import kernel32, ExitProcess, 'ExitProcess'
 import msvcrt, _printf, 'printf',\
                _getchar, 'getchar'

I'm trying to copy one string to another. But it's not works, string don't changes.

12 Dec 2014, 14:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17270
Location: In your JS exploiting you and your system

revolution

I assume you actually want to use LEA instead of MOVSX.

Code:

lea eax, [Text] ;and the same for the other movsx

12 Dec 2014, 14:09

rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

Can you explain the difference, please?

UPD: This isn't works. _memcpy won't change variable Text :/

12 Dec 2014, 14:18

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17270
Location: In your JS exploiting you and your system

revolution

You are putting the arguments for memcpy in the wrong place. You can use the macro cinvoke to do the work for you.

Code:

cinvoke _memcpy,Text,addr Text2,9

12 Dec 2014, 14:28

rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

I don't understand. Why ebp is the wrong place? I found it in tutorials.
And I tried to use "push" and proc _memcpy, arg1,arg2,arg3...

cinvoke is fast? What is the difference from "invoke" and "call"?

12 Dec 2014, 14:40

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17270
Location: In your JS exploiting you and your system

revolution

Your parameters need to follow ESP not EBP.

invoke is a macro, call is a CPU instruction.

cinvoke (ccall convention) and invoke (stdcall convention) only differ in the way the stack is restored when the procedure returns. cinvoke will place the "add esp,12", but invoke won't. This is because of the stdcall and ccall convention differences.

12 Dec 2014, 14:44

rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

cinvoke _memcpy, Text, addr Text2, 9
Error: invalid value : pushd addr ..var?TK

cinvoke _memcpy, Text, dword[Text2], 9
Error: operand size not specified: call[_memcpy]

12 Dec 2014, 14:45

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17270
Location: In your JS exploiting you and your system

revolution

Oh, sorry, your _memcpy is not the MSVCRT version. In that case use "stdcall _memcpy,...".

Also, don't put the ! in front of the names, it has no meaning unless you define your variable names with the ! also.

12 Dec 2014, 14:48

rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

I use it for reason, believe me Very Happy

stdcall _memcpy, !Text, dword[!Text2], 9
Result: Crash

12 Dec 2014, 14:53

rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

I use it for reason, believe me Very Happy

stdcall _memcpy, Text, dword[Text2], 9
Result: Crash

12 Dec 2014, 14:53

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17270
Location: In your JS exploiting you and your system

revolution

Use "addr" not "dword":

Code:

stdcall _memcpy, Text, addr Text2, 9

12 Dec 2014, 15:09

rinart73

Joined: 11 Dec 2014
Posts: 16

rinart73

all is working Smile

thnx

12 Dec 2014, 15:18

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