1. GDB Introduction And Lab 2
CSE 351
GDB Introduction
And
Lab 2

2. Lab 2: Bomb Lab Lab 2 has been assigned as of yesterday
You will be disassembling programs and trying to defuse artificial “bombs” by determining certain codes
You likely haven’t covered enough in lecture to do the entire lab right now
Start early!
You have a week and a half to complete it
Some people finish it quickly, some people take a long time

3. Lab 2 Requires you to defuse “bombs” by entering a series of passcodes
Not real bombs/viruses/etc!
Each passcode is validated by some function
You only have access to the assembly code
It’s your job to determine what passcodes will prevent the program from ever calling the explode_bomb() function
Each student has a different bomb
Show them how to download the files

4. Lab 2 Files bomb bomb.c defuser.txt The executable bomb program
This is the entry point for the bomb program, and it calls functions whose source code is not available to you
defuser.txt
Contains passcodes, each separated by a newline
Place your passcodes here once you solve each phase
Can be passed as an argument to prevent you from entering the passcodes manually each time
Open bomb.c: show them the phases
Defuser.txt : remind them to have a line break at the end or the grading script might go wrong

5. Loading the Program to GDB
In order to load a binary into GDB, you simply pass the name of the executable to the gdb program
Try this on your machine
gdb bomb
You should see a bunch of version and license information
The last line before the (gdb) prompt is always the symbol loading status
If you ever see (no debugging symbols found) you may have a problem
In this case, you should see no such message

6. Exiting GDB
Before we go any further, it might be helpful to understand how to exit GDB
There are a few ways to exit:
Ctrl-D
Typing quit
Typing q
Many GDB commands can simply be abbreviated to their first letter, as you will see
If you ever want to stop the current GDB command, just use Ctrl-C
Then run gdb again

7. Running the executable
There are a couple ways you can begin execution of a program in GDB
The run command will start your program and keep running until it hits a critical error or the program finishes
Try entering run, or just r
Run the program
Input some strings when asked. Watch the world burn (exploded)
Next set up the breakpoint

8. Setting breakpoints
In order to step through code, we need to be able to stop our code first
GDB allows you to set breakpoints, just like when you debugged Java programs in Eclipse, for example
The break command will set breakpoints for you (b for short)
Let’s set a breakpoint at the entry to phase_1() to prevent explosion
Each breakpoint is given a number
Our breakpoint is given the number 1
To disable our breakpoint temporarily, enter disable 1
To enable our breakpoint again, enter enable 1
To delete our breakpoint, enter delete 1
If you ever want to see a summary of all your breakpoints, just enter info break
Set some more breakpoints, clear some breakpoints, get them comfortable with the concept

9. Disassemble your program
Dump the assembly code of the current function we are in
Use disas command
Can also passed in a specific function name
disas function_name
It will dump the assembly code of the given function name
Try disas main for example

10. Stepping through code There are next and step
Stepping through one line of C code
What is the difference?
For this lab, you want to step through each line of assembly
Use stepi or nexti
Can also mention how many steps: stepi 2
If you have halted execution and wish to continue running the program, use the continue command
Use that now to run the program to completion
Explain difference between next and step : step get inside the function

11. Examining Register/Memory
x/[NUM][SIZE][FORMAT] [Register/Memory]
NUM = number of objects to display
SIZE = size of each object (b=byte, h=half-word, w=word, g=giant (quad- word))
FORMAT = how to display each object (d=decimal, x=hex, o=octal, etc.)
Show an example:
Try to examine the Answer string

12. Examining Register/Memory

13. Lab 2 Notes The bomb uses sscanf, which parses a string into values
Example:
int a, b;
sscanf(“123, 456”, “%d, %d”, &a, &b);
The first argument is parsed according to the format string
After this code is run, a = 123 and b = 456

14. Lab 2 Tips Print out the disassembled phases
To disassemble a program, run objdump -d bomb > bomb.s
You can then print out bomb.s
Mark the printouts up with notes
Try to work backwards from the “success” case of each phase
Remember that some addresses are pointing to strings located elsewhere in memory
Print them out in GDB