The names all the functions the bomb calls

1 Introduction

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

• bomb: The executable binary bomb.

• bomb.c: Source file with the bomb’s main routine.

You can use many tools to help you with this; 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 staff, and you lose 1/4 point (up to a max of 10 points) in the final score for the lab. So there are consequences to exploding the bomb. You must be careful! Note that only whole negative points will be noted, so the first 3 explosions will be free.

gnulinux> ./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.

You should do the assignment on assembler.nada.kth.se. 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 they say.

2

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.

• gdb
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 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.

• objdump -d
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 scanf might appear as:

• strings
This utility will display the printable strings in your bomb.

Looking for a particular tool? How about documentation? Don’t forget, the commands apropos and man are your friends. In particular, man ascii might come in useful. Also, the web may also be a treasure trove of information. If you get stumped, feel free to ask Stefan for help.