Defining Bytes in GCC Inline Assembly in Dev-C++(.ascii in AT&T syntax on Windows)

Question

Hey guys,

The code below is just showing a Message Box on the screen.
The addresses are hardcoded to facilitate:

int main ()
{
    asm("xorl %eax, %eax        \n"
        "xorl %ebx, %ebx        \n"
        "xorl %ecx, %ecx        \n"
        "xorl %edx, %edx        \n"
        "pushl %ecx             \n" //$0x0
        "pushl $0x20206c6c      \n" //"  ll"
        "pushl $0x642e3233      \n" //"d.23"
        "pushl $0x72657375      \n" //"resu"
        "movl %esp, %ecx        \n" //store "user32.dll" address in %ecx
        "movl $0x7c801d7b, %ebx \n" //store address of LoadLibraryA in %ebx
        "pushl %ecx             \n"
        "call *%ebx             \n"
        "movl $0xef30675e, %ecx \n"
        "addl $0x11111111, %ecx \n"
        "pushl %ecx             \n"
        "pushl $0x42656761      \n"
        "pushl $0x7373654d      \n"
        "movl %esp, %ecx        \n"
        "pushl %ecx             \n"
        "pushl %eax             \n"
        "movl $0x7c80ae40, %ebx \n"
        "call *%ebx             \n"
        "movl %esp, %ecx        \n"
        "xorl %edx, %edx        \n"
        "pushl %edx             \n"
        "pushl %ecx             \n"
        "pushl %ecx             \n"
        "pushl %edx             \n"
        "call *%eax             \n"
        "xorl %eax, %eax        \n"
        "pushl %eax             \n"
        "movl $0x7c81cb12, %eax \n"
        "call *%eax             \n"
    );
}

(I didn't comment all the code because my question is not really about the code)

My question is: Is there a way to write the string "user32.dll" in assembly inline without pushing manually to the stack? I mean like this in NASM: db 'Hello'

I know that in AT&T syntax I could do .ascii 'Hello' or .string 'Hello' but how about in gcc inline?

Please note that I'm using Dev-C++ on Windows XP SP3

Thanks!

Answer 1

Yes, by making use of assembler directives inside your inline assembler. The trick is in putting the string in the right place (the data section), which you can do by switching using .section .data, and then switching back again with .section .text.

You must give the data a label so that you can refer to it; I would recommend using the local label syntax here (where the label is a number, e.g. 1:, and you reference it as either 1b for the first 1: label backwards, or 1f for the first 1: label forwards - see the GNU assembler documentation for more details).

Like this:

int main(void)
{
  asm(".section .data      \n"
      "1: .asciz \"Hello\" \n"
      ".section .text      \n"
      "pushl $1b           \n"
      "call _puts          \n"
      "add $4, %esp        \n"
     );
  return 0;
}

I don't have a Windows system handy to test this on, but it compiles OK and looks like it should be doing the right thing using a MinGW cross-compiler on Linux (I believe Dev-C++ is based on MinGW).

Note: this technique is generally applicable when using a GNU toolchain. If you're building ELF binaries (e.g. native Linux), there is a neater way to switch back to the text section, which is to use .previous, which means "whatever the section before the previous .section was". (The above example works on Linux if you change _puts to puts to account for different symbol prefixing conventions.)