Reverse Engineering for Beginners

CHAPTER 21. STRUCTURES CHAPTER 21. STRUCTURES

unix_time=time(NULL);

localtime_r (&unix_time, &t);

printf ("Year: %d\n", t.tm_year+1900);
printf ("Month: %d\n", t.tm_mon);
printf ("Day: %d\n", t.tm_mday);
printf ("Hour: %d\n", t.tm_hour);
printf ("Minutes: %d\n", t.tm_min);
printf ("Seconds: %d\n", t.tm_sec);
};

Let’s compile it in GCC 4.4.1:

Listing 21.6: GCC 4.4.1

main proc near
push ebp
mov ebp, esp
and esp, 0FFFFFFF0h
sub esp, 40h
mov dword ptr [esp], 0 ; first argument for time()
call time
mov [esp+3Ch], eax
lea eax, [esp+3Ch] ; take pointer to what time() returned
lea edx, [esp+10h] ; at ESP+10h struct tm will begin
mov [esp+4], edx ; pass pointer to the structure begin
mov [esp], eax ; pass pointer to result of time()
call localtime_r
mov eax, [esp+24h] ; tm_year
lea edx, [eax+76Ch] ; edx=eax+1900
mov eax, offset format ; "Year: %d\n"
mov [esp+4], edx
mov [esp], eax
call printf
mov edx, [esp+20h] ; tm_mon
mov eax, offset aMonthD ; "Month: %d\n"
mov [esp+4], edx
mov [esp], eax
call printf
mov edx, [esp+1Ch] ; tm_mday
mov eax, offset aDayD ; "Day: %d\n"
mov [esp+4], edx
mov [esp], eax
call printf
mov edx, [esp+18h] ; tm_hour
mov eax, offset aHourD ; "Hour: %d\n"
mov [esp+4], edx
mov [esp], eax
call printf
mov edx, [esp+14h] ; tm_min
mov eax, offset aMinutesD ; "Minutes: %d\n"
mov [esp+4], edx
mov [esp], eax
call printf
mov edx, [esp+10h]
mov eax, offset aSecondsD ; "Seconds: %d\n"
mov [esp+4], edx ; tm_sec
mov [esp], eax
call printf
leave
retn
main endp

Somehow,IDAdid not write the local variables’ names in the local stack. But since we already are experienced reverse
engineers :-) we may do it without this information in this simple example.

Please also pay attention to thelea edx, [eax+76Ch]—this instruction just adds0x76C(1900) to value inEAX, but
doesn’t modify any flags. See also the relevant section aboutLEA(A.6.2 on page 887).