CSCI 341: Lab 1

The Elf Format
Due: 11:59 PM on Monday, February 22nd

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

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. 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!

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

Your readelf output will vary depending on your compiler. I recommend doing all programming for this class on its dedicated server, cs341.cs.oberlin.edu, to ensure you get the results listed here. You can log in with your CS username and password.

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 full name 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. Size needs to be the same for Objects, but not for functions. Everything that should match is bolded below.

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 7
7: 0000000000000008 4 OBJECT LOCAL DEFAULT 3 order
8: 0000000000000000 4 OBJECT LOCAL DEFAULT 4 isnt
9: 0000000000000000 58 FUNC LOCAL DEFAULT 1 important
10: 0000000000000010 8 OBJECT LOCAL DEFAULT 3 for_these
11: 0000000000000008 8 OBJECT LOCAL DEFAULT 4 symbols
12: 0000000000000000 0 SECTION LOCAL DEFAULT 11
13: 0000000000000000 0 SECTION LOCAL DEFAULT 12
14: 0000000000000000 0 SECTION LOCAL DEFAULT 10
15: 0000000000000020 100 OBJECT GLOBAL DEFAULT COM but_the
16: 0000000000000000 8 OBJECT GLOBAL DEFAULT 3 types
17: 0000000000000000 8 OBJECT GLOBAL DEFAULT 5 and_the
18: 0000000000000000 15 OBJECT GLOBAL DEFAULT 7 scopes
19: 000000000000003a 32 FUNC GLOBAL DEFAULT 1 matter
20: 000000000000005a 44 FUNC GLOBAL DEFAULT 1 main
21: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _GLOBAL_OFFSET_TABLE_
22: 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] .data.rel.local
			[ 6] .rela.data.rel.local
			[ 7] .rodata
			[ 8] .comment
			[ 9] .note.GNU-stack
			[10] .eh_frame
			[11] .rela.eh_frame
			[12] .symtab
			[13] .strtab
			[14] .shstrtab

Hints:

  • Section names will correspond to different numbers depending on the compiler you use and will also change as you add symbols to different functions. Use readelf -S to check which sections correspond to which section number. Make sure you are working on the cs341 server
  • 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.

How to Submit The Lab

You will need to submit to Gradescope by using github. Instructions on how to do this are in Worksheet 3. 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 me.

Due Date

This assignment is due 02/22/2021 11:59 PM. 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

Lab written by C. Taylor, C. Kanich