CPSC 213 – Assignment 7 Stacks and Polymorphism


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The first part of the assignment is about the stack. You will mount a buffer-overflow, stacksmash attack on a SM213 program.
The second part of the assignment is about dynamic flow control. You’ll also do a bit more with
dynamic control flow next week. First, you will implement the last two instructions in the
SM213 ISA. By now this should be quite straight forward.
Then, you will extend a C program that uses java-style polymorphism, but implemented
explicitly using C function pointers as we have discussed in class. In doing so, it will be helpful
to review the code we covered in class, which is posted to Piazza and included with this
assignment as Snippet A.
Part 1: Stack Smash Attack
Questions 1 and 2: The Stack Smash attack [45%]
For the next two questions, refer to the file copy.c found in www.ugrad.cs.ubc.ca/~cs213/cur/
1. [5%] Using copy.c as a guide, write a simple SM213 assembly-language program that
copies a null-terminated array of integers (use Snippets 8 or 9 from Assignment 6 as a
guide). Call this program copy.s.
In copy.c, which is reproduced below, the input array is stored in a global variable
named src and the destination array is in a local variable (i.e., stored on the stack). Your
assembly code must do the same.
As in copy.c, you will need two procedures: one that copies the array and one that
initializes the stack pointer and calls the copy procedure. Ensure that the copy procedure
saves r6 (the return address) on the stack in its prologue and restores it from the stack in
its epilogue, as shown in class.
Note that this code contains a buffer-overflow bug. That is intentional. Be sure your
assembly code has this bug so that you will be able to attack it in Question 2. Another
thing you’ll want to do is to keep the value of i in a register in the body of the loop. If
you were to read/write it from/to the stack on every iteration, you’ll find that the buffer
overflow will overwrite the value of i and thus change the way the attack string is written
to the stack.
2. [40%] Modify copy.s to devise a buffer-overflow attack on this program. The attack
should set the value of every register to -1 and then halt.
You are stuck with a similar set of restrictions that a real attacker confronts. You may not
modify the program you have just written in any way other than to change its input
(i.e., src). Change src to make it bigger to contain virus program and other values as
needed so that copy executes the virus program when it returns, instead of actually
returning to main.
You must specify the attack string (the value of src) using a sequence of .long
directives. Recall that each .long specifies the value of 4 bytes of memory. The string
will contain the virus program as machine instructions, which are either 2 bytes or 6 bytes.
You will thus need to compact multiple instructions into a single .long and possibly
also split a 6-byte instruction across two .long’s.
Remember that the only change you are permitted to make to the program you wrote for
Question 1 is to specify a different value for src.
Run your attack in the simulator to be sure that it works.
Part 2: Polymorphism
Question 3: Implement and Test Double-Indirect Jumps [5%]
There are two remaining instructions to implement in the simulator, described below. Implement
Instruction Assembly Format Semantics
dbl ind jmp b+d j *o(rt) dtpp pc ← m[r[t] + (o == pp*4)]
dbl ind jmp indx j *(rb,ri,4) ebi- pc ← m[r[b] + r[i]*4]
Then, use the simulator to examine the snippet SA-dynamic-call that you will find in this
week’s code file at www.ugrad.cs.ubc.ca/~cs213/cur/assignments/a7/code.zip. There you will
also find the CPU.java solution from Assignment 6, which you can use as a starting point for
this question, if you like. Replace the “TODO” comments with your code.
Question 4: Modelling Polymorphism in C [50%]
In www.ugrad.cs.ubc.ca/~cs213/cur/assignments/a7/code.zip you will find two directories:
polytree-java and polytree-c that implement a binary search tree.
Carefully examine the Java code; compile and run it. You will see that it consists of a base class
called Node that implements the tree and two subclasses called IntegerNode and
StringNode use Node to stored sorted lists of integers and strings respectively. Look at the
main method to figure out how to call the PolyTree main from the command line with lists
of either integers or strings.
Now examine the C code. It has complete implementations of Node and StringNode and
templates for the rest of the work you will do. Examine this code and the Makefile carefully.
Compile it by typing make and run it. You will notice that the implementation consists of both c
(source code) and h (header / interface) files. One new thing you’ll see in the header files is a
variable declared with the extern keyword added to its type; this is how you allow multiple
files to reference a global variable declared in one of them.
Now you are going to extend this code in various ways. Take this step by step as described
below. Get each step working before moving to the next one.
1. Create a new class called ReverseStringNode that extends StringNode by
overriding only the compareTo method to sort strings in reverse. All of
ReverseStringNode’s functions except for compareTo must come from (i.e.,
delegate to) its super classes: StringNode and Node. Place your solution in the files
reversestringnode.c and reversestringnode.h and modify polytree.c
in the TODO location to use the ReverseStringNode class when the “r” option is
specified on the command line.
2. Create a new class called LoggingStringNode that extends StringNode by
overriding the insert method to print “insert x” each time insert is called
(where “x” is value of the node begin inserted without the quotes). Note that this means
that a node may print multiple times in the process of being inserted since insert is
For example, if the tree contains a single node with value “D”, inserting a second node
with value “B” would print “B” once: the call from polytree and no recursive call.
Inserting a third node with the value “A” or “C” would print the value twice (one
recursive call), but inserting “E” would print “E” only once.
Your solution should call Node’s implementation of insert. In Java you would do this
by calling super.insert(node). Note that this is actually a static call (i.e., not
polymorphic). The Java compiler implements this call by locating the nearest ancestor
class that implements the target method, which in this case will be Node_insert. So,
your solution can just call this method directly.
Place your solution in to files provided and update polytree.c to use this class with
the “l” option.
3. Implement the class IntegerNode ensuring that every instance method in the Java
version of the class is implemented using the C-based polymorphic structure so that, for
example, another class could someday extend IntegerNode and override or inherit any
of these methods.
4. If you examined polytree.c very carefully will you have noticed that it has a memory
leak. It never frees the tree nodes that it allocates inside of the for loop. Fix this problem
by adding a delete method to the Node class that calls free on a node and all of its
descendants in the tree. Then call this method at the end of polytree main. Your code
must be free of all memory leaks (check with valgrind).
What to Hand In
Use the handin program.
The assignment directory is ~/cs213/a7, it should contain the following plain-text files.
5. PARTNER.txt containing your partner’s CS login id and nothing else (i.e., the 4- or 5-
digit id in the form a0z1). Your partner should not submit anything.
6. For Question 1 and 2: copy.s.
7. For Question 3: CPU.java.
8. For Question 4: all of the c and h files in polytree-c, modified appropriately. DO
NOT PLACE THEM IN A SUBDIRECTORY as they are in the code pack. Please just
place them directly in your handin directory. You handin directory should have no
subdirectories (i.e., there is no polytree-c).