[2021] UIC - CS 361: Systems Programming - Homework1 The ELF Format
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 through this GitHub Classroom assignment. 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!
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!
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.
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:
- cause your UIC netID (and nothing else) to be printed on the first line of output when the program is run.
- cause
gcc -c -Wall hw1.c and gcc -Wall -o hw1 *.c to issue zero warnings (and zero errors). - 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:
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.
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:
- cause your UIC netID (and nothing else) to be printed on the first line of output when the program is run.
- cause
gcc -c -Wall hw1.candgcc -Wall -o hw1 *.cto issue zero warnings (and zero errors). - cause Type, Bind, and Ndx of each Symbol to be the same as in the example below. E.g.
importantmust have TypeFUNC, BindLOCAL, and Ndx1.
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:
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.
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.
