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Index > Main > Variables on the stack buffer - the virtual directive

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pete



Joined: 20 Apr 2009
Posts: 110
pete
Hello!

I have a problem with the virtual directive used to access buffers on the stack, or more concrete: to push the address of "virtual-named" buffers on the stack, please see below:

Code:
function_start:
        sub     esp,0x28
    push    ebp esi edi ebx
     mov     ebp,esp
     virtual at ebp+0x10
         buffer1 dd ?
                buffer2 dd ?
                ...
 end virtual
 mov     [buffer1],0x0000'0033
      push    buffer1         ;I want to push the address of buffer1
                              ;on the stack... how?

    


Thanks in advance for any help!
Post 27 Jan 2010, 11:12
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17450
Location: In your JS exploiting you and your system
revolution
Code:
lea eax,[buffer1]
push eax    
Post 27 Jan 2010, 11:14
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pete



Joined: 20 Apr 2009
Posts: 110
pete
Thanks revolution but why?
I thought "buffer1" is always replaced with "ebp+0x10", "buffer2" with "ebp+0x10+4" and so on. Why can't "buffer1" be pushed?
Post 27 Jan 2010, 11:51
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17450
Location: In your JS exploiting you and your system
revolution
You can't do this:
Code:
push ebp+0x10    
Post 27 Jan 2010, 11:57
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pete



Joined: 20 Apr 2009
Posts: 110
pete
I can compile "push ebp+0x10" with fasm 1.68 but it produces "push 0x10" without any error or warning.
Post 27 Jan 2010, 12:03
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pete



Joined: 20 Apr 2009
Posts: 110
pete
Well, thanks for your help revolution!
Post 27 Jan 2010, 12:08
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17450
Location: In your JS exploiting you and your system
revolution
Actually it compiles to 'push ebp' and 'push 0x10'. Which is clearly not what you want from the above code.
Post 27 Jan 2010, 12:08
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pete



Joined: 20 Apr 2009
Posts: 110
pete
Ah, yep!
Post 27 Jan 2010, 12:09
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Borsuc



Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania
Borsuc
I always hated that 'feature' to push multiple symbols on one line, it causes confusion sometimes. Confused
Post 27 Jan 2010, 17:12
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MazeGen



Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia
MazeGen
(vid here) That's weird. If buffer1 was preprocessor symbol that equates to ebp+0x10 (three tokens), then okay. But why does this work that way with label, which should be a single token???
Post 27 Jan 2010, 22:28
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
vid, the "push ebp+0x10" was tried literally, "push buffer1" in the first posted code completely fails compilation with "Error: invalid value".
Post 28 Jan 2010, 00:19
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17450
Location: In your JS exploiting you and your system
revolution
Think of it like "push (ebp+0x10)" And perhaps you see the reason better.
Post 28 Jan 2010, 02:05
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