Reverse Engineering for Beginners

CHAPTER 46. VARIADIC FUNCTIONS CHAPTER 46. VARIADIC FUNCTIONS

; get pointer to the 2nd argument
lea eax, DWORD PTR _fmt$[esp]
push eax ; pass pointer
push ecx
call _vprintf
add esp, 8
push 0
call _exit
$LN3@die:
int 3
_die ENDP

We see that all our function does is just taking a pointer to the arguments and passing it tovprintf(), and that function is
treating it like an infinite array of arguments!

Listing 46.5: Optimizing MSVC 2012 x64

fmt$ = 48
die PROC
; save first 4 arguments in Shadow Space
mov QWORD PTR [rsp+8], rcx
mov QWORD PTR [rsp+16], rdx
mov QWORD PTR [rsp+24], r8
mov QWORD PTR [rsp+32], r9
sub rsp, 40
lea rdx, QWORD PTR fmt$[rsp+8] ; pass pointer to the 1st argument
; RCX here is still points to the 1st argument (format-string) of die()
; so vprintf() will take it right from RCX
call vprintf
xor ecx, ecx
call exit
int 3
die ENDP