Although it is a powerful tool, gdb is pretty cumbersome to use by itself. Even simple tasks such as execution tracing are made difficult by the lack of “friendliness”. To overcome this, we're going to use a wrapper over gdb that greatly enhances its default functionality. This tool is called “Python Exploit Development Assistance for GDB”, in short: peda. Even though there are lots of goodies included in it we're going to go only over what we need right now.

You can download peda using:

git clone https://github.com/longld/peda.git ~/peda

To set it up add the following to your ~/.gdbinit file and then run gdb as usual:

.gdbinit

# Source all settings from the peda dir
source ~/peda/peda.py
 
# These are other settings I have found useful
 
# When inspecting large portions of code the scrollbar works better than 'less'
set pagination off
 
 
# Keep a history of all the commands typed. Search is possible using ctrl-r
set history save on
set history filename ~/.gdb_history
set history size 32768
set history expansion on
 
# By default peda clears the screen after most commands, displaying a single
# context frame at a time and allowing you to access the previous/next frame
# using Shift+PageUp/Shift+PageDown. However, that might not work in your
# terminal, leaving you unable to access any older information. If that is the
# case, uncomment the following line:
#
#pset opt clearscr off

The most common actions done in gdb are: setting breakpoints, stepping through program execution and examining memory. The following are commands you need to know:

• run [args] ⇒ restart the program with [args] as args
• stepi (or simply si) ⇒ execute the current instruction and go to the next one - if it's a call instruction go to that subroutine (step into)
• nexti (or simply ni) ⇒ execute the current instruction and go to the next one - if it's a call instruction execute the whole subroutine in the background (step over)
• break (or simply b) ⇒ set a permanent breakpoint on an address or function
• info break ⇒ display all current breakpoints set
• delete 2 ⇒ delete the breakpoint with index 2 (from the list of current breakpoints)
• continue (or simply c) ⇒ continue execution after hitting a breakpoint (or receiving a signal)
• hexdump <addr> [/NR] ⇒ dump NR lines of memory starting from <addr>. (by default NR is 1)
• x /s <addr> ⇒ dump a string starting from <addr> (/100s would dump 100 strings)
• x /wx <addr> ⇒ dump a dword starting from <addr> (/100wx would dump 100 dwords)

gdb test
... # GDB banner is skipped
gdb-peda$ run arg0 arg1 arg2 

gdb test
... # GDB banner is skipped
gdb-peda$ run < inputfile > outputfile

gdb test
... # GDB banner is skipped
gdb-peda$ run arg0 arg1 arg2 < inputfile > outputfile

Let's find out how to do these on a previous crackme from session 01.Remember that the point was that it implemented a custom my_strcmp function such that ltrace/strace did not work. We now redo that task using gdb-peda:

First we start it and investigate what happens in main using pdis (the peda enhanced version of dis - disassemble)

$ gdb ./crackme3
gdb-peda$ pdis main
Dump of assembler code for function main:
   0x080485a7 <+0>:	push   ebp
   0x080485a8 <+1>:	mov    ebp,esp
   0x080485aa <+3>:	and    esp,0xfffffff0
   0x080485ad <+6>:	sub    esp,0x400
   0x080485b3 <+12>:	mov    DWORD PTR [esp],0x804a02c
   0x080485ba <+19>:	call   0x804855a <deobf>
   0x080485bf <+24>:	mov    DWORD PTR [esp],0x80486e0
   0x080485c6 <+31>:	call   0x80483b0 <puts@plt>
   0x080485cb <+36>:	mov    eax,ds:0x804a044
   0x080485d0 <+41>:	mov    DWORD PTR [esp+0x8],eax
   0x080485d4 <+45>:	mov    DWORD PTR [esp+0x4],0x3e8
   0x080485dc <+53>:	lea    eax,[esp+0x18]
   0x080485e0 <+57>:	mov    DWORD PTR [esp],eax
   0x080485e3 <+60>:	call   0x80483a0 <fgets@plt>
   0x080485e8 <+65>:	test   eax,eax
   0x080485ea <+67>:	jne    0x80485f8 <main+81>
   0x080485ec <+69>:	mov    DWORD PTR [esp],0xffffffff
   0x080485f3 <+76>:	call   0x80483d0 <exit@plt>
   0x080485f8 <+81>:	lea    eax,[esp+0x18]
   0x080485fc <+85>:	mov    DWORD PTR [esp],eax
   0x080485ff <+88>:	call   0x80483e0 <strlen@plt>
   0x08048604 <+93>:	sub    eax,0x1
   0x08048607 <+96>:	mov    BYTE PTR [esp+eax*1+0x18],0x0
   0x0804860c <+101>:	mov    DWORD PTR [esp+0x4],0x804a02c
   0x08048614 <+109>:	lea    eax,[esp+0x18]
   0x08048618 <+113>:	mov    DWORD PTR [esp],eax
   0x0804861b <+116>:	call   0x80484fc <my_strcmp>
   0x08048620 <+121>:	test   eax,eax
   0x08048622 <+123>:	jne    0x8048632 <main+139>
   0x08048624 <+125>:	mov    DWORD PTR [esp],0x80486ea
   0x0804862b <+132>:	call   0x80483b0 <puts@plt>
   0x08048630 <+137>:	jmp    0x804863e <main+151>
   0x08048632 <+139>:	mov    DWORD PTR [esp],0x80486f3
   0x08048639 <+146>:	call   0x80483b0 <puts@plt>
   0x0804863e <+151>:	mov    eax,0x0
   0x08048643 <+156>:	leave  
   0x08048644 <+157>:	ret    
End of assembler dump.

