macro local

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system error

Joined: 01 Sep 2013
Posts: 670

system error 01 Sep 2013, 14:07

hi. this my first post.

just want to ask how to properly declare local variables in macro in protected mode. I read the manual but not that satisfying. lets say i have an array of 20 chars like

hello db 20 dup ('F')

macro foo
{
local hello

hello db 20 dup ('F')

mov eax, 'ABCD'
mov byte [hello], al ; not error, but stuck here
}

i don't know whats wrong there. This is just an example code.

regards

01 Sep 2013, 14:07

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 01 Sep 2013, 16:45

The declaration is correct but foo will expand to the buffer you created and the code afterwards, so if you tried to execute foo, you're actually executing the local data as code. A possible correction would be:

Code:

macro foo 
{ 
 local hello 
 local skip

 jmp skip
 hello db 20 dup ('F') 
 skip:
 mov eax, 'ABCD' 
 mov byte [hello], al ; not error, but stuck here 
}

01 Sep 2013, 16:45

system error

Joined: 01 Sep 2013
Posts: 670

system error 01 Sep 2013, 18:19

Hello cod3b453, appreciate your reply but that code didn't work either. This is my updated code. Just a test code with no specific purpose.

Code:

format PE console
include 'win32ax.inc'
entry main
macro foo
{
        local hello
        local skip

        jmp skip
            hello db 10 dup ('XyXyXyXyXy')
        skip:
        mov eax, 'ABCD'
        mov byte [hello], al ; not error, but stuck here
        cinvoke printf, "%c", dword [hello]
}

section '.code' code executable
main:
foo
invoke system, "pause"
invoke exit, 0

section '.data' data readable writable
anything db ?

section '.idata' import data readable
          library msvcrt, 'msvcrt.dll'
          import msvcrt,\
                 system, 'system',\
                 printf,'printf',\
                 exit, 'exit'

I opened the VC++ debugger and it gave me:
Unhandled exception in foo32exe: 0xC0000005:Access Violation.

This is the problematic region I got from the debugger

00401001 pop eax
00401003 jns 0040105D
00401005 jns 0040105F
00401007 jns 00401061
---cut---
0040105F jns 004010B9
00401061 jns 004010BB
00401063 jns 004010BD
00401065 jns 0040101F
00401067 inc ecx
00401068 inc edx
00401069 inc ebx
0040106A inc esp
0040106B mov [00401002],al ;<<--- Debugger stops here.

01 Sep 2013, 18:19

bitRAKE

Joined: 21 Jul 2003
Posts: 4060
Location: vpcmpistri

bitRAKE 01 Sep 2013, 20:14

"Access Violation" means that permission to write is not allowed. So, we would look at the permissions of memory location. Code section is read only (default). Need to add WRITEABLE to the SECTION definition.

01 Sep 2013, 20:14

system error

Joined: 01 Sep 2013
Posts: 670

system error 01 Sep 2013, 20:27

bitRAKE wrote:

"Access Violation" means that permission to write is not allowed. So, we would look at the permissions of memory location. Code section is read only (default). Need to add WRITEABLE to the SECTION definition.

Great! I got it working now. Thanks to you and cod3b45e. I didn't know FASM macro is that 'powerful'.

01 Sep 2013, 20:27

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