Segfault when loading function parameter into a register

问题

I'm quite new to x86 assembly, and I'm trying to build off a hello world program. I'm trying to make a subroutine, that writes a single byte to stdout, but i've hit a problem. The line mov ebx, [esp+1] (to load the byte passed, when I call the subroutine) causes a segfault.

I've tried xoring the ebx register with itself, to make sure that it is empty, to make sure, that it doesn't mess with the syscall

_start:
    push 32h
    call _writeByte

    ; This just jumps to an exit routine
    jmp  _exit

_writeByte:
    ; This line causes the problem. If I remove it the program works fine
    mov  ebx, [esp+1]
    xor  ebx, ebx

    mov  eax, 1
    mov  edi, 1
    mov  esi, tmp
    mov  edx, 1
    syscall

    ret

Why is the program segfaulting?

回答1:

I'm in x64 mode, and like a bunch of people suggested in the comments using mov ebx, [rsp+8] worked, because esp are just the 4 lower bytes of the register.

来源：https://stackoverflow.com/questions/54498071/segfault-when-loading-function-parameter-into-a-register