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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?
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
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
