(N)ASM windows MessageBox , import dll

Mostly i use NASM, GCC, LD for programming practices! Because I use Linux as my primary Operating system, So i love to use cross-platform application.

I am not an asm coder, in past i have searched a lots for ASM(nasm) code of MessagBox() function Example just to get started. Because everything was fairly new to me(i had no clue!). So i just decided to post a simple example code which was my first assembly program for windows, in case someone is searching for basic example for getting started. I hope it will be useful to someone who is in same situation as i was!

First Example:

extern _ExitProcess@4
extern _MessageBoxA@16

global _main

section .data
msgb db "pusheax.com!",0
title db "Security Research!",0
section .text

_main:

    push dword 0x00
    ;mov     esi,msgb
    ;push esi
    push dword title
    push dword msgb
    push dword 0
    call _MessageBoxA@16
    
    push 0
    call _ExitProcess@4

"extern" is importing symbol from other module. In our case the symbols are _ExitProcess@4 and _MessageBoxA@16. There are three things we see are:

1. underscore before MessageBoxA .
2. @4/16 

The underscore used for calling the function in C style, Linux does not have underscore(_). And @4/16 indicating that how many parameter for the calling function. Such as MessageBox has 4 parameters. Each parameters are 4 bytes so 4 parameters are (4*4) 16bytes=4 . Extra "A" for ANSI-C .

"global _main" , declaring it as startup of our asm instructions(C style).



"Section .data" , you know what is it! Declaring uninitialized data such as variable.

in "section .text" (our codes) there are all parameters pushed to stack in reverse mode. I have called MessageBox(see here: http://msdn.microsoft.com/en-us/library/windows/desktop/ms645505%28v=vs.85%29.aspx ). The MessageBox in C should be like this:

MessageBox(NULL,msgb,title,0x00000000L)


in ASM it is opposite:

First pushing the 0x00000000L(MB_OK) to stack. Currently top of the stack!
Then pushing "title" . "title" is now top of the stack.
Then pushing "msgb". Same as above. "msgb(string)" top of the stack .
And last push is 0 . Same as above.
At last call the function.

Stack is LIFO(Last in first out). So it is now:

MessageBox(NULL,msgb,title,0x00000000L)

 It is always always good idea terminating the current process so ExitProccess() function has been called when MessageBox() operation is completed.

Compile the code :
nasm -fwin32 msg.asm
gcc msg.obj -o msg.exe

But I want to import specific dll because all dlls are not loaded so some API function may not work if i can't load the dll in my code. How i do this ? "import MessageBoxA@16 user32.dll" ? I think this is not going to happen for me because nasm will not generate win32 object file(Perhaps issue). So i need to work with obj (nasm -fobj msg.asm) but another issue is gcc won't compile the obj file.. I used ALINK (Download: http://alink.sourceforge.net/), This what we want instead using gcc/LD (maybe)? But still LD can be used to compile it by linking library with (-l) . Here is the code i have assembled with nasm and compiled with alink.exe:

import MessageBoxA user32.dll ;Include the dll user32.dll
extern MessageBoxA          ; Now calling external symbol without underscore.... 

section .data               
msgme   db "Hi",0           ;Say "Hi" to pusheax.com

section .text use32 CLASS=CODE          ;"use32 CLASS=CODE for telling the other linker(Such as alink.exe) that program for 32bit

..start:                    ; ..start (not _start) for other linker for start of the code

    push    dword   0x00    ; MB_OK
    mov     esi,msgme       ;esi="Hi"
    push    esi             ;"Hi" is now top of the stack, second paramaters title
    push    dword   msgme   ; Say "Hi"
    push    dword   0       ;Reserve 
    call    [MessageBoxA]   ;Call the Function

;nasm -fobj msg2.asm
;alink -oPE msg2.obj

 If we use ld(with gcc) then our command should be ld -o what.exe what.obj -luser32.dll whereas alink.exe -oPE what.exe but nasm. For alink we don't need to declare how many parameters , underscore etc and for ld we need to declare all the required things and extra option "-l" to link dll.  

Which Linker you will use?