Question

I am trying to convert an assembly program I wrote into NULL-free shellcode.

However, I am unsure how to go about this for certain instructions.
Some of them (in Intel syntax) include:

push 0x1000

and

mov BYTE [eax],0x31

I want to avoid using thousands of inc eax instructions. I was thinking maybe something creative with xor-ing values, and for the second, maybe if there was a flag to set to make it take a constant of only 8 bits.

Answer 1

push 0x1000

If you can spare a register (and you don’t mind clobbering the flags), how about something like:

xor eax, eax
inc eax
shl eax, 12
push eax

---

mov BYTE [eax],0x31

The zero here does not come from the constant, but from the addressing mode. Try:

xchg eax, ecx
mov BYTE [ecx],0x31
xchg eax, ecx