Homework 1: The ELF format

This homework is meant to get you familiarized with the concept of symbols and how they relate to compiling and linking executables.

We covered using git in the week 1 video and discussion section, 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!

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 an Ubuntu 18.04 based environment. The skeleton code comes with a devcontainer setup script so that running it in a remote container through Visual Studio Code will put you in the same environment. This assignment is simple enough that any Linux environment with an up to date gcc should be sufficient, including systems1.cs.uic.edu. If you aren’t/can’t/don’t want to complete this assignment using vscode remote containers, 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 UIC netID (and nothing else) to be printed on the first line of output when the program is run.
  2. cause gcc -c -Wall hw1.c and gcc -Wall -o hw1 *.c to issue zero warnings (and zero errors).
  3. cause Type, Bind, and Ndx of each Symbol to be the same as in the example below. E.g. important must have Type FUNC, Bind LOCAL, and Ndx 1.

Running readelf -sW hw1.o on the solution results in:

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 0 SECTION LOCAL DEFAULT 5  
6: 0000000000000000 0 SECTION LOCAL DEFAULT 6  
7: 0000000000000000 0 SECTION LOCAL DEFAULT 8  
8: 0000000000000008 4 OBJECT LOCAL DEFAULT 3 order
9: 0000000000000000 4 OBJECT LOCAL DEFAULT 4 isnt
10: 0000000000000000 58 FUNC LOCAL DEFAULT 1 important
11: 0000000000000010 8 OBJECT LOCAL DEFAULT 3 for_these.3824
12: 0000000000000008 8 OBJECT LOCAL DEFAULT 4 symbols.3825
13: 0000000000000000 0 SECTION LOCAL DEFAULT 11  
14: 0000000000000000 0 SECTION LOCAL DEFAULT 12  
15: 0000000000000000 0 SECTION LOCAL DEFAULT 10  
16: 0000000000000020 100 OBJECT GLOBAL DEFAULT COM but_the
17: 0000000000000000 8 OBJECT GLOBAL DEFAULT 3 types
18: 0000000000000000 15 OBJECT GLOBAL DEFAULT 5 scopes
19: 0000000000000000 8 OBJECT GLOBAL DEFAULT 6 and_the
20: 0000000000000000 8 OBJECT GLOBAL DEFAULT 8 indices
21: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND definitely
22: 000000000000003a 32 FUNC GLOBAL DEFAULT 1 matter
23: 000000000000005a 44 FUNC GLOBAL DEFAULT 1 main
24: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND GLOBAL_OFFSET_TABLE
25: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND puts

As a further hint, here’s the mapping between the Section Index and the Section Names:

Section Headers:
  [Nr] Name              
  [ 0]                   
  [ 1] .text             
  [ 2] .rela.text        
  [ 3] .data             
  [ 4] .bss              
  [ 5] .rodata           
  [ 6] .data.rel.local   
  [ 7] .rela.data.rel.local
  [ 8] .data.rel         
  [ 9] .rela.data.rel    
  [10] .comment          
  [11] .note.GNU-stack   
  [12] .eh_frame         
  [13] .rela.eh_frame    
  [14] .symtab           
  [15] .strtab           
  [16] .shstrtab

Hints:

  • 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.
  • Order for the symbols does not matter. Take a close look at Chapter 7 and think about what properties of a variable put it in a different section, or would give it a different Binding or Type. Everything you need to know to solve this puzzle is in the book.

Template

There is a very bare bones skeleton and a Makefile provided for this assignment. The skeleton file is worth two points 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 with submitting your assignment is not an acceptable excuse for submitting the assignment improperly or late.

Due Date

This assignment is due Fri, 11 Sep 2020 17:00:00 -0500. See the syllabus for the late turnin policy. This assignment is worth just as much as every other homework, but is easier than all the rest (in our opinion) so getting as much credit on it as possible is important!

helpful documents

Chapter 7 in the book