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Computer Systems I Laboratory Exercise 6

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CSCI 247 Computer Systems I
Laboratory Exercise 6
Introduction
The nefarious Dr. Evil has planted a slew of “binary bombs” on our class machines. A
binary bomb is a program that consists of a sequence of phases. Each phase expects
you to type a particular string on stdin. If you type the correct string, then the phase is
defused and the bomb proceeds to the next phase. Otherwise, the bomb explodes by
printing “BOOM!!!” and then terminating. The bomb is defused when every phase has
been defused. There are too many bombs for us to deal with, so we are giving each
student a bomb to defuse. Your mission, which you have no choice but to accept, is to
defuse your bomb before the due date. Good luck, and welcome to the bomb squad!
Step 1: Get Your Bomb
You can obtain your bomb by pointing your Web browser at:
http://247.cs.wwu.edu:15213/ <Katie is going confirm this location>
This will display a binary bomb request form for you to fill in. Enter your user name and
email address and hit the Submit button. The server will build your bomb and return it to
your browser in a tar file called bombk.tar, where k is the unique number of your bomb.
Save the bombk.tar file to a (protected) directory in which you plan to do your work.
Then give the command:
tar -xvf bombk.tar
This will create a directory called ./bombk with the following files:
• README: Identifies the bomb and its owners.
• bomb: The executable binary bomb.
• bomb.c: Source file with the bomb’s main routine and a friendly greeting from Dr. Evil.
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If for some reason you request multiple bombs, this is not a problem. Choose one bomb
to work on and delete the rest.
Step 2: Defuse Your Bomb
Your job for this lab is to defuse your bomb.
You must do the Lab on one of the class machines. In fact, there is a rumor that Dr. Evil
really is evil, and the bomb will always blow up if run elsewhere. There are several other
tamper-proofing devices built into the bomb as well, or so we hear.
You can use many tools to help you defuse your bomb. Please look at the hints section
for some tips and ideas. The best way is to use your favorite debugger to step through
the disassembled binary.
Each time your bomb explodes it notifies the bomblab server, and you lose 1/2 point (up
to a max of 20 points) in the final score for the lab. So there are consequences to
exploding the bomb. You must be careful!
The first four phases are worth 10 points each. Phases 5 and 6 are a little more difficult,
so they are worth 15 points each. So the maximum score you can get is 70 points.
Although phases get progressively harder to defuse, the expertise you gain as you
move from phase to phase should offset this difficulty. However, the last phase will
challenge even the best students, so please don’t wait until the last minute to start.
The bomb ignores blank input lines. If you run your bomb with a command line
argument, for example,
linux> ./bomb psol.txt
then it will read the input lines from psol.txt until it reaches EOF (end of file), and then
switch over to stdin. In a moment of weakness, Dr. Evil added this feature so you don’t
have to keep retyping the solutions to phases you have already defused.
To avoid accidentally detonating the bomb, you will need to learn how to single-step
through the assembly code and how to set breakpoints. You will also need to learn how
to inspect both the registers and the memory states. One of the nice side-effects of
doing the lab is that you will get very good at using a debugger. This is a crucial skill
that will pay big dividends the rest of your career.
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Logistics
This is an individual project. All handins are electronic. Clarifications and corrections will
be posted on the course message board. Initially, the bombs can only be worked on in
CF 416. Other labs will be added.
Handin
There is no explicit handin. The bomb will notify your instructor automatically about your
progress as you work on it. You can keep track of how you are doing by looking at the
class scoreboard at:
http://247.cs.wwu.edu:15213/scoreboard (Links to an external site.)Links to an external
site.
This web page is updated continuously to show the progress for each bomb.
Hints (Please read this!)
There are many ways of defusing your bomb. You can examine it in great detail without
ever running the program, and figure out exactly what it does. This is a useful
technique, but it not always easy to do. You can also run it under a debugger, watch
what it does step by step, and use this information to defuse it. This is probably the
fastest way of defusing it.
We do make one request, please do not use brute force! You could write a program that
will try every possible key to find the right one. But this is no good for several reasons:
• You lose 1/2 point (up to a max of 20 points) every time you guess incorrectly and the
bomb explodes.
• Every time you guess wrong, a message is sent to the bomblab server. You could very
quickly saturate the network with these messages, and cause the system administrators
to revoke your computer access.
• We haven’t told you how long the strings are, nor have we told you what characters
are in them. Even if you made the (incorrect) assumptions that they all are less than 80
characters long and only contain letters, then you will have 2680 guesses for each
phase. This will take a very long time to run, and you will not get the answer before the
assignment is due.
There are many tools which are designed to help you figure out both how programs
work, and what is wrong when they don’t work. Your main tool will be gdb.
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The GNU debugger, this is a command line debugger tool available on virtually every
platform. You can trace through a program line by line, examine memory and registers,
look at both the source code and assembly code (we are not giving you the source code
for most of your bomb), set breakpoints, set memory watch points, and write scripts.
Here are some other tips for using gdb.
– To keep the bomb from blowing up every time you type in a wrong input, you’ll want to
learn how to set breakpoints.
– For online documentation, type “help” at the gdb command prompt, or type “man
gdb”, or “info gdb” at a Unix prompt.

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