Assembly to C

Package: asm-and-c-package.tar

Upload via Moodle as: asm-and-c.tar

NOTE: this assignment has two due dates. For Tuesday, April 28, submit work for puzzle1.asm, puzzle2.asm, and puzzle3.asm, including one reverse-engineered puzzle. For Thursday, April 30, submit work for puzzle4.asm, puzzle5.asm, and puzzle6.asm, including one reverse-engineered puzzle.

You may work alone or with one other person for this assignment. I encourage you to collaborate on this one; it's much more fun to talk to each other and to have friends to share your befuddlement.

Goals

• Get familiar with x86_64 assembly language basics
• Identify assembly language patterns representing common structures from higher-level languages
• Trace possible execution flows through assembly code

Rubric

Assignment overview

For this assignment, you are going to practice understanding the correspondence between simple C code and its equivalent assembly language by studying a sequence of puzzles.

For each puzzle, you will read some assembly language and try to identify the assembly structures that correspond to C-language structures like loops, if/else statements, function calls, etc. For two of the puzzles, you will also try to write C code that compiles to the puzzle's assembly code. Overall, for this assignment, you will be doing a simple form of reverse engineering.

To help us study assembly code, we will use Compiler Explorer, as described briefly in this getting-started document. You'll put some C code into the input panel, and the output panel will show you the assembly language generated by the selected compiler. As you adjust your C code, you'll be able to watch the changes in the assembly language, and then compare your assembly code to the puzzle's code.

What you should do?

In the asm-and-c-package.tar package, you will find several files named puzzle0.asm, puzzle1.asm, etc. For each puzzle, your job will go like this:

• Study puzzleN.asm to understand what it does. Although it's good to try to understand it line-by-line, you will also want to understand the puzzle holistically. Each puzzle's code performs a computationally familiar task, and can be described in a single short sentence.
• Fill in the comment at the top of the puzzleN.asm file, indicating which structures are present in the puzzle:
  • conditional branching (if, if/else)
  • loop (for, while, do-while)
  • nested loop
  • function call
  • recursive function call
• For each # Next: TODO marked in the inline comments, provide the label(s) of the instruction(s) that could possibly be executed immediately after the instruction on the TODO line.
• Fill in the Possible order comments at the top of the assembly file, giving three possible orders in which the labels may be executed in during a single invocation of the function function1.
• Fill in the Registers and sizes comment at the top of the assembly file, listing the registers used for the parameters passed to function1, and explicitly indicating the sizes of the registers. For example, if the only parameter to function1 is found in %rdi and it is always referenced as %edi, you’d write something like "%edi (4 bytes)". If there are two parameters, with the first being an 8-byte parameter and the second being a 2-byte parameter, you’d write "%rdi (8 bytes)" and "%si (2 bytes)".
• For your choice of two puzzles (one from 1-3 and another from 4-6), write a C file puzzleN.c containing a function that, when entered into Compiler Explorer, produces the assembly language code found in puzzleN.asm. At the top of puzzleN.c, include a comment containing a one-sentence description of the purpose of function1.

NOTE: when you are writing your two puzzleN.c files, you may find that some of the labels in the puzzleN.asm will not show up in the assembly generated by Compiler Explorer. For example, puzzle4.asm has labels CA, CB, CC, and CD, all of which were added by the CS faculty after compiling was finished. Those extra labels are there to help you trace through the code. Similarly, if you write code in Compiler Explorer and it gives you the same code as one of the puzzles but the L* labels have different numbers in different places than the puzzle, that's fine--the numbers don’t have to exactly match, but they should be in the right places.

Hand it in

For each of the two deadlines, combine your edited puzzleN.asm files and your puzzleN.c file in a tar file named asm-and-c.tar and submit it via Moodle.

One last note...

This is weird stuff at first. We'll spend a lot of time in class giving you the tools you need to complete this assignment. Be patient and persistent. The pieces will fall into place before long.

Have fun!