辅导案例-COMP202

  • May 15, 2020

Bomb Lab – Assignment 2 COMP202 Revised 27 August 2019 Len Hamey, [email protected] Adapted from Bryant and O’Hallaron’s Bomb Lab This lab is the second assignment in COMP202. Commences: See ilearn for notification when it is available; no later than start of the mid-semester break. Progress: Weeks 9, 10, 11, 12. For specific dates use: $ lab -d Due: Week 13. For specific due date use: $ lab -d Value: 20% (16% for task, 4% weekly progress) Introduction The nefarious Dr. Evil has planted a slew of “binary bombs” on our servers. 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!!!” (or something similar) 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, 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!  BREAKING NEWS!! Dr. Evil has evidently become aware of Internet sites that provide help for defusing his earlier binary bombs. The most recent binary bombs are very different. Specific Learning Outcomes The specific learning outcomes of this lab exercise include the following. You will encounter these learning outcomes at various phases of the lab. • Using debugging tools is a major learning outcome. You cannot achieve anything worthwhile in this lab without using debugging tools. • Interpret data values in memory and registers. • Experience with ASCII character conversion. • Interpret assembly code corresponding to numeric and logical operators. • Interpret addressing modes including immediate, register, and register indirect modes. • Recognise and interpret control constructs expressed in assembly including if-else and loops. • Recognise and interpret procedure calls, parameters, and returns expressed in assembly. • Interpret information stored on the stack. • Interpret simple data structures in memory. • Experience with buffer overflow exploit. Step 1: Get Your Bomb You can obtain your bomb by using the lab command to get lab 2.1. $ lab –g 2.1 A binary bomb has already been placed in the server for you, and the lab command will download it in a tar file. Save the tar file to a protected directory in which you plan to do your work. When you extract the tar file, it will create a directory called bombk where k is your unique bomb ID. The directory contains the following files: • README: Identifies the bomb and its owner. • bomb: The executable binary bomb. • bomb.c: Source file with the bomb’s main routine and a friendly greeting from Dr. Evil. We have verified that this source code is genuine. This file contains information that may be useful, although how much can you trust someone with the name `Dr. Evil’? In particular, we do not adhere to his licensing terms so you should feel free to use all the tools he mentions, and any other tools you like, to disassemble, dump and debug the bomb. If for some reason you request the bomb again, this is not a problem. You will receive the same bomb. Step 2: Defuse Your Bomb Your job for this lab is to defuse your bomb. You must do the assignment on one of the machines iceberg or ash. In fact, there is a rumour 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 we hear. You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas, and there are additional documents on iLearn providing further assistance. The best way to defuse the bomb is to use your favourite debugger to step through the disassembled binary. We suggest gdb. We have provided a separate document giving you some help to defuse your bomb with gdb. Each time your bomb explodes it notifies the bomblab server, and you lose 0.1 points (up to 2.0 marks for each bomb) in the final score for the lab. There is only one way to recover lost points – to start again with a new bomb which we will offer to you later in the lab. So there are consequences to exploding the bomb. You must be careful! There are six phases. The first four phases are worth 3 marks each and the last two phases are worth 2 marks each. There are also four progress marks. The maximum score you can get is 20 marks. The minimum score is -2.0 if you explode the bombs 20 times each and do not solve any phases. Negative marks in this assignment will count as zero in your final grade for the unit. 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 even the best students, 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 EOF (end of file), and then switch over to reading from 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 can store your earlier phase solutions in the text file. You can also use this feature when running your bomb in the debugger gdb – simply type the file name as the parameter of gdb’s run command. 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 for the rest of your career. Logistics This is an individual project. All submissions are electronic and automatic – you do not need to use the lab command to submit anything because your bomb notifies the server of your successes and failures. Clarifications and corrections will be posted in iLearn. Achievement Marks Achievement marks in this lab are earned for defusing stages of the bomb. The first four phases are worth 3.0 marks each and the last two phases are worth 2.0 marks each. Phases 1, 2, 3, 4 and 5 have been heavily influenced by Scott Evil. As you know, Scott is Dr Evil’s son, and he has a good heart. Out of kindness to the world, Scott introduced a simpler solution to each of these bomb phases than Dr. Evil’s solution. Scott Evil’s solution successfully defuses the bomb, but is only worth 2.0 achievement marks in phases 1 through 4, and 1.5 in phase 5. To obtain the full marks for each phase you need to dig deeper into the assembly code. When your phase is defused using Dr. Evil’s solution, the bomb will print a congratulatory message, as you can see by examining bomb.c. Strategy It is not possible to attempt to defuse a bomb phase until the preceding phases have been defused, so you have to work on the phases in sequence. However, once you have found Scott Evil’s defusion string for a particular phase, then you can choose whether to work on the next phase or to continue working on the current phase in order to achieve Dr. Evil’s solution. You might find it helpful to move on to the next phase and then return later, when you are more experienced, to try to find Dr. Evil’s solution to the earlier phase.  