Local variable structure FASM

Question

So I was reading the manual on how to create structures..

At first, I did:

struc point X, Y
{
   .X dw X
   .Y dw Y
}



section '.code' code readable executable
main:
   push ebp
   mov ebp,esp

   PP Point 0, 0   ;Crashes here..

   mov esp, ebp
   pop ebp
   mov eax, 0x0
ret

So I thought well since its a local variable, I need to sub esp, sizeof(Point) and I did and it still crashed..

and so I did it within the Point structure and instead of crashing, it now gets deleted:

section '.data' data readable writeable
Response db 13, 10, "Value: %d", 13, 10, 0

struc Point X, Y
{
   sub esp, 0x8
   mov dword[esp + 0x00], X
   mov dword[esp + 0x04], Y
}



section '.code' code readable executable
main:
   push ebp
   mov ebp,esp

   PP Point 0, 0        ;create a point
   push PP
   call Point_Valid
   add esp, 0x04



   mov esp, ebp
   pop ebx

   call [getchar]

   mov eax, 0x00
ret


Point_Valid:
   push ebp
   mov ebp, esp

   mov eax, [ebp + 0x8]
   push eax              ;Not sure how to get Point from eax or Point.X..
   push Response
   call [printf]
   add esp, 0x08


   mov esp, ebp
   pop ebp
ret

However, not only does the above fail, it also triggers this:

enter image description here From within Malware Defender for Windows 8.1

I do not understand why :S But it only happens when I use the sub esp within the struct. If I use the .X dw X it just crashes but does not trigger malware defender.

Any ideas how I can achieve passing a point to a function? Or creating a local point and passing it to a function as well as accessing its fields?

score 1 · Answer 1 · answered Mar 18 '14 at 12:43

In these two variants, you define different things. In the first variant, you create data inside your code and then tries to execute it. As long as the variables contain random instructions (check in the debugger) the code crashes. I would suggest moving the variable in the data section of the program - where it must reside by default.

In the second try, you replace the data with the instructions (although it is not necessary to use structure here) and then the code is properly executed. And it is valid code.

The fact your AV don't like this code is IMHO problem of the AV maker. My suggestion is to uninstall it at all.

In fact FASM often have problems with the AV assumes what is "good code" and what is a virus. Notice, that all these are false positives.

Michael · Accepted Answer · 2014-03-18T12:54:52.427

In the first example you had misspelt point as Point, so it didn't assemble (unless you told the assembler to ignore case).

If we ignore that, what you're doing here:

main:
push ebp
mov ebp,esp

PP point 0, 0   ;Crashes here..

mov esp, ebp
pop ebp
mov eax, 0x0

is pretty much the same as if you had written:

main:
push ebp
mov ebp,esp

dw 0,0

mov esp, ebp
pop ebp
mov eax, 0x0

It just inserts those two 0-words into the instruction "stream" where you put them. If you assemble this and then disassemble the output you get:

00000000  6655              push ebp
00000002  6689E5            mov ebp,esp
00000005  0000              add [bx+si],al  ; <- the first 0
00000007  0000              add [bx+si],al  ; <- the second 0
00000009  6689EC            mov esp,ebp
0000000C  665D              pop ebp
0000000E  66B800000000      mov eax,0x0
00000014  C3                ret

Your pp variable would have to be declared somewhere else so that it never ends up in the code path (i.e. eip / rip shouldn't reach your variable).

If you really need a stack-allocated point you could use virtual at:

virtual at esp-0x10  ; use whatever address is appropriate in your case
  pp point 1,2
end virtual

mov ax,[pp.X]

Note that this neither allocates space for nor initializes a point; it just sets up the name-address mapping.

Local variable structure FASM

2 Answers2