unexpected code

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Corsair59

Joined: 23 May 2016
Posts: 2

Corsair59 07 Nov 2016, 08:24

I was testing some disassemblers and used fasmw17157. I noticed the code right after the exitprocess instruction was supposed to push several values and call instruction, then push several registers. What I found is nothing of the sort.

end_loop:
invoke ExitProcess,[msg.wParam]

proc MainWindow hwnd,wmsg,wparam,lparam
push ebx esi edi
;--------------------------------------------------

call dword ptr [exitprocess]
push ebp
mov ebp,esp
push ebx
push esi
push edi

several confirmations prompted me to point this out.
I'm just getting back into assembly programming again, so maybe the assembler knows something I don't.

07 Nov 2016, 08:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20300
Location: In your JS exploiting you and your system

revolution 07 Nov 2016, 09:27

Immediately after the call to ExitProcess the next procedure "MainWindow" begins. That is exactly what you coded. If you put an intervening instruction like int3 you would see where the function begins.

Code:

end_loop:
invoke ExitProcess,[msg.wParam]

int3

proc MainWindow hwnd,wmsg,wparam,lparam
push ebx esi edi

07 Nov 2016, 09:27

Corsair59

Joined: 23 May 2016
Posts: 2

Corsair59 07 Nov 2016, 10:18

The upper code is from the fasmw.asm file. below the dotted line is what was generated. note that the data pushes and procedure call are absent. exit process and the register pushes frame the abhorrent code. the push and move instructions were not supposed to be there. You can check this yourself by matching the fasmw.asm file with the disassembled program. this was 1.71.57 new release.

07 Nov 2016, 10:18

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20300
Location: In your JS exploiting you and your system

revolution 07 Nov 2016, 10:33

AFAIKT everything is there as you coded:

Code:

invoke ExitProcess,[msg.wParam]                 ;push dword ptr [msg.wParam]
                                                ;call dword ptr [exitprocess]

proc MainWindow hwnd,wmsg,wparam,lparam         ;push ebp
                                                ;mov ebp,esp

push ebx esi edi                                ;push ebx
                                                ;push esi
                                                ;push edi

07 Nov 2016, 10:33

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