Process command line in Linux 64 bit

Question

I have problems accessing the process command line from Linux 64 bit Assembly program. To reproduce this with minimal code, I made this 32-bit program which prints first 5 characters of the program name:

.section .text

.globl _start
_start:
 movl  %esp, %ebp

 movl $4, %eax        # write
 movl $1, %ebx        # stdout
 movl 4(%ebp), %ecx   # program name address (argv[0])
 movl $5, %edx        # hard-coded length
 int  $0x80

 movl $1, %eax
 movl $0, %ebx
 int  $0x80

This program is working. When I translate it to 64 bit and run in Linux 64, it doesn't print anything:

.section .text

.globl _start
_start:
 movq  %rsp, %rbp

 movq $4, %rax
 movq $1, %rbx
 movq 8(%rbp), %rcx       # program name address ?
 movq $5, %rdx
 int  $0x80

 movq $1, %rax
 movq $0, %rbx
 int  $0x80

Where is my mistake?

Answer 1

As stated in the X86_64 ABI: Use the syscall instruction instead of int $0x80. The Kernel uses different registers in 64 Bit as syscall arguments, and the number assigned for a syscall function varies between i386 and x86_64, too.

An example - in german, sorry - can be found here:
http://zygentoma.de/codez/linux_assembler.php

Answer 2

You are loading the correct address into %rcx.

int 0x80 then invokes the 32-bit syscall interface. That truncates the address to 32 bits, which makes it incorrect. (If you use a debugger and set a breakpoint just after the first int 0x80, you will see that it returns with -14 in %eax, which is -EFAULT.)

The second syscall, exit, works OK because the truncation to 32 bits doesn't do any harm in that case.

---

If you want to pass a 64-bit address to a system call, you will have to use the 64-bit syscall interface:

• use syscall, not int 0x80;
• different registers are used: see here;
• the system call numbers are different as well: see here.

Here is a working version of your code:

.section .text

.globl _start
_start:
 movq  %rsp, %rbp

 movq $1, %rax
 movq $1, %rdi
 movq 8(%rbp), %rsi       # program name address ?
 movq $5, %rdx
 syscall

 movq $60, %rax
 movq $0, %rdi
 syscall