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Index > Windows > Stack shift under win64 confused

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Wztml



Joined: 08 Mar 2012
Posts: 6
Location: China
Wztml
Here is the code:

Code:
format PE64 GUI
entry start

include 'win64ax.inc'

section '.text' code readable executable

        start:
                sub     rsp,8*5
                mov     r9d,0
                lea     r8,[_caption]
                lea     rdx,[_text]
                mov     rcx,0
                call    [MessageBoxA]
                add     rsp,8*5

                nop
                nop
                nop
                nop

                sub     rsp,8*5
                invoke  MessageBoxA,0,_text,_caption,0,MB_OK
                invoke  ExitProcess,0

section '.data' data readable writeable

        _caption        db  'Test',0
        _text           db  'Hello World',0

section '.idata' import data readable writeable

        library kernel32,'KERNEL32.DLL',\
                user32,'USER32.DLL'

        import kernel32,\
               ExitProcess,'ExitProcess'

        import user32,\
               MessageBoxA,'MessageBoxA'
    


What's wrong with macro 'invoke'? Why there is an additional shift?

I will show the screenshot in IDA


Description: sub rsp,28h+30h ????????????
Filesize: 20.58 KB
Viewed: 1217 Time(s)

Capture.PNG



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Post 09 Mar 2012, 12:43
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
invoke macro takes care of allocating stack space on each call, to avoid that, use frame/endf ( http://flatassembler.net/docs.php?article=win32#1.4 )
Post 09 Mar 2012, 17:34
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shutdownall



Joined: 02 Apr 2010
Posts: 518
Location: Munich
shutdownall
I didn't know that invoke is a macro, thought it was an instruction inside fasm.
Wink
Post 09 Mar 2012, 18:31
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17278
Location: In your JS exploiting you and your system
revolution
Wztml wrote:
Code:
                invoke  MessageBoxA,0,_text,_caption,0,MB_OK    
You have one too many parameters there. MessageBox only takes four parameters.
Post 09 Mar 2012, 20:40
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Wztml



Joined: 08 Mar 2012
Posts: 6
Location: China
Wztml
@Rev
If I give 4 parameters, the code will be
sub rsp,28h
sub rsp,20h
..
..

SO WHAT'S WRONG?
Post 10 Mar 2012, 04:58
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17278
Location: In your JS exploiting you and your system
revolution
Nothing is wrong. You have manually specified the sub rsp,28h, and the invoke macro has generated the sub rsp,20h. Your rsp adjustment is aligning the stack (with an extra 20h bytes for unknown purpose). If you do not want invoke to allocate stack space then follow LocoDelAssembly's suggestion.
Post 10 Mar 2012, 05:02
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Wztml



Joined: 08 Mar 2012
Posts: 6
Location: China
Wztml
@Rev @LocoDelAssembly
Thank you guys
Post 10 Mar 2012, 08:00
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