Rellocate stack to the heap (evil)

realloc-stack.asm

format ELF64 executable 3;
entry start;

segment readable writeable

; copied verbatim from my lib
SYS.mmap:
  .id        = $09
  .proto_r   = $01
  .proto_rw  = $03
  .proto_rx  = $05
  .anon      = $22
  .shared    = $01
  .nofd      = -1

SYS.munmap.id=$0B
BUFF_SZ=1
; ---   *   ---   *   ---
; setup

segment readable executable
start:

  ; get N pages
  mov rsi,BUFF_SZ
  shl rsi,12

  ; linux boilerpaste
  mov rdx,SYS.mmap.proto_rw
  mov r10,SYS.mmap.anon
  mov r8 ,SYS.mmap.nofd
  xor rdi,rdi
  xor r9 ,r9

  ; ^call mmap
  mov rax,SYS.mmap.id
  syscall

  ; ^get end of buff (recomputing size seems faster)
  mov rdx,BUFF_SZ
  shl rdx,12
  
  ; THE BIT: use heap as stack because we can
  lea rsp,[rax+rdx]
  push rax

  ; once you're done it'd be a good idea to reset rsp/rbp
  ; back to what they where. rather, it'd be a good idea to
  ; never do this, in fact. but because the program ends here,
  ; it doesn't matter

  ; cleanup
  pop rdi
  mov rsi,rdx
  mov rax,SYS.munmap.id
  syscall

  ; exit
  mov rax,60
  syscall
  
; ---   *   ---   *   ---