Virtual directive

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RKT878

Joined: 05 May 2012
Posts: 2

RKT878 07 May 2012, 13:34

Hello, I have recently started writing in FASM and I haven't understood yet how "Virtual" directive exactly works. I believe it is just a... *virtual* directive to help programmer use stuff from registers/pointers (like in "virtual at eax a1 db 1 a2 db 2 end virtual" a2 points to [eax+1]).
But whats the difference between

Code:

virtual at eax
a1 db 12 dup(?)
end virtual

and

Code:

virtual at eax
a1 rb 12
end virtual

?
And I am encountering the problem with it at the moment: lets say I want to get UserName without using actual variables:

Code:

format PE GUI 4.0
include 'win32ax.inc'
virtual at ebp
usrn db 128 dup(?)
usrn2 db 128 dup(?)
end virtual
section '.code' code readable writeable  executable 
start:
invoke GlobalAlloc,GPTR,1024
push dword[eax]
invoke GetUserName,  dword[usrn], usrnsz
invoke MessageBox,0,dword[usrn],dword[usrn2],MB_OK
endl:
invoke ExitProcess,0
endcl:
usrnsz dd 127
.end start

It works "ok", but if I use the latter "variable" (usrn2) it leads to application crash.

07 May 2012, 13:34

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 07 May 2012, 13:45

If you want to push the address of a pointer then you would not place it in brackets.

Code:

invoke MessageBox,0,addr usrn,addr usrn2,MB_OK

Note the use of the addr prefix to allow the invoke macro to properly generate the ebp based address.

Also when using ebp in a virtual block you will need to make memory space for it somewhere else you risk overwriting the stack.

Code:

sub esp,256 ;make space for our structure
mov ebp,esp ;set ebp to point to our allocated memory

07 May 2012, 13:45

RKT878

Joined: 05 May 2012
Posts: 2

RKT878 07 May 2012, 14:15

Thanks Revolution, it took me some time experimentating though, but it works now.
Sorry for taking your time Smile

07 May 2012, 14:15

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