Lab 3 -- Exploring Executable Code.

You may work in groups for this lab. Each group can turn in one writeup.

First download the executable file: test. This program has been compiled on the jazz machines and will error out if you use it on another machine.

Once you have the program, you can use gdb, objdump, and strings on the executable file.

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 (in this case, you do not have any source code), set breakpoints, set memory watch points, and write scripts. Here are some tips for using gdb.
The class website now has a single-page gdb summary.
For other 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 symbol table. The symbol table includes the names of all functions and global variables, the names of all the functions, and their addresses. You may learn something by looking at the function names!

objdump -d
Use this to disassemble all of the code. You can also just look at individual functions. Reading the assembler code can tell you how the program 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 code.

Once you have explored the software, try to answer the following questions.

1. What procedure calls were implemented in the original C source code, including parameters used.
2. What does each function do?
3. To the best of your ability, try to recreate the C source code.
4. The linux memory map is divided into regions. In what regions are the procedures and global variables stored. You should be able to intuit this information by reading about how the objdump utility works. Hint: look at -h
5. What are the memory locations of all the variables indicated in the C source file, this will require some creative thought.

Write up your answers and email them to me with your name in the file, on the file and subject line.