Introduction

The goal of this project is to get used to Linux development environment and modify Linux kernel source code. The project is split into two parts. The first part is analyzing a given user-space socket application and adding printk() to the system call entries in Linux kernel. The second part is to add one new system call which encrypts a given string and prints out encrypted string to kernel message.

Make a folder named with your SBU ID (e.g., 112233445), put all your submission files in the folder, create a single gzip-ed tarball named [SBU ID].tar.gz, and turn the gzip-ed tarball to Blackboard.

$ tar czvf 112233445.tar.gz 112233445/

112233445/

112233445/code-socket.tar.gz

112233445/printk.patch

112233445/printk.png

112233445/syscall.patch

112233445/syscall.tar.gz

112233445/syscall.png

Recommended Background Reading

• Socket Programming in C/C++
• GNU Make in Detail for Beginners
• printk()
• git diff
• ERRNO(3)
• GETOPT(3)

Part 1. Adding printk()

The attached source code code-socket.tar.gz is a simple socket application that exchanges messages between a server and client. It consists of three files as shown in below.

$ cd code-socket $ ls

client.c Makefile server.c

P1.1: Understanding source code

Carefully read above three source file to understand how a simple network client/server works. Once you completely understand each file, add comments of each line (M1-M10, S1-S9, and C1-C7) explaining what the line means. Turn in the gzip-ed tarball named code-socket.tar.gz.

$ tar czvf code-socket.tar.gz code-socket/ code-socket/ code-socket