CS 213, Fall 2000Lab Assignment L2: Defusing a Binary BombAssigned: Sept. 21, Due: Wed., Oct. 4, 11:59PMSeptember 20, 2000Dave O’Hallaron ([email protected]) is the lead person and bomb squad chief for this lab.1 IntroductionThe nefarious Dr. Evil has planted a slew of “binary bombs” on the fish cluster. A binary bomb is a programthat consists of a sequence of phases. Each phase expects you to type a particular string on stdin. If youtype the correct string, then the phase is defused and the bomb proceeds to the next phase. Otherwise, thebomb explodes by printing "BOOM!" and then terminating. The bomb is defused when every phase hasbeen defused.There are too many bombs for us to deal with, so we are giving each group 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, andwelcome to the bomb squad!Step 1: Get Your BombEach group of students will attempt to defuse their own personalized bomb. Each bomb is an Intel Red HatLinux 5.2 binary executable file that has been compiled from a C program. Each bomb consists of 6 phases.To obtain your group’s bomb, follow these steps carefully and in order:1. IMPORTANT: First, each group member must login to an Andrew machine (not a fish machine)and type the following:% aklog cs.cmu.edu2. Next, one of the group members should send mail to [email protected] with the string “Lab 2bomb request” in the subject header, and the names and Andrew ID’s of the group members in themessage body. Don’t do this until each group member has completed Step 1.Dave will then send you the location of your bomb and your group’s ID number.1Step 2: Defuse Your BombOnce you have received your bomb from Dave, copy it to your personal directory. Your job is to defuse thebomb.You can use many tools to help you with this; please look at the hints section for some tips and ideas. Thebest 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 thefinal score for the lab. So there are consequences to exploding the bomb. You must be careful!Each phase is worth 10 points, for a total of 60 points. (Remember that each lab is worth about 8% of yourfinal grade.)The phases get progressively harder to defuse, but the expertise you gain as you move from phase to phaseshould offset this difficulty. However, the last phase will challenge even the best students, so please don’twait until the last minute to start.To avoid accidently detonating the bomb, you will need to learn how to single-step through the assemblycode and how to set breakpoints. You will also need to learn how to inspect both the registers and thememory states. One of the nice side-effects of doing the lab is that you will get very good at using adebugger. This is a crucial skill that will pay big dividends the rest of your career.LogisticsAs usual, you may work in a group of up to 2 people.Any clarifications and revisions to the assignment will be posted on the class bboard and Web page.You should do the assignment on the class machines (the fish cluster). In fact, there is a rumor that Dr. Evilreally is evil, and the bomb will always blow up if run elsewhere. There are several other tamper proofingdevices built into the bomb as well.Hand InThere is no explicit hand-in. The bomb will notify your TA automatically after you have successfullydefused it. You can keep track of how you (and the other groups) are doing by looking at:http://www.cs.cmu.edu/afs/cs/academic/class/15213-f00/www/bombstat.htmlThis web page will be updated every 5 minutes or so to show the progress of each group.Hints (Please read this!)There are many ways of defusing your bomb. You can examine it in great detail without ever running theprogram, and figure out exactly what it does. This is a useful technique, but it not always easy to do. You2can also run it under a debugger, watch what it does step by step, and use this information to defuse it. Thisis 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 everypossible key to find the right one. But this is no good for several reasons:You lose 1/4 point (up to a max of 10 points) every time you guess incorrectly and the bomb explodes.Every time you guess wrong, a message is sent to the staff. You could very quickly saturate thenetwork 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. Evenif you made the (wrong) assumptions that they all are less than 80 characters long and only containletters, then you will have guesses for each phase. This will take a very long time to run, andyou 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 wrongwhen they don’t work. Here is a list of some of the tools you may find useful in analyzing your bomb, andhints on how to use them.gdbThe GNU debugger, this is a command line debugger tool available on virtually every platform. Youcan trace through a program line by line, examine memory and registers, look at both the source codeand 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 learnhow to set breakpoints.– The Documents link on the course Web page has a very handy single-page gdb summary.– For other documentation, type “help” at the gdb command prompt, or type “man gdb”, or “infogdb” at a Unix prompt. Some people also like to run gdb under gdb-mode in emacs.objdump -tThis will print out the bomb’s symbol table. The symbol table includes the names of all functions andglobal variables in the bomb, the names of all the functions the bomb calls, and their addresses. Youmay learn something by looking at the function names!objdump -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.stringsThis utility will display the printable strings in your bomb.Looking for a
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