NASM x86_64 open file and write 'Hello world'

hello_world_into_file.asm

section .text
global _start          ;must be declared for linker (ld)

_start:                ;tell linker entry point
    mov rdi, filename
    mov rsi, 0102o     ;O_CREAT, man open
    mov rdx, 0666o     ;umode_t
    mov rax, 2
    syscall

    mov [fd], rax
    mov rdx, len       ;message length
    mov rsi, msg       ;message to write
    mov rdi, [fd]      ;file descriptor
    mov rax, 1         ;system call number (sys_write)
    syscall            ;call kernel

    mov rdi, [fd]
    mov rax, 3         ;sys_close
    syscall

    mov rax, 60        ;system call number (sys_exit)
    mov rdi, 0         ;do not return error code
    syscall            ;call kernel

section .data
    msg db 'Hello, world', 0xa
    len equ $ - msg
    filename db 'test.txt', 0
    lenfilename equ $ - filename
    fd dq 0