The interesting function is my_strcmp. Next we set a breakpoint on it and start the program:

gdb-peda$ break *my_strcmp
Breakpoint 1 at 0x80484fc
gdb-peda$ run
Password:
bla bla bla
[----------------------------------registers-----------------------------------]
EAX: 0xffffc9e8 ("bla bla bla")
EBX: 0xf7f94e54 --> 0x1a6d5c 
ECX: 0x28 ('(')
EDX: 0xc ('\x0c')
ESI: 0x0 
EDI: 0x0 
EBP: 0xffffcdd8 --> 0x0 
ESP: 0xffffc9cc --> 0x8048620 (<main+121>:	test   eax,eax)
EIP: 0x80484fc (<my_strcmp>:	push   ebp)
EFLAGS: 0x202 (carry parity adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x80484e3:	mov    ebp,esp
   0x80484e5:	sub    esp,0x18
   0x80484e8:	mov    DWORD PTR [esp],0x8049f08
   0x80484ef:	call   eax
   0x80484f1:	leave  
   0x80484f2:	jmp    0x8048470
   0x80484f7:	jmp    0x8048470
=> 0x80484fc <my_strcmp>:	push   ebp
   0x80484fd <my_strcmp+1>:	mov    ebp,esp
   0x80484ff <my_strcmp+3>:	sub    esp,0x28
   0x8048502 <my_strcmp+6>:	mov    eax,DWORD PTR [ebp+0x8]
   0x8048505 <my_strcmp+9>:	mov    DWORD PTR [esp],eax
   0x8048508 <my_strcmp+12>:	call   0x80483e0 <strlen@plt>
   0x804850d <my_strcmp+17>:	mov    DWORD PTR [ebp-0x10],eax
   0x8048510 <my_strcmp+20>:	cmp    DWORD PTR [ebp-0x10],0x0
   0x8048514 <my_strcmp+24>:	jne    0x804851d <my_strcmp+33>
[------------------------------------stack-------------------------------------]
0000| 0xffffc9cc --> 0x8048620 (<main+121>:	test   eax,eax)
0004| 0xffffc9d0 --> 0xffffc9e8 ("bla bla bla")
0008| 0xffffc9d4 --> 0x804a02c ("WXXHYIWE5yWic9vnmMGlA")
0012| 0xffffc9d8 --> 0xf7f95a80 --> 0xfbad2288 
0016| 0xffffc9dc --> 0x4 
0020| 0xffffc9e0 --> 0x4 
0024| 0xffffc9e4 --> 0x7 
0028| 0xffffc9e8 ("bla bla bla")
0032| 0xffffc9ec ("bla bla")
0036| 0xffffc9f0 --> 0x616c62 ('bla')
0040| 0xffffc9f4 --> 0x0 
0044| 0xffffc9f8 --> 0x40 ('@')
0048| 0xffffc9fc --> 0x4 
0052| 0xffffca00 --> 0x4 
0056| 0xffffca04 --> 0x6474e550 
0060| 0xffffca08 --> 0x170960 
 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
 
Breakpoint 1, 0x080484fc in my_strcmp ()

If you remember from the last session, the parameters passed to a function are on the stack. Because we have just arrived at this function using a call instruction, the return address is placed at the top of the stack (0x8048620). Immediately afterwards are the two parameters to the function: with “bla bla bla” being my input and “WXXHYIWE5yWic9vnmMGlA” the correct input (obviously, the two should match, therefore your job now is to input the value that the program expects).

