Read a File via syscalls

; cat_file(path)   rdi = path (C string)
section .rodata
    buf_len: equ 4096
section .bss
    buf: resb buf_len

section .text
global cat_file
cat_file:
    ; open(path, O_RDONLY, 0)  syscall #2
    mov     rax, 2
    mov     rsi, 0           ; O_RDONLY
    xor     rdx, rdx
    syscall
    test    eax, eax
    js      .fail            ; negative -> error
    mov     r12, rax         ; save fd in callee-saved r12

    push    r12              ; preserve r12 across our own calls
.read_loop:
    ; read(fd, buf, buf_len)  syscall #0
    mov     rax, 0
    mov     rdi, r12
    lea     rsi, [rel buf]
    mov     rdx, buf_len
    syscall
    test    rax, rax
    jle     .close           ; 0 -> EOF, <0 -> error
    mov     r13, rax         ; save bytes read

    ; write(1, buf, n)        syscall #1
    mov     rax, 1
    mov     rdi, 1
    lea     rsi, [rel buf]
    mov     rdx, r13
    syscall
    jmp     .read_loop

.close:
    ; close(fd)  syscall #3
    mov     rax, 3
    mov     rdi, r12
    syscall
    pop     r12
    xor     eax, eax
    ret
.fail:
    mov     rax, -1
    ret