辅导案例-CSC 322

  • May 15, 2020

CSC 322 Systems Programming Fall 2019Lab Assignment L2: Defusing a Binary BombDue: Thursday October 17Note: NOT allowed to submit late1 IntroductionThe nefarious Dr. Evil has planted a slew of “binary bombs” on our class machines. A binary bomb is aprogram 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 phasehas been defused.There are too many bombs for us to deal with, so we are giving each student a bomb to defuse. Yourmission, 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!Step 1: Get Your BombYou can obtain your bomb by pointing your Web browser at:http://pi.cs.oswego.edu:11322/This will display a binary bomb request form for you to fill in. Enter your user name and email address andhit the Submit button. The server will build your bomb and return it to your browser in a tar file calledbombk.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 thecommand: tar -xvf bombk.tar. This will create a directory called ./bombk with the followingfiles:• README: Identifies the bomb and its owners.• bomb: The executable binary bomb.• bomb.c: Source file with the bomb’s main routine and a friendly greeting from Dr. Evil.1• writeup.{pdf,ps}: The lab writeup.If for some reason you request multiple bombs, this is not a problem. Choose one bomb to work on anddelete the rest.Step 2: Defuse Your BombYour job for this lab is to defuse your bomb.You must do the assignment on one of the class machines. In fact, there is a rumor that Dr. Evil really isevil, and the bomb will always blow up if run elsewhere. There are several other tamper-proofing devicesbuilt 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 andideas. 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, and you lose 1/2 point (up to a max of 20points) 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 worth15 points each. So 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 phaseshould offset this difficulty. However, the last phase will challenge even the best students, so please don’twait until the last minute to start.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 overto stdin. In a moment of weakness, Dr. Evil added this feature so you don’t have to keep retyping thesolutions to phases you have already defused.To avoid accidentally 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.2 LogisticsThis is an individual project. All handins are electronic. Clarifications and corrections will be posted on thecourse message board.23 HandinThere is no explicit handin. The bomb will notify your instructor automatically about your progress as youwork on it. You can keep track of how you are doing by looking at the class scoreboard at:http://pi.cs.oswego.edu:11322/scoreboardThis web page is updated continuously to show the progress for each bomb. The maximum point is 70.This lab does NOT accept the late submission. The web server will be closed after the due.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. Youcan 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 1/2 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 saturatethe 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. Evenif you made the (incorrect) assumptions that they all are less than 80 characters long and only containletters, then you will have 2680 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.The CS:APP web sitehttp://csapp.cs.cmu.edu/public/students.html3has a very handy single-page gdb summary that you can print out and use as a reference. Here aresome other 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.– 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 inemacs.• 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.Although objdump -d gives you a lot of information, it doesn’t tell you the whole story. Calls tosystem-level functions are displayed in a cryptic form. For example, a call to sscanf might appearas:8048c36: e8 99 fc ff ff call 80488d4 To determine that the call was to sscanf, you would need to disassemble within gdb.• stringsThis utility will display the printable strings in your bomb.Looking 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 youmore than you ever wanted to know about the GNU Assembler. Also, the web may also be a treasure troveof information. If you get stumped, feel free to ask your instructor for help.4

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