strange problem with messagebox

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shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 30 May 2012, 16:15

So maybe someone could help.
I have an application and there is a function called on keystroke (F12).

If I press the key, the function is executed but not as expected.

Code:

        invoke  MessageBox,[hwnd],_welcome,_caption,MB_ICONERROR+MB_OK
initusb:
        invoke  FT_CreateDeviceInfoList,ftdevices
        mov     eax,[ftdevices]
        call    printeax   ; messagebox appears first time
        call    printeax   ; but not second time
        jmp  finished

  printeax:
        cinvoke  wsprintf,my_string_buffer,fmtprt,eax
        invoke  MessageBox,[hwnd],my_string_buffer,_caption,MB_ICONERROR+MB_OK
        ret

fmtprt  db '%#p',0

So the first messagebox is printed on screen when key pressed.
The second messagebox is printed after it in function printeax.
But when I call printeax second time (or third or whatever) no more messagebox appears.
If I copy the "welcome" messagebox of the first line, it's executed multiple times.
If I double the invoke of messagebox directly in the printeax function, it also is printed twice.
But why is the messagebox not shown when I repeat the call of the function ???

Things can get mad when you don't understand why. Shocked

30 May 2012, 16:15

typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 30 May 2012, 20:58

try using

Code:

jmp printeax

instead of

Code:

call printeax

30 May 2012, 20:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 30 May 2012, 22:19

You didn't show all of your code so we have to assume some things not given. Is your code section posted inside a proc/endp section? If it is then the problem is caused by the ret. Use retn to avoid instantiating the ret macro:

Code:

  printeax:
        cinvoke  wsprintf,my_string_buffer,fmtprt,eax
        invoke  MessageBox,[hwnd],my_string_buffer,_caption,MB_ICONERROR+MB_OK
        retn ; <--- don't use the ret macro here

30 May 2012, 22:19

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 31 May 2012, 11:59

revolution wrote:

You didn't show all of your code so we have to assume some things not given. Is your code section posted inside a proc/endp section? If it is then the problem is caused by the ret. Use retn to avoid instantiating the ret macro:

Okay you are right.
It was in a proc section (Window Proc) and I didn't expect that ret is a macro instruction as it is also an official assembling instruction.

Works fine now with retn. Wink

31 May 2012, 11:59

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 31 May 2012, 12:02

typedef wrote:

try using
Code:
jmp printeax    
instead of
Code:
call printeax    

That didn't help and I want to call a simple subroutine.
Okay now it's not much code which could be copied several times but it was more to understand principals than getting some workaround. Wink

31 May 2012, 12:02

typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 31 May 2012, 17:14

Then set up your calling right like revolution suggested.

31 May 2012, 17:14

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 31 May 2012, 21:38

As you can read - i did and it worked.
I just wanted to let you know, that simply jmp instead of call is not working - with ret. Very Happy

31 May 2012, 21:38

typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 01 Jun 2012, 02:39

shutdownall wrote:

As you can read - i did and it worked.
I just wanted to let you know, that simply jmp instead of call is not working - with ret.

lol. Dude, I assumed that you knew you don't mix jmp with ret.

If you JMP to X you have to JMP back to Y and not RET.

I thought you knew that. You fixed it so let's move on.

01 Jun 2012, 02:39

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 01 Jun 2012, 09:57

As I know the difference between GOSUB/RETURN and GOTO I assume it is similar to call and jmp. Razz

01 Jun 2012, 09:57

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