Assignment 1 (20 pts)

Deadline: January 13, 2021 by 11:59:59 PM

Early Turn-In: January 10, 2021 by 11:59:59 PM (10% bonus)

The goal of this assignment is to become familiar with the setup that will be used for future assignments, such as the use of a virtual machine and the included turn-in script, as well as the basics of working with gdb and writing programs in x86 assembly.

Getting Started

To complete this assignment, you will be provided with a VirtualBox VM pre-populated with the assignment files.

VM Image

In order to match the environment in which your submission will be graded, all work for this assignment must be done on the VirtualBox VM we provide, named pa1box. You can download the VM image here.

The VM is configured with two users: student, with password hacktheplanet; and root, with password hackallthethings. The VM is configured with SSH on port 2222. Please note that SSH is disabled for root, so you can only SSH in as the student user. You can still log in as root using su or by logging into the VM directly.

To SSH into the VM:

ssh -p 2222 student@127.0.0.1

To copy files from your computer to the VM:

scp -P 2222 -r /path/to/files/ student@127.0.0.1:/home/student

To copy files from the VM to your computer:

scp -P 2222 student@127.0.0.1:/path/to/files/ /destination/path

Part 1: Using GDB (10 pts)

Files for this sub-assignment are located in the gdb subdirectory of the student user's home directory in the VM image; that is, /home/student/gdb. SSH into the VM and cd into that directory to begin working on it.

Inside the gdb directory, you'll find fib.c, a C program demonstrating the Fibonacci sequence; a Makefile; and hw1.txt, in which you'll record your responses to the questions below. The first step is to compile fib by running make on the command line.

To run the fib executable in GDB, run gdb fib. I recommend the following workflow in GDB:

1. Starting. Set breakpoints that you can later use for analysis:
   • b foo — break at function foo
   • b *0x08048489 — break at the instruction at address 0x08048489
   • r — run the executable
2. Analyzing. Examine memory, registers, etc; disassemble code; show stack frames, backtrace, etc; and more:
   • disas foo — disassemble function foo
   • i r — view registers
   • where — view stack frames
   • x <loc> — examine memory
   • x $eip — examine current instruction pointer
   • x /10x $esp — examine 10 words at top of stack
   • x /10x buf — examine 10 words in buf
   • x /10i $eip — examine 10 instructions starting at instruction pointer
   • x /10i foo — examine 10 instructions starting at foo
3. Continuing. Continue analysis:
   • c — continue execution until next breakpoint/watchpoint
   • si — step to the next instruction
   • s — step to the next line of source code

Note that this is only a cursory overview of GDB; much more info is available from online resources.

Assignment Instructions

Complete the following exercises and fill out hw1.txt with your answers. Follow the directions in the template, do not delete the square brackets

1. What is the value, in hex, of the ecx register when the function f is called? (2 pts)
2. Which register stores the value of the variable i in the function main? (2 pts)
3. What is the address, in hex, of the function f? (2 pts)
4. What is the name of the 6th instruction of the function f? (2 pts)
5. When f completes after being called from main, to which address in main does control return? Write your answer in hex form. (2 pts)

Submission

Submit hw1.txt to "1a. gdb" on Gradescope. Gradescope will check that it has successfully detected your answers, but it will not give you your grade until the due date.

Part 2: echo in x86 (10 pts)

Files for this sub-assignment are located in the x86 subdirectory of the student user's home directory in the VM image; that is, /home/student/x86. SSH into the VM and cd into that directory to begin working on it.

For this part, you will be implementing a simplified version of the familiar echo command, using raw x86 assembly code. The goal of this assignment is to familiarize you with writing programs directly in x86.

Your echo command must behave as follows:

• When run with a single command line argument (e.g., ./echo Hello):
  1. Prints that argument back to the console's standard output (stdout).
  2. Prints a trailing newline (\n).
  3. Exits with code 0.
• When run with too few command line arguments (e.g., ./echo) or too many (e.g., ./echo Hello World):
  1. Prints exactly the error message This command expects exactly one argument. — followed by a trailing newline (\n) — to the console's standard error (stderr).
  2. Exits with code 1.

Your code should be written in the file echo.s inside the x86 directory. A heavily-commented example echo.s is provided, which simply prints the message Hello World (followed by a trailing newline) to stdout. Your job is to modify this program to meet the specification for echo above.

A Makefile is included, so you can build the echo binary by running make from the command line.

Helpful Hints

• In a Linux program, stdout is file descriptor number 1 and stderr is number 2.
• Linux programs start with argc at the top of the stack, accessible at 0(%esp) from x86-assembly programs. Below it is argv, the array of pointers to (null-terminated) strings passed into the program as arguments. So argv[0] can be accessed from x86-assembly programs at 4(%esp), argv[1] at 8(%esp), argv[2] at 12(%esp), and so on.
• argv[0] is the name of the program executed, not the first argument, which is argv[1] if supplied (or 8(%esp) for our purposes). Then argc (or 0(%esp)) will be 1 if the program was not passed any arguments, 2 if it was passed 1 argument, and so on.
• This table of Linux system calls may come in handy.

Submission

Submit echo.s to "1b. x86 echo" on Gradescope.