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Archive for January 29th, 2019

The basic idea of using debugger is that you can stop the program and continue the process before it terminates so you examine and identify if there is a problem with program.

In this tutorial, I will show few stepping process after breakpoints.

  1. Continue or c
  2. Next or n
  3. Step or s
  4. Nexti or ni
  5. Stepi or si

To check if the program has already run or not, type: info program.

(gdb) info program
The program being debugged is not being run.

1. Continue
It will resume the program execution until complete or stop at next breakpoint.

Before we start, type the code below and save it to ‘break.asm’.

1 section .text
2 global _start
3 _start:
4
5 mov eax,1111
6 mov ebx,2222
7 call do_nothing1
8 call do_nothing2
9 call do_nothing3
10 mov ecx,3333
11 mov edx,4444
12 call exit
13
14
15 do_nothing1:
16 nop
17 nop
18 ret
19
20 do_nothing2:
21 nop
22 nop
23 ret
24
25 do_nothing3:
26 nop
27 nop
28 ret
29
30 exit:
31 mov eax,1
32 int 0x80

Compile the program without debug function.

$ nasm -f elf32 break.asm -o break.o
$ ld break.o -o break
$ gdb ./break --silent
Reading symbols from /home/darklinux/break...(no debugging symbols found)...done.

(more…)

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Reverse the program process is very useful when you are debugging and realize that you need to go backward. Instead of re-run the whole process, starting GDB version 7, you can go to the previous process.

There are few reverse that you can do:

  1. reverse-continue or rc
  2. reverse-step or reverse-step count
  3. reverse-stepi
  4. next or reverse-next count
  5. reverse-nexti

Before you use ‘reverse’ command, you need to activate the recording process otherwise you will get an error below:
“Target child does not support this command”.

Let’s try.
Type the code below and save it to ‘break.asm’.

1 section .text
2 global _start
3 _start:
4
5 mov eax,1111
6 mov ebx,2222
7 call do_nothing1
8 call do_nothing2
9 call do_nothing3
10 mov ecx,3333
11 mov edx,4444
12 call exit
13
14
15 do_nothing1:
16 nop
17 nop
18 ret
19
20 do_nothing2:
21 nop
22 nop
23 ret
24
25 do_nothing3:
26 nop
27 nop
28 ret
29
30 exit:
31 mov eax,1
32 int 0x80

reverse01
(more…)

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