Debugging using GDB

So here I am, up with my first blog post. This post is basically an intro on how to debug a program using gdb.

GDB or GNU Debugger is extensively used in debugging programs written using mainly Ada, C, C++, Objective-C, Free Pascal, Fortran, Go, Java. In this post I am going to explain how to get started with gdb.

Prerequisites:

A basic knowledge about x32 assembly and C language.

Installation:

First to check whether gdb is installed in you system or not try this command in you terminal

gdb --version

If that doesn’t work then you can probably check out this link. It explains how to install gdb very clearly.

Getting Started:

Now the next task is to compile the program and disassemble it using gdb.

Command for compiling the program:

gcc -g -o program program.c

Command to open the program in gdb :

gdb executable

Useful commands while using gdb:

disas main : to disassemble the main of the program

set disassembly flavor-intel : to change the disassembly to intel syntax

info registers (i r) : to view the current status of registers

help (h) : to get help on gdb commands

b (break) <address> : to set a breakpoint at the specified address

r (run) : run the program until it encounters a breakpoint or to end

s (step) : single step to descend into the functions

n (next) : single step without descending into the functions

c (continue) : continue till the next breakpoint or end

l (list) : to list the codes around the current point

i b (info b) : list all the breakpoints

p (print) <address/variable> : to print the value at the specified address or variable.

p/x <variable> : prints the value at the given address in hex form

delete <number> : to delete all breakpoints or the specified one

dis <number> : disable the specified breakpoint

en <number> : to enable the specified breakpoint

clear <function> : to clear all breakpoints set in that function

GDB-PEDA

PEDA – Python Exploit Development Assistance for GDB.

PEDA is a Python GDB script with many handy commands and a user friendly display. It helps in speeding up the exploit development process.

Installation :

git clone https://github.com/longld/peda.git ~/peda
echo "source ~/peda/peda.py" >> ~/.gdbinit

Useful commands while using gdb-peda :

pd <function> : to print the disasembly of the specified function

b *(function/address) : to set a breakpoint at the specified address or function

r : to run the program till the breakpoint or end

ni : single step to without descending into any functions

si : single step to descend into the functions

help (h) : to get help on gdb commands

c (continue) : continue till the next breakpoint or end

delete : to delete all breakpoints or the specified one

print <$register/address/variable> : to print the value at the specified address or variable.

x/s <$register/address> : to print the string at the given address

[s in the above command can be replaced with x (hex), wx (integer) as preferred]

So these were some basic commands which will help you in getting started with gdb or gdb-peda. I hope you will find this article useful. If you have any questions or queries feel free to contact me. Happy debugging 😉 !!

Thank you!!