This page does not represent the most current semester of this course; it is present merely as an archive.
A Mad Programmer got really mad and 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 is to defuse your bomb before the due date. Good luck, and welcome to the bomb squad!
This lab is intended to get you thinking in assembly again. As such, you will get full credit if you can diffuse phase 1. I will give a small amount of extra credit for each additional phase you diffuse.
There will not be a homework associated with this lab.
Once the lab is live, you can obtain your bomb by pointing your Web browser at:
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 mad greeting from the Mad Mad Programmer.
If for some reason you request multiple bombs, this is not a problem. Choose one bomb to work on and delete the rest.
Almost no students succeed unless they use
gdb. See the "hints" and "getting started" sections below for more.
Your job for this lab is to defuse your bomb. Each has a few tamper-proofing elements.
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. If you get to 20 explosions I'll start removing points.
Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty. The last phase will challenge even the best students.
The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,
linux> ./bomb psol.txtthen it will read the input lines from psol.txt until it reaches EOF (end of file), and then switch over to stdin. This will keep you from having re-type solutions.
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.
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:
This web page is updated continuously to show the progress for each bomb. It also shows a "score" based on how many phases are diffused and how many explosions you've had. This score is not how I grade your submission; your grade will be
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 points if you have too many bomb explosions.
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. Here is a list of some of the tools you may find useful in analyzing your bomb, and hints on how to use them.
gdbThe 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.
The CS:APP web site
http://csapp.cs.cmu.edu/public/students.htmlhas 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 -tobjdump -dUse 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.
stringsLooking for a particular tool? How about documentation? Don’t forget, the commands apropos, man, and info are your friends. In particular, man ascii might come in useful. info gas will give you more than you ever wanted to know about the GNU Assembler. Also, the web may also be a treasure trove of information. If you get stumped, feel free to ask course staff for help.
Try running objdump -t bomb. This will show you the symbols in the executable, including the names of all methods. Look for one that looks dangerous, as well as some that looks like interesting methods (perhaps something like "Phase 1").
Don't run the program directly; instead run it from a debugger like gdb. For example, if I decide I never want the method to run, I would invoke
linux> gdb bomb
(gdb) b theMethodName
(gdb) runThen the code will stop before it runs theMethodName
You can objdump -d but it might be easier to use the gdb command disas methodName or the like. You might also like some memory examining gdb commands like
call (void)puts(0x...)x/20b 0x...The textbook has a nice summary of useful gdb commands on page 255.