Assembly Challenge

From MRL Wiki

Challenge 1

Originally published 3/13/2006.

[edit] Challenge

Write a stub program in assembler that returns "1" as the return code. Test it with this shell script:

#!/bin/sh
./stub
if test "$?" -ne 1; then
    echo "You lose"
else
    echo "You win"
fi

Rules:

• Must be written in assembly language. Processor and OS don't matter.
• Must be 100% original code.
• Either Intel or AT&T syntax is fine for x86 machines.

Some helpful pages:

[edit] Resources

FreeBSD application binary interface for x86

Mac OS X application binary interface (all platforms)

Linux standard base specification (all platforms)

[edit] Official Solutions

Here is a NASM version for Linux on x86:

SECTION .text
GLOBAL main
main
        mov     eax,1
        mov     ebx,1
        int     80h

Here is a GNU as version for FreeBSD on x86:

.globl _start

.text
_start:
        pushl $1
        movl $1, %eax
        int $0x80

Here is a GNU as version for Darwin (Mac OS X) on x86, using the new sysenter trap found on Darwin 8.4 (OS X 10.4.4) and up:

.globl _main

.text
_main:
        movl $0x1, %eax
        pushl $1
        pushl $0x31337 # ignored
        movl %esp, %ecx
        pushl %ecx
        sysenter

[edit] Contributed Solutions

Below is the solution using Linux exit system call. Compiled and tested using GNU Assembler on Intel platform.

#stub.s Sample program that returns 1
.section .text
.globl _start
_start:
        movl $1, %eax
        movl $1, %ebx
        int $0x80

A solution that literally prints "1", without using exit codes.

.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        andl    $-16, %esp
        movl    $0, %eax
        addl    $15, %eax
        addl    $15, %eax
        shrl    $4, %eax
        sall    $4, %eax
        subl    %eax, %esp
        subl    $12, %esp
        pushl   $49
        call    putchar
        addl    $16, %esp
        subl    $12, %esp
        pushl   $0
        call    exit
        .size   main, .-main

And now for something completely ridiculous. Completes with nasm -f; ld -s -o.

SECTION .data .text

GLOBAL _start

_start:
        mov     eax,1
        mov     ebx,1
        int     80h