Reverse Engineering for Beginners

CHAPTER 21. STRUCTURES CHAPTER 21. STRUCTURES

GDB

Let’s try to load the example into GDB^4 :

Listing 21.7: GDB

dennis@ubuntuvm:~/polygon$ date
Mon Jun 2 18:10:37 EEST 2014
dennis@ubuntuvm:~/polygon$ gcc GCC_tm.c -o GCC_tm
dennis@ubuntuvm:~/polygon$ gdb GCC_tm
GNU gdb (GDB) 7.6.1-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
http://www.gnu.org/software/gdb/bugs/...
Reading symbols from /home/dennis/polygon/GCC_tm...(no debugging symbols found)...done.
(gdb) b printf
Breakpoint 1 at 0x8048330
(gdb) run
Starting program: /home/dennis/polygon/GCC_tm

Breakpoint 1, __printf (format=0x80485c0 "Year: %d\n") at printf.c:29
29 printf.c: No such file or directory.
(gdb) x/20x $esp
0xbffff0dc: 0x080484c3 0x080485c0 0x000007de 0x00000000
0xbffff0ec: 0x08048301 0x538c93ed 0x00000025 0x0000000a
0xbffff0fc: 0x00000012 0x00000002 0x00000005 0x00000072
0xbffff10c: 0x00000001 0x00000098 0x00000001 0x00002a30
0xbffff11c: 0x0804b090 0x08048530 0x00000000 0x00000000
(gdb)

We can easily find our structure in the stack. First, let’s see how it’s defined intime.h:

Listing 21.8: time.h

struct tm
{
int tm_sec;
int tm_min;
int tm_hour;
int tm_mday;
int tm_mon;
int tm_year;
int tm_wday;
int tm_yday;
int tm_isdst;
};

Pay attention that 32-bitintis used here instead of WORD in SYSTEMTIME. So, each field occupies 32-bit.

Here are the fields of our structure in the stack:

0xbffff0dc: 0x080484c3 0x080485c0 0x000007de 0x00000000
0xbffff0ec: 0x08048301 0x538c93ed 0x00000025 sec 0x0000000a min
0xbffff0fc: 0x00000012 hour 0x00000002 mday 0x00000005 mon 0x00000072 year
0xbffff10c: 0x00000001 wday 0x00000098 yday 0x00000001 isdst0x00002a30
0xbffff11c: 0x0804b090 0x08048530 0x00000000 0x00000000

Or as a table:

(^4) Thedateresult is slightly corrected for demonstration purposes. Of course, it’s not possible to run GDB that quickly, in the same second.