Cat Execve NASM

Question

I am trying to output a file in Assembly using cat with the execve syscall, but I am having a bit of trouble and I don't know why.

section .text
global  _start

_start:
    mov  al,  59  ; Syscall number
    xor  rdx, rdx ; No enviornment variables
    push rdx

    ; Add arg1
    mov  rdi, "/bin/cat"
    push rdi
    mov  rdi, rsp
    push rdx

    ; Add arg2
    mov  si,  "xt"
    push rsi
    mov  rsi, "//file.t"
    push rsi
    mov  rsi, rsp

    ; Call execve
    syscall      

I checked to make sure /file.txt exists. Then looked at the stack with GDB before the syscall is done, and everything also looks correct:

0x00007fffc25f8628│+0x0000: "//file.txt"         ← $rsp, $rsi
0x00007fffc25f8630│+0x0008: 0x0000000000007478 ("xt"?)
0x00007fffc25f8638│+0x0010: 0x0000000000000000
0x00007fffc25f8640│+0x0018: "/bin/cat"   ← $rdi

How can I call execve correctly?

Answer 1

Here is an 64-bit example, written in nasm, that runs an external command with arguments and this works for me.

; ---------------------------------------------------------
; Executes external command 
; ---------------------------------------------------------

SECTION .data

    ; Important: The strings must be null-terminated
    bin   db '/usr/bin/nano',0x0
    arg1  db '-w', 0x0
    arg2  db '/etc/fstab', 0x0

    args  dq arg1, arg2, 0x0

SECTION .text
global _start
_start:

    ; Invoke SYS_EXECVE
    mov rax, 59
    mov rdi, bin
    mov rsi, args
    xor rdx, rdx                    ; envp[] is not used
    syscall

    ; Quit
    mov rax, 60
    mov rdi, 0
    syscall

Best regards.