Ancient CS 61 Content Warning!!!!!1!!!
This is not the current version of the class.
This site was automatically translated from a wiki. The translation may have introduced mistakes (and the content might have been wrong to begin with).

Assignment 2: Binary Bomb

Introduction

The nefarious Dr. Evil is going to tickle us to death unless we defuse a huge number of “binary bombs” he's planted on our class server. A binary bomb is a program that consists of a sequence of phases. Each phase expects you to type a particular string on the standard input. 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 different bomb to defuse. Your mission, which you have no choice but to accept, is to defuse your bomb before the due date. Good luck!

Download

Obtain your bomb by pointing your Web browser here:

http://cs61.seas.harvard.edu:6161/

Enter your code.seas.harvard.edu username into the form and hit Submit. The server will return your bomb 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. This must be on a 32-bit Linux host, such as a CS 50 Appliance. Then give the command: tar -xvf bombk.tar. This will create a directory called ./bombk with the following files:

If you lose the bomb, no problem; just go back to http://cs61.seas.harvard.edu:6161/ and request it again.

Defuse

Your job is to defuse your bomb. This involves supplying it with just the right input.

The bombs seem to be tamper-proofed in a couple ways. For one, they can only be defused when the computer is connected to the Internet. Running the bomb on a machine without Internet connectivity won’t do anything. There are several other tamper-proofing devices built into the bomb as well, we hear!

You can use many tools to help you defuse your bomb. Probably the best way is to use your favorite debugger to step through the disassembled binary.

Each time your bomb explodes it notifies Dr. Evil, who tells us, 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. 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 everyone, 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 psol.txt

then it will read the input lines from psol.txt until it reaches the end, 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 registers and memory state. One of the nice side effects of doing the lab is that you will get very good at using a debugger!

Handin

The bomb notifies us automatically of your progress as you work on it. You can keep track of how you are doing, and compare (anonymously) with everyone else, by looking at the class scoreboard at:

http://cs61.seas.harvard.edu:6161/scoreboard

This web page is kept updated to show the progress for each bomb. It may take up to a minute for new explosions and defusings to show up on the scoreboard.

Hints

There are many ways of defusing your bomb. Hypothetically, you could even figure out the bomb without ever running the program, just from the machine code (and various tools like objdump). But it's much easier to run the bomb under a debugger, watch what it does step by step, and reverse-engineer the input it wants.

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. Here is a list of some of the tools you may find useful in analyzing your bomb, and hints on how to use them.

Understanding instructions There are lots of ways to puzzle out instruction meanings: lecture, the book, even the examples distributed as part of our lectures. (For example, if you're curious about the leal instruction, go to the cs61-lectures repository, and try “grep leal */*.s”. This reports a match in l09/f36.s, so look at l09/f36.c.)

Also try searching for the instruction name on Google: “movl instruction”. But be careful. There are two syntaxes used for x86 assembly language. We use “AT&T syntax” in class and in the book, but many online references use “Intel syntax,” which switches the order of arguments and is different in other annoying ways. For instance, Intel calls the %eax register EAX (all caps, no percent). Read about the differences in syntaxes in the Aside on p166 of CS:APP2e, or here or here.

gdb

The GNU debugger 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. The CS:APP2e web site has a handy single-page gdb summary (PDF) that you can print out and use as a reference. Here are some other tips for using gdb:

objdump -t

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 (or objdump -S) Use this to disassemble all of the code in the bomb. You can also just look at individual functions.

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

This utility will display the printable strings in your bomb.

For more, don’t forget your friends the commands man and info, and the amazing Google and Wikipedia. In particular, info gas has more than you might ever want to know about assembler.

This lab was originally created for CMU’s CS:APP course.