1   Introduction

The nefarious Dr. Evil has planted a slew of“binary bombs”on our ilab 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!

2   Step 1: Get Your Bomb

You can obtain your bomb by pointing your Web browser at:

http://ilab4.cs.rutgers.edu:23211/

This will display a binary bomb request form for you to ill in. Enter your NetID and rutgers.edu email address and hit the Submit button. The server will build your bomb and return it to your browser in a tar ile called bomb[ID].tar, where [ID] is the unique number of your bomb.

The server may be unresponsive for a short while if many people are trying to download their bomb at the same time.  If you see a“Failed - Network error”error message when trying to download your bomb using Chrome, consider using Incognito mode.

Save the bomb[ID].tar ile to your home directory on the ilab machines in which you plan to do your work. Then give the command:

$ tar -xvf bomb[ID].tar

This will create a directory called ./bomb[ID]with the following iles:

• README: Identiies the bomb and its owners.

• bomb: The executable binary bomb.

• bomb.c: Source ile with the bomb’s main routine and a friendly greeting from Dr. Evil.

If for some reason you request multiple bombs, this is not a problem. Choose one bomb to work on and delete the rest.

3   Step 2: Defuse Your Bomb

Your job for this lab is to defuse your bomb.

You must do the assignment on one of the ilab machines:

•  ilab1.cs.rutgers.edu

•  ilab2.cs.rutgers.edu

•  ilab3.cs.rutgers.edu

•  ilab4.cs.rutgers.edu

If you see a“Initialization error: Running on an illegal host”error when you run your bomb, it means you are running on some machine other than the ilab 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-prooing 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 notiies the bomblab server, and you lose points in the inal score for the lab. So there are consequences to exploding the bomb. You must be careful!

The irst four phases are worth 10 points each. Phases 5 and 6 are a little more dificult, so they are worth 15 points each.  So the maximum score you can get is 70 points.  The 70 points on the assignment will then get scaled up by 15/7 so that this assignment is equal weight as other programming assignments in CS 211.

Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this dificulty. 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,

$ ./bomb mysolution.txt

then it will read the input lines from mysolution.txtuntil it reaches EOF (end of ile), 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.

4   Logistics

This is an individual project.  All handins are electronic.  Clariications and corrections will be posted on the course Piazza.

5   Scoreboard

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://ilab4.cs.rutgers.edu:23211/scoreboard

This web page is updated continuously to show the progress for each bomb.

6   Hints (Please read this!)

The materials posted on the class Canvas in the“Machine-level representation of programs”mod- ule will be your main reference for this assignment.

There are many ways of defusing your bomb.  You can examine it in great detail without ever running the program, and igure 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 bruteforce! You could write a program that will try every possible key to ind 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 igure out both how programs work, and what is wrong when they don’t work. Here is a list of some of the tools you may ind useful in analyzing your bomb, and hints on how to use them.

• gdb

The GNU debugger, this is a command line debugger tool available on virtually every plat- form. 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.

The CS:APP website

http://csapp.cs.cmu.edu/public/students.html

has a very handy single-page gdb summary that you can print out and use as a reference. 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.  Some people also like to run gdb under gdb-mode in emacs.

• objdump -t bomb

This will print out the bomb’s symbol table.  The symbol table includes the names of all functions and global variables in the bomb, the names of all the functions the bomb calls, and their addresses. You may learn something by looking at the function names!

• objdump -d bomb

Use this to disassemble all of the code in the bomb.  You can also just look at individual functions. Reading the assembler code can tell you how the bomb works.

Although objdump -d gives you a lot of information, it doesn’t tell you the whole story. Calls to system-level functions are displayed in a cryptic form. For example, a call to sscanf might appear as:

8048c36:  e8 99 fc ff ff  call   80488d4 <_init+0x1a0>

To determine that the call was to sscanf, you would need to disassemble within gdb.

•  strings -t x bomb

This utility will display the printable strings in your bomb.