CSCI 237

Computer Organization

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Lab 2: Bomb Lab

Overview

The purpose of this assignment is twofold: first, you will gain basic familiarity with x86-64 assembly instructions and how they are used to implement comparison, loops, switch statements, recursion, pointers, and arrays; and second, you will gain experience using the gdb debugger and other tools to step through assembly code.

Note: You must ssh into lohani or limia to complete this lab.

Instructions

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 (standard input). 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. Each time the bomb explodes you will lose 0.25 points, so be careful! The bomb is defused when every phase has been defused.

There are too many bombs for us to deal with, so we are giving everyone 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!

The bombs were constructed specifically for 64-bit machines. You should do this assignment on a 64-bit lab Linux machine. Be sure to test your solution on one of those platforms before submitting it, to make sure it works when we grade it! In fact, there is a rumor that Dr. Evil has ensured 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.

Everyone gets a unique bomb to defuse. You can obtain your bomb by pointing your Web browser here. 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 bomb$NUM.tar, where $NUM is the unique number of your bomb. If your browser tries to block the download, make sure you allow it to proceed.

If you are not directly logged into a lab machine, you will have to copy your file to a lab machine. You can do this using the following command:

scp bomb$NUM.tar 25abc1@lohani.cs.williams.edu:

Insert your bomb number for $NUM and your username for 25abc1. You can use either lohani or limia as the destination machine. This command will copy the tarball to your home directory on the Linux machines. Next, you should ssh to either lohani or limia and untar your bomb using the following command:

tar xvf bomb$NUM.tar

These commands will create a directory called bomb$NUM (where $NUM is the ID of your bomb) with the following files:

• bomb: The executable binary bomb
• bomb.c: Source file with the bomb's main routine
• README: A mostly empty file that just shows your name

Your job is to defuse the bomb. You can use many tools to help you with this; please look at the tools section for some tips and ideas. Two of the best ways are to (a) use a debugger to step through the disassembled binary and (b) print out the dissassembled code and step through it by hand.

The bomb has 5 regular phases. The first four are worth 10 points, and the fifth phase is worth 15, for a total of 55 points. The 6th phase is extra credit, and rumor has it that a secret 7th phase exists. If it does and you can find and defuse it, you will receive additional extra credit points. The phases get progressively harder to defuse, but the expertise you gain as you move from phase to phase should offset this difficulty. Nonetheless, the latter phases are not easy, so please don't wait until the last minute to start. (If you're stumped, check the hints section at the end of this document, and come see me and/or the TAs.)

Here are some hints for what to think about at each stage: (1) comparison, (2) loops, (3) switch statements, (4) recursion, (5) pointers and arrays, (6) sorting linked lists.

The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,

./bomb defuser.txt

then it will read the input lines from defuser.txt until it reaches EOF (end of file), and then switch over to stdin (standard input from the terminal). 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, instead you can put them in defuser.txt.

When formatting your defuser.txt file, please remember to:

• Put your answer for each phase in one line. Your answer for phase 1 should be in the first line, answer for phase 2 on the second line, and so on.
• Do not put your name or other information at the top of the file. Again, you want the first line in the file to be your answer for phase 1.
• Do not add numbering or other “comments” for your answers (e.g., 1. This is my answer for phase 1).
• Make sure all your answers, including the last one, have a newline character afterwards, so even your last-phase answer is a complete line with a newline. This last newline is important for our grading script even though you will not notice the difference in your own testing.

To avoid accidentally detonating the bomb, you will need to learn how to single-step through the assembly code in gdb 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.

Resources

There are a number of online resources that will help you understand any assembly instructions you may encounter while examining the bomb. In particular, the programming manuals for x86-64 processors distributed by Intel and AMD are exceptionally valuable. They both describe the same ISA, but sometimes one may be easier to understand than the other. In addition, you can also refer to your textbook.

• Intel Instruction Reference
• AMD Instruction Reference
• x86-64 Cheat Sheet (from Brown)

Important Note: The instruction format used in the Intel and AMD manuals is known as “Intel format”. This format is very different than the format used in our text, in lecture slides, and in what is produced by gcc, objdump and other tools (which is known as “AT&T format”. You can read more about these differences in our textbook (on p. 177 of your book) or on Wikipedia. The biggest difference is that the order of operands is SWITCHED. This also serves as a warning that you may see both formats come up in web searches.

Speaking of web searches, under no circumstances are you to search for anything involving the words "binary bomb" on the Internet. This would obviously be a violation of the Honor Code. There are many incorrect solutions out there, and they would truly be detrimental in the long run.

Tools and Hints (Read This!!)

The x86-64 ISA passes the first six arguments to a function in registers. Registers are used in the following order: rdi, rsi, rdx, rcx, r8, r9. The return value for functions is passed in rax.

There are many ways of defusing your bomb. You can print out the assembly and 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. Both are useful skills to develop.

Please do not use brute force! You could write a program that will try every possible key to find the right one, but the number of possibilities is so large that you won't be able to try them all in time, and you'll lose points each time you blow up the bomb.

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.

• gdb: 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. 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.
  • The CS:APP Student Site has a very handy gdb summary (there is also a more extensive tutorial, and this video).
  The Labs page also include a few pointers to useful GDB cheat sheets.
  • For other 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 -t bomb: This 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 bomb: 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 may look cryptic. 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.
• strings -t x bomb: This utility will display the printable strings in your bomb and their offset within the bomb.

Looking for a particular tool? How about documentation? Don't forget, the commands apropos and man are your friends. In particular, man ascii is more useful than you'd think.

Scoreboard

Track the progress of your classmates. Check out the scoreboard for more info.

Final scores available here.