Once you have a solution to a phase you should enter it into a text file that you use when you run the bomb, as explained above. You can name the text file as a parameter when you run the bomb inside the debugger. This will move through the phases that you have already solved, leaving you ready to work on your current phase. Submission There is nothing for you to submit explicitly, however you should keep any written notes and a copy of your solution file just in case there is a need to verify your results. 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 using the lab command to examine your marks for lab 2: $ lab –m 2 The report is reasonably detailed but may take some time because it is generated automatically. The bomb lab will close automatically on the due date. You can use the lab command for free extensions. Progress Marks Automatic progress marks are awarded for defusing each of the first four bomb phases. Scott Evil’s solution is sufficient to achieve the progress mark. The progress mark schedule is lenient, but you should aim to be ahead of schedule, because the last phases are considerably more difficult, and also more interesting! The automatic progress mark schedule is shown by the command $ lab -d The final deadline for this lab is in week 13, and is shown by the command $ lab -d If you wish to claim a free extension for a progress mark, you must claim the extension before you defuse the relevant bomb stage. Once the stage is defused, the server will register the progress mark that is due in the future, and it will not be possible to extend the progress mark that is overdue. If you affected by significant disruption and request an extension of a progress mark due date, please also email the unit convenor. Extensions of progress marks can be tricky but we will do our best to give you an appropriate extension if the disruption justifies one. After the Lab Closes After the lab closes, it will still be possible to run your bomb. Every defusion and explosion that you cause after the lab closes will still be recorded, but it will not affect your mark unless you are given an extension. If you claim a free extension, or receive an extension due to special consideration, then all your bomb activity up to the new closing date will be counted towards your mark. For this reason we recommend that you be as careful with your bomb after the closing date as you were beforehand. If you are affected by significant disruption and request an extension of the closing date, please also email the unit convenor. You can continue to work on the lab as you are able, even before an extension is granted. Every defusion and explosion will be recorded by the automarker. Your marking report will not show your new work until an extension is granted, so you should keep your own notes. If you wish to use a free extension for the closing date, you can minimise your risk by working on the bomb lab after closing without claiming the extension. This only applies to an extension of the closing date. You will need to keep your own notes of your defusions and explosions. If you achieve a worthwhile improvement, then you can claim the free extension and immediately be rewarded with the marks for the new work. However, be warned: Explosions take away marks, so if you lose more through explosions than you gain through improved defusion results, your total mark could possibly go down when you claim a free extension. Remember that it is only possible to cancel future free extension days. It is also not possible to claim free extensions up to a time in the past – the end of your free extension must always be in the future when you are claiming it. 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.  Using a debugger you can step through the bomb. If you find a point at which it wants to explode the bomb, you can look at registers and memory and determine what is needed to avoid exploding the bomb at that point. Then, tracing through the program code, you work out what input string you need to provide in order to not explode the bomb at that point. 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 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 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. For more hints and help, see the Lab Notes section on iLearn. Gdb The document “Bomb Defusing with gdb” provides a lot more information about how to use gdb to defuse your bomb. Please read it! Here is a very brief introduction. 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:APP3e web site http://csapp.cs.cmu.edu/3e/students.html has 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. Even better, learn about .gdbinit to protect yourself against typing mistakes. • iLearn has lab notes that provide more information about gdb and helpful hints for using it. • For online documentation, type “help” at the gdb command prompt, or type “man gdb”, or “info gdb” at a Unix prompt. objdump -t This Unix command 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 Use this Unix command 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 something like: 8048c36: e8 99 fc ff ff call 80488d4 To determine that the call was to sscanf, you would need to disassemble within gdb. Gdb recognises system calls, although the names appear somewhat strange in gdb. strings This utility will display the printable strings in your bomb. Documentation Looking for a particular tool? How about documentation? iLearn contains some useful information. Don’t forget, the commands apropos, man, and info are your friends. In particular, man ascii might come in useful and 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 your instructor for help.

LATEST POSTS
MOST POPULAR

ezAce多年来为广大留学生提供定制写作、留学文书定制、语法润色以及网课代修等服务,超过200位指导老师为您提供24小时不间断地服务。