Execing Processes in Assembly Language

Assembly language doesn’t have direct equivalents for many high-level concepts in the original code. However, we can create a similar program that executes an external command using system calls. This example will be for x86 assembly on a Linux system.

section .data
    command db '/bin/ls', 0
    arg1 db 'ls', 0
    arg2 db '-a', 0
    arg3 db '-l', 0
    arg4 db '-h', 0
    args dq command, arg1, arg2, arg3, arg4, 0
    env dq 0

section .text
    global _start

_start:
    ; Execute the command using execve system call
    mov rax, 59          ; syscall number for execve
    mov rdi, command     ; command to execute
    mov rsi, args        ; array of argument pointers
    mov rdx, env         ; environment variables (null in this case)
    syscall

    ; If execve returns, it means there was an error
    mov rax, 60          ; syscall number for exit
    mov rdi, 1           ; exit status 1 (error)
    syscall

This assembly code attempts to replicate the functionality of the original program. Here’s a breakdown of what it does:

1. We define the command (/bin/ls) and its arguments in the .data section.

2. In the _start function (the entry point for assembly programs), we use the execve system call to execute the ls command with the specified arguments.

3. If execve returns, it means there was an error (because on success, execve doesn’t return). In this case, we exit the program with an error status.

To assemble and link this program:

$ nasm -f elf64 exec_process.asm
$ ld exec_process.o -o exec_process

When we run our program, it should be replaced by ls:

$ ./exec_process
total 16
drwxr-xr-x  4 user 136B Oct 3 16:29 .
drwxr-xr-x 91 user 3.0K Oct 3 12:50 ..
-rw-r--r--  1 user 1.3K Oct 3 16:28 exec_process.asm
-rwxr-xr-x  1 user 8.0K Oct 3 16:29 exec_process

Note that assembly language doesn’t offer high-level abstractions like environment variable handling or path lookup. In a real-world scenario, you might need to implement these features yourself or use library functions through system calls.