Quick introduction to reverse engineering for beginners

#include
#include
struct float_as_struct
{
unsigned int fraction : 23; // fractional part
unsigned int exponent : 8; // exponent + 0x3FF
unsigned int sign : 1; // sign bit
};
float f(float _in)
{
float f=_in;
struct float_as_struct t;
};
assert (sizeof (struct float_as_struct) == sizeof (float));
memcpy (&t, &f, sizeof (float));
t.sign=1; // set negative sign
t.exponent=t.exponent+2; // multiple d by 2^n (n here is 2)
memcpy (&f, &t, sizeof (float));
return f;
int main()
{
printf ("%f\n", f(1.234));
};
float_as_struct structure occupies as much space is memory as float, e.g., 4 bytes or 32 bits.
Now we setting negative sign in input value and also by addding 2 to exponent we thereby multiplicating
the whole number by 2 2 , e.g., by 4.
Let’s compile in MSVC 2008 without optimization:
_t$ = -8 ; size = 4
_f$ = -4 ; size = 4
__in$ = 8 ; size = 4
?f@@YAMM@Z PROC ; f
push ebp
mov ebp, esp
sub esp, 8
fld DWORD PTR __in$[ebp]
fstp DWORD PTR _f$[ebp]
push 4
lea eax, DWORD PTR _f$[ebp]
push eax
lea ecx, DWORD PTR _t$[ebp]
push ecx
call _memcpy
add esp, 12 ; 0000000cH
76