Homework 1: The ELF format

git, personal and public repositories

The primary objective of this homework is to get you familiarized with the ELF file format and the process of linking executables. The secondary goal of this assignment is to give you some experience using the Gradescope autograder and turnin system.

We covered using git in the first lab, but if you’re still not that familiar with it, this assignment is a second chance to get up to speed. Remember: git gives you warnings and errors for good reason, if it complains at the command line when you run a command, don’t just assume it’s completed correctly!

Setting git to the simple push is a helpful quality of life improvement, this is recommended but not required:

git config --global push.default simple

You should also tell git who you are, otherwise it will complain that you are some anonymous coder. Do this, but use your own name and email. Note that if you do not change your username and email properly, you won’t get credit.

$ git config --global user.name "Copypasting Carol"
$ git config --global user.email copypasta@domain.invalid

The skeleton code for this assignment is available at this link. You must use GitHub classroom to write your code and keep a commit log on GitHub. You can submit the code via Gradescope.

Now that you have the skeleton code, you can start coding. You should commit early and often, and push to your remote repository whenever is convenient to back up your work!

Programming environment

This assignment was created in a Debian-derived Linux environment. This assignment is simple enough that any Linux environment with an up to date gcc will be sufficient, including systems1.cs.uic.edu. If you aren’t seeing the line with __GLOBAL__OFFSET__TABLE__ in it, don’t worry. The autograder is written to the correct specification for the autograding environment. Feel free to complete this on a lab machine, a local Linux VM, or elsewhere.

The Programming Part!

This part will give you a quick introduction to using readelf to better understand the linking process.

In this assignment, you must fill hw1.c with code which will:

  1. cause your username (and nothing else) to be printed on the first line of output when the program is run.
  2. cause gcc -Wall hw1.c to issue zero warnings (and zero errors, duh).
  3. cause the output of readelf -s hw1.o to have an identical number of symbol table entries as shown below.
  4. cause the output of readelf -s hw1.o to have identical values in the red+bold sections of the output below:

Symbol table ‘.symtab’ contains 24 entries:

Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND  
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS hw1.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1  
3: 0000000000000000 0 SECTION LOCAL DEFAULT 3  
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4  
5: 0000000000000000 11 FUNC LOCAL DEFAULT 1 I_have_written
6: 0000000000000000 12 OBJECT LOCAL DEFAULT 3 the_code
7: 000000000000000b 26 FUNC LOCAL DEFAULT 1 that_you_needed
8: 0000000000000018 4 OBJECT LOCAL DEFAULT 3 to_compile.2213
9: 0000000000000000 0 SECTION LOCAL DEFAULT 5  
10: 0000000000000058 78 FUNC LOCAL DEFAULT 1 and_which
11: 000000000000001c 4 OBJECT LOCAL DEFAULT 3 has_a_bunch_of.2220
12: 0000000000000020 4 OBJECT LOCAL DEFAULT 3 ridiculous.2221
13: 0000000000000000 8 OBJECT LOCAL DEFAULT 4 symbols.2222
14: 0000000000000000 0 SECTION LOCAL DEFAULT 7  
15: 0000000000000000 0 SECTION LOCAL DEFAULT 8  
16: 0000000000000000 0 SECTION LOCAL DEFAULT 6  
17: 0000000000000025 51 FUNC GLOBAL DEFAULT 1 main
18: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND __GLOBAL__OFFSET__TABLE__
19: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND printf
20: 00000000000000a6 47 FUNC GLOBAL DEFAULT 1 Forgive_me
21: 0000000000000004 4 OBJECT GLOBAL DEFAULT COM they_are_arbitrary
22: 0000000000000010 8 OBJECT GLOBAL DEFAULT 3 so_random
23: 00000000000000d5 41 FUNC GLOBAL DEFAULT 1 and_so_varied

Hints:

  • Are you seeing puts instead of printf? Check the man pages for what the difference is, and make sure that the compiler can’t optimize away your call to printf. Remember, requirement #1 only mentions the first line of output.
  • The four digit numbers do not need to made identical, but you do need to make them show up. Keep experimenting with different variable types until you find how to create variables with periods and numbers on them.
  • Function lengths are very difficult to reproduce - note that for every FUNC, you do not have to duplicate the length (the length is the number of bytes of assembly code chosen by the compiler to execute the body of each function).

Template

There is a very bare bones skeleton and a Makefile provided for this assignment. The skeleton file is worth one point because it compiles with no warnings and no errors.

Grading

Grading will be done automatically using Gradescope. Submitting to GitHub is not sufficient - your code must be submitted to Gradescope. If you have issues with the autograder, please contact us via Piazza ASAP. Technical issues within 36 hours of the deadline will not be an excuse for submitting the assignment improperly or late.

Due Date

This assignment is due Monday, September 17th at 11:59 PM. See the syllabus for the late turnin policy. This assignment is worth just as much as every other homework, so getting as much credit on it as possible is important (don’t turn it in late!).

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