CS31 Lab 5: Binary Maze

(or getting out of Parrish Hall in a CS31 kind-of way)

Checkpoint: due by 11:59pm, Wed Oct. 30
Complete Solution (floors 1, 2, 3, 4 (5 extra credit)): due before 11:59pm Wednesday Nov. 06

This lab should be done with your partner.

Lab 5 Goals:

Note: I am assigning this lab now and introducing material you need to solve it this week in lab. However, if you don't start really working on lab 5 until after the midterm, that is fine; you have enough time after the midterm to start on this lab and complete it (the checkpoint isn't due until week 8). I want you to focus first on studying for the midterm.

Lab 5 Intro
You have been sleep walking again, and you wake up on the roof of Parrish Hall. You need to find your way through its maze of floors and out of the building in time for your first class. The problem is that due to construction, there is no stairwell that connects more than two floors. As a result, you need to travel along each floor to find the next open stairwell down to the next floor below. However, there are people or things along your path that can trip you up and impede your progress, forcing you to run back to the roof to try again.

In this assignment, you and your partner are going to receive a binary maze program. Your maze has 5 phases, one for each floor of Parrish Hall (from the roof to out the door). Each floor's phase is a binary puzzle that needs to be solved to move on to the next floor. To solve a puzzle you need to enter a correct phrase on stdin (you can also have your maze read phrases from a file given as a command line argument).

Your goal is to solve all phases/floors of your maze, limiting the number of times you trip-up along the way and have to start all over.

Your maze will automatically notify me every time you trip-up and whenever you have solved a puzzle on a particular floor. In addition, your maze will only run on one of the CS lab machines.

You will submit your lab5 solution in two parts:

Part 1: The Checkpoint (due Wed Oct. 30 before 11:59pm):

Part 2: The complete solution (due Wednesday Nov. 6):

Getting Your maze: we will do this together in lab Thurs
  1. First, both you and your partner should run update31, which will create the directory structure for your lab 5 assignment and copy over starting point files:

      $ update31
      $ cd cs31/labs/05
      $ pwd
      /home/your_user_name/cs31/labs/05
      $ ls
      README  maze_ID  maze_how_we_solved_it  soln.txt
    

  2. Next, follow these steps to get your maze:
    1. In firefox on the CS system, one of you or your partner should enter this url:
      http://zucchini.cs.swarthmore.edu:15213/
      
    2. Enter your user name and email address on our system (username@cs.swarthmore.edu), and choose Submit.
    3. Choose to save the mazeX.tar file in the dialog box that pops up.
    4. move the file into your labs/ directory (do not put it in your labs/05 directory as I do not want you to submit your maze binary with your lab 5 solution):
      mv ~/Desktop/mazeX.tar ~/cs31/labs/.
      
    5. cd into your labs directory, and scp the tar file to your partner
      cd ~/cs31/labs
      ls
      scp mazeX.tar partner_username@cs.swarthmore.edu:cs31/labs/.
      # partner enters his/her password, and the mazeX.tar file should be 
      # scp'ed into their cs31/labs/ subdirectory
      
    6. both you and your partner can untar the mazeX.tar file in your private labs/ subdirectory:
      cd ~/cs31/labs
      ls     # you should see your mazeX.tar file
      tar xvf mazeX.tar
      
    7. if you cd into your mazeX subdirectory, you will see 3 files:
      README  maze*  main.c
      
      Do not run the maze program yet!

Checking the status of your maze
From firefox running on a CS lab machine, enter this url (you cannot connect to this url from outside our network):
http://zucchini.cs.swarthmore.edu:15213/scoreboard
You will see your maze in this list. The scoreboard displays how many stages you have solved, how many total trip-ups you have had, and your score so far.

This information is updated about every 40 seconds. Re-load the page to see the latest information.

Details of the maze lab

Introduction

A binary maze is a program that consists of a sequence of puzzles, one corresponding to each floor of Parrish that you need to pass through to get out the door. Each puzzle expects you to type a particular string on stdin. If you type the correct string, then the puzzle is solved and the maze program proceeds to the next floor. Otherwise, the maze program issues a trip-up message and terminates. The maze program is solved when every puzzle on every floor has been solved. You will be penalized for every trip-up that you let fully happen (1/4 a point for each one), so you need to be careful not to trip-up too many times.

You can receive up to 71 points out of 66 total for this lab (5 points are extra credit):

  1. Solving the first 4 puzzles are each worth 10 points (40pts total)
  2. Solving the 5th puzzle is worth 5 points (this is not required, and you must include in your write-up a description of how you solved it to receive all 5 points).
  3. Your write-up of how you solved each floor is worth 4 points for each floor (16 points total). Your write-up should be in the file maze_how_we_solved_it in your cs31/labs/05 subdirectory.
  4. The checkpoint is worth 10 points.
In addition, I will take up to 5 points off total for trip-ups. You will lose a point for each 4th trip-up (number 4, 8, 12, ...), so you get a few for "free". I will not take off more than 5 points total for trip-ups, unless it is clear that you are trying a brute force approach, in which case I will.

