Assigned | Friday, January 27, 2017 |
---|---|
Due Date | Monday, February 13, 2017 at 11:59pm (23:59) |
Files | Available
at https://courses.cs.washington.edu/courses/cse410/17wi/labs/lab2/<username>/lab2-bomb.tar
(Note: substitute your UWNetID for <username> )
|
Video | You may find this video helpful for getting started with the lab. |
Submissions | Submit your completed defuser.txt
file using the course's Assignment Drop Box.
Make sure the file format of your file is correct, so that it can be used as submitted on a CSE VM to defuse your bombs. Details below.
|
gdb
debugger to step
through assembly code and other tools such
as objdump
.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!
Everyone gets a unique bomb to difuse!
Use wget
to download from the command line.
In a terminal, run the following command, making sure to
substitute your UWNetID for <username>
:
wget https://courses.cs.washington.edu/courses/cse410/17wi/labs/lab2/<username>/lab2-bomb.tar
Your binary bomb (lab2-bomb.tar
) has already been copied into your home directory!
Once you have a local copy, extract its contents using the following command:
tar xf lab2-bomb.tar
Note the extra v
flag used this time!
This will create a directory called bomb$NUM (where $NUM is the ID of your bomb) with the following files:
bomb
: The executable binary bombbomb.c
: Source file with the bomb's main routinedefuser.txt
: File in which you write your defusing solutionThe bombs were constructed specifically for 64-bit machines. You should do this assignment on a 64-bit CSE Linux VM or on klaatu. 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 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 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 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.
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 (p.177) 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.
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
.
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 by the additional arguments:
int i;
printf("Enter a number: ");
scanf("%d", &i);
char *mystring = ā123, 456ā
int a, b;
sscanf(mystring, ā%d, %dā, &a, &b);
mystring
, is parsed according to the format string, "%d, %d"
.a = 123
and b = 456
.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.
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
.
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 emacsobjdump -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 assembly 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. If you get stumped, use the course's discussion
board.
Hints
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.
All you submit is your defuser.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, otherwise
our script is likely to conclude you defused zero bombs:
1. This is my answer for phase 1
).