Note that peda automatically telescopes addresses (dereferences and interprets the data)

Returning into main we see that there is something similar to an if:

   0x0804861b <+116>:	call   0x80484fc <my_strcmp>
   0x08048620 <+121>:	test   eax,eax
   0x08048622 <+123>:	jne    0x8048632 <main+139>
   0x08048624 <+125>:	mov    DWORD PTR [esp],0x80486ea
   0x0804862b <+132>:	call   0x80483b0 <puts@plt>
   0x08048630 <+137>:	jmp    0x804863e <main+151>
   0x08048632 <+139>:	mov    DWORD PTR [esp],0x80486f3
   0x08048639 <+146>:	call   0x80483b0 <puts@plt>

If my_strcmp returns 0 then the Zero Flag is set and jne does not determine a jump. Afterwards, a parameter is pushed on the stack and puts is called. Let's dump the two strings:

gdb-peda$ x/s 0x80486ea
0x80486ea:	"Correct!"
gdb-peda$ x/s 0x80486f3
0x80486f3:	"Nope!"

In peda you have quick access to information that you would otherwise have to obtain using other tools as presented before:

gdb-peda$ vmmap
Start      End        Perm	Name
0x08048000 0x08049000 r-xp	/tmp/black/crackmes/crackme3
0x08049000 0x0804a000 r--p	/tmp/black/crackmes/crackme3
0x0804a000 0x0804b000 rw-p	/tmp/black/crackmes/crackme3
0xf7ded000 0xf7dee000 rw-p	mapped
0xf7dee000 0xf7f93000 r-xp	/lib32/libc-2.17.so
0xf7f93000 0xf7f95000 r--p	/lib32/libc-2.17.so
0xf7f95000 0xf7f96000 rw-p	/lib32/libc-2.17.so
0xf7f96000 0xf7f99000 rw-p	mapped
0xf7fda000 0xf7fdb000 rw-p	mapped
0xf7fdb000 0xf7fdc000 r-xp	[vdso]
0xf7fdc000 0xf7ffc000 r-xp	/lib32/ld-2.17.so
0xf7ffc000 0xf7ffd000 r--p	/lib32/ld-2.17.so
0xf7ffd000 0xf7ffe000 rw-p	/lib32/ld-2.17.so
0xfffdd000 0xffffe000 rw-p	[stack]
gdb-peda$ elfheader
.interp = 0x8048174
.note.ABI-tag = 0x8048188
.hash = 0x80481a8
.gnu.hash = 0x80481e0
.dynsym = 0x8048204
.dynstr = 0x8048294
.gnu.version = 0x80482f6
.gnu.version_r = 0x8048308
.rel.dyn = 0x8048328
.rel.plt = 0x8048338
.init = 0x8048368
.plt = 0x8048390
.text = 0x8048400
.fini = 0x80486c4
.rodata = 0x80486d8
.eh_frame_hdr = 0x80486fc
.eh_frame = 0x8048738
.init_array = 0x8049f00
.fini_array = 0x8049f04
.jcr = 0x8049f08
.dynamic = 0x8049f0c
.got = 0x8049ffc
.got.plt = 0x804a000
.data = 0x804a024
.bss = 0x804a044
gdb-peda$ elfsymbol
Found 6 symbols
fgets@plt = 0x80483a0
puts@plt = 0x80483b0
__gmon_start__@plt = 0x80483c0
exit@plt = 0x80483d0
strlen@plt = 0x80483e0
__libc_start_main@plt = 0x80483f0

You can also search for strings in the mapped regions:

gdb-peda$ find "Correct"
Searching for 'Correct' in: None ranges
Found 2 results, display max 2 items:
crackme3 : 0x80486ea ("Correct!")
crackme3 : 0x80496ea ("Correct!")
 
gdb-peda$ find "/bin/sh"
Searching for '/bin/sh' in: None ranges
Found 1 results, display max 1 items:
libc : 0xf7f53be6 ("/bin/sh")

• Download level01 from Smash the stack and solve it using peda. Break on *main, step through the execution and figure out what it does and how to crack it.

$ scp level1@io.netgarage.org:/levels/level01 . # Password is level1