Machine Problem: Binary Bomb Solution

   Objectives

1. Learn to use GDB and other command line debugging utilities
2. Practice debugging and tracing binary executables
3. Learn to read and understand x86 assembly, including:
    - addressing modes
    - procedure call conventions
    - control structure implementation


   Introduction

The nefarious *Dr. Evil* has planted a slew of "binary bombs" on our
class machines. A binary bomb is a program that consists of a sequence
of phases. Each phase expects you to type a particular string on
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 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!


   Step 1: Get Your Bomb

You should now have a directory named "mp-bomb-USER". `cd` into it and obtain
your bomb by running the command `./getbomb.sh`.

This command will download a bomb from a remote server and create a directory
named "bomb*N*", where *N* is the unique number of your bomb. In that directory
you'll find 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.

If for some reason you request multiple bombs, this is not a problem.
Choose one bomb to work on and delete the rest.


   Step 2: Defuse Your Bomb

Your job for this lab is to defuse your bomb.

For this assignment you **must** do the assignment on `fourier.cs.iit.edu`. In
fact, there is a rumor 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. The best way is to use gdb to step
through the disassembled binary.

Each time your bomb explodes it notifies the bomblab server, and you
lose 1/4 point (up to a max of 10 points) 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 6 points each. Phases 5 and 6 are a
little more difficult, so they are worth 8 points each. So the maximum
score you can get is 40 points.

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 `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.

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 the rest of your career (and this class).


   Submission

There is no explicit handin for this lab. You do not need to commit or push any
files to GitHub. 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 looking
at the class scoreboard at <http://moss.cs.iit.edu:35101/scoreboard>. This web
page is updated continuously to show the progress for each bomb.


   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.

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 1/4 point (up to a max of 10 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.

-   `gdb `

    The GNU debugger, this 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:APP web site has a [handy `gdb`
    summary](http://csapp.cs.cmu.edu/2e/docs/gdbnotes-ia32.txt) that you can use
    as a reference and guide.  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.

    -   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` in `emacs`.

-   `objdump -t`

    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`

    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 are
    displayed in a cryptic form. 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 `

    This 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 you more than you ever wanted
to know about the GNU Assembler. If you get stumped, feel free to ask for help.