Solve Your Maze

You must run your maze on one of the class machines; the maze will always trip-up if run elsewhere. There are several other tamper-proofing devices built into the maze binary as well. In particular, using the gdb set command while trying to solve your maze will cause a trip-up.

To kill your maze executable (to make it exit without tripping-up), type CNTRL-C. This way you can run your maze, solve a puzzle on a floor, and then exit and come back later to try the puzzle on the next floor.

You can use many tools to help you solve your maze. Look at the hints section below for some tips and ideas. The best way is to use ddd or gdb to step through the disassembled binary.

Although the puzzles on each floor get progressively harder to solve, the expertise you gain as you move from floor to floor should offset this difficulty.

Once you have solved the puzzle on a floor, I encourage you to run your maze with a soln.txt file containing the input phrases for the floors you have solved. The format of the file should be one phrase per line, in order of the maze floors. Using an input file will help to prevent you from accidentally tripping up in the maze on a previously solved floor. For example:

./maze soln.txt
will read the input lines from soln.txt until it reaches EOF (end of file), and then switch over to stdin for the remaining input. This feature is also nice so you don't have to keep retyping the solutions to floors you have already solved. The maze ignores blank input lines, both in the file and on stdin.

To avoid accidentally tripping up in the maze, 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 the rest of your career.

The Write-up
Edit the file cs31/labs/05/maze_how_we_solved_it in vim (or emacs) to include a short explanation of how you solved each floor and a short explanation of what the floor is doing.

Describe at a high-level what the original C code is doing for each floor. For example, is it doing some type of numeric computation, string processing, function calls, etc. and describe the specific computation it is doing (i.e. what type of string processing and how is that being used?).

Do not list registers and assembly code for this part, but describe what the puzzle on each floor is doing at a higher-level in terms of C semantics. You do not need to reverse engineer the IA32 code and translate every part of it to equivalent C code. Instead, give a rough idea of equivalent C or pseudo code for the main part of the puzzle on each floor. For example, something like "uses an if-else to choose to do X or Y based on the input value Z" is an appropriate right level of explanation. Something like "moves the value at %ebp-8 into register %eax" is way too low-level.

This part of the lab should not be onerous; you should be able to explain each puzzle in a short paragraph or two (maybe with a few lines of C or pseudo code to help explain). I recommend doing the write-ups for each floor as you solve them.

Excessively verbose, low-level descriptions will be penalized, as will vague descriptions; you want to clearly demonstrate to me that you figured out what that floor is doing by examining the IA32 code for each floor in your maze executable.

If you are unable to solve a floor, you can still receive partial credit for it in the write-up part by telling me what you have determined about that floor.

Hints
There are many ways of solving your maze. 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 solve it. This is probably the fastest way of solving it.

We do make one request, please do not use brute force! You could write a program that will try every possible input string to find the right one. But this is no good for several reasons:

  1. You lose 1/4 point (up to a max of 5 points) every time you guess incorrectly and the maze trips-up.

  2. Every time you guess wrong, a message is sent to the mazelab server. You could very quickly saturate the network with these messages, and cause the system administrators to revoke your computer access.

  3. 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 26^80 guesses for each floor. This will take a very 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 maze, and hints on how to use them. And refer to the week05 and week06 weekly lab pages for information on using gdb and tools for examining binaries: Looking for documentation about a particular tool? The the commands apropos and man will help you find documentation about unix utilities, and in gdb the help command will explain gdb commands:
$ man objdump
(gdb) help ni


What to submit
Whether or not you have solved a maze floor, and how many times you have tripped-up your maze is automatically submitted to the maze server by your maze program.

Checkpoint: For the checkpoint, you need do nothing to "submit" it. I'll check the scoreboard at the due date to verify whether or not you met the checkpoint.

Complete Solution: For the complete solution you will run handin31 to submit the contents of your cs31/labs/05 directory. In particular, make sure that it contains these three files:

  1. maze_how_we_solved_it: the write-up part for your maze.
  2. soln.txt: your maze's solution input file: ./mazeX soln.txt
  3. maze_ID: this file just lists your maze ID
You do not need to submit your binary maze file (I have a copy of it). I'll also check the scoreboard at the due date to determine which floor you solved and how many times you trip-up in your maze.
Submit

Once you are satisfied with your solution, hand it in by typing handin31 at the unix prompt.

Only one of you or your partner should run handin31 to submit your joint solutions If you accidentally both run it, send me email right away letting me know which of the two solutions I should keep and which I should discard (you don't want the grader to just guess which joint solution to grade).

You may run handin31 as many times as you like, and only the most recent submission will be recorded. This is useful if you realize, after handing in some programs, that you'd like to make a few more changes to them.