Question

I’ve been relearning and writing some assembly code, very basic stuff for starters. I’m on running Ubuntu on x86_64, but the tutorials I’m following along with were done on 32-bit x86.

I’ve included the short assembly program below, which assembles and links fine, but segfaults when running it. I’m betting my error is some mixup/confusion with respect to the lines between 32 and 64 bit instructions and memory usage. For example, I am using movq instructions on all the r** registers (x86_64 version of 32 bit registers from what I’ve gathered) – but perhaps that is in error.

I’d really appreciate it if someone is able to explain the problem here, I feel like if I better understood the cause it would really help avoid these types of problems in the future.

 .data
Bash:
    .asciz "/bin/bash"
Null1:
    .int 0
AddrToBash:
    .int 0
Null2:
    .int 0

.text
.globl _start

_start:
movq $Bash, AddrToBash
movq $59, %rax
movq $Bash, %rbx
movq $AddrToBash, %rcx
movq $Null2, %rdx
syscall 

Exit:
movq $60, %rbx
movq $1, %rax
syscall

Thanks in advance!

Answer 1

1. Syscall arguments are in %rsi, %rdi, %rdx. See What are the calling conventions for UNIX & Linux system calls on x86-64.
2. .int 0 is too short, should be .quad 0 in all three places.