x64 helloworld shellcode not printing anything

Question

I am learning x64 shellcode on exploit.courses linux containers and I have a problem running a hello world x64 shellcode I wrote. I am trying to move the "Hi there" buffer directly to a register so I don't use the .data section.

section .data

;msg db "Hi there"

section .text

global _start
_start:
;zeroed out these registers
xor rax, rax
xor rbx, rbx
xor rsi, rsi
xor rdi, rdi
xor rdx, rdx

;write (int fd, char *msg, unsigned int len);
mov rax,1 ; syscall 1 is write in 64bit arch
mov rdi,1 ; rdi is fd
mov rbx, 0x6572656874206948
mov rdx, 9; rdx is size (9 for null byte)
syscall ; instead of int 0x80 for 32 bit architecture

;exit (int ret)
mov rax, 60 ; syscall 60 is exit in 64bit arch
mov rdi, 0 ; rdi is error code
syscall

I assemble the code and run it:

$nasm -f elf64 -o print2.o print2.asm
$ld -o print2 print2.o               
$./print2

And nothing happens though print2 seems to exit normally... Could someone explain why?

Sorry if this question has already been asked. I tried looking for a similar one but couldn't find anything.

Answer 1

As first steps look at the write documentation

   ssize_t write(int fd, const void *buf, size_t count);

The second arguments must be a const void *

But for linux the calling convention is:

 RDI, RSI, RDX, RCX, R8, R9, XMM0–7

Then your implementation is not correct.

You should do something like this

global _start


_start:

    jmp short msg

    routine:

    ...
    pop rsi         ;address of the string from the stack
    ...


    msg:
    call routine    
    db 'Hi here'