CSE 351: The Hardware/Software Interface

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Lab 2: Disassembling and Defusing a Binary Bomb

Assigned Friday, April 13 [! :)]
Due Date Wednesday, April 25
Files Available at https://courses.cs.washington.edu/courses/cse351/18sp/uwnetid/<username>/lab2-bomb.tar
Note: substitute your UWNetID for <username> and use a web browser (not wget since you will need to authenticate using your browser).
Video You may find this video (or with captions) helpful for getting started with the lab.
Submissions Submit defuser.txt and lab2reflect.txt via the Canvas assignments page (go to the Labs section, not the Homeworks section). Be sure to follow the detailed formatting rules described below.

Learning Objectives


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. 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!

Code for this lab

Everyone gets a different bomb to diffuse! Substitute <username> in the URL below with your UWNetID in order to find yours.

Go to https://courses.cs.washington.edu/courses/cse351/18sp/uwnetid/<username>/lab2-bomb.tar with your web browser to download lab2-bomb.tar. You need to use your browser because you will authenticate with your UW NetID.

If you are using attu, first download the tar file to your local machine and then use scp to move the file to attu (use the scp command on terminal for Mac and Linux users and Windows with bash users; for a user interface use WinSCP on Windows and Cyberduck on Mac). For help on scp, visit https://courses.cs.washington.edu/courses/cse391/16au/handouts/MovingAndEditingFiles_12sp.pdf.

Running tar xvf lab2-bomb.tar from the terminal in the directory where lab2-bomb.tar is located will extract the lab files to a directory called bomb$NUM (where $NUM is the ID of your bomb) with the following files:


You should do this assignment on a 64-bit CSE Linux VM or a CSE lab Linux machine or on attu. 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.

Your job is to find to correct strings to defuse the bomb. Look at the Tools section for ideas and tools to use. Two of the best ways are to (a) use a debugger to step through the disassembled binary and (b) print out the disassembled code and step through it by hand.

The bomb has 5 regular phases. 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.)

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.

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.


There are many online resources that will help you understand any assembly instructions you may encounter. In particular, the instruction references 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.

Useful for this Lab

Important Note: The instruction format used in these 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 the 3e) 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.

Not Directly Useful, but Good Brainfood Nonetheless

x86-64 Calling Conventions

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.

Using sscanf

It will be helpful to first familiarize yourself with scanf ("scan format"), which reads in data from stdin (the keyboard) stores it according to the parameter format into the locations pointed to by the additional arguments:

int i;
printf("Enter a number: ");
scanf("%d", &i);
Lab 2 uses sscanf ("string scan format"), which is similiar to scanf but rather than read in data from stdin it parses a string that is provided as an argument:
char *mystring = "123, 456"
int a, b;
sscanf(mystring, "%d, %d", &a, &b);

Documentation for sscanf, scanf, and printf

Tools (Read This!!)

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.

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 the number of possibilities is so large that you won't be able to try them all in time.

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.

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. If you get stumped, use the course's discussion board.


If you're still having trouble figuring out what your bomb is doing, 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.

Lab 2 Reflection

REMINDER: You will need to be on the CSE VM or attu in order to get addresses that are consistent with our solutions.

Start with a fresh copy of lab0.c and examine part2() using the following commands:

$ wget https://courses.cs.washington.edu/courses/cse351/18sp/labs/lab0.c
$ gcc -g -std=c99 -o lab0 lab0.c
$ gdb lab0
(gdb) layout split
(gdb) break fillArray
(gdb) break part2
(gdb) run 2

Now answer the following questions:

  1. What address is the variable value stored at in memory?  [0.5 pt]
  2. What is the relative address (i.e. how many bytes forwards or backwards) of the variable array compared to the variable value?  [0.5 pt]
  3. Consider the lea instruction at address 0x400842. Replace it with another lea instruction that has the same effect at this position in this program but does not use %rbp.  [1 pt]
  4. Give the lowest and highest addresses of the instructions that perform the i * 3 + 2 calculation within the loop (do not include the assert and do not include any instructions that store the result somewhere).  [1 pt]

You will find the following GDB commands useful: nexti, finish, print, and refresh.

Submission Instructions (!)

All you submit is your defuser.txt and lab2reflect.txt files. So that our grading scripts can use your file as-is to defuse your bombs, please make sure it obeys these formatting rules, else our script is likely to conclude you defused zero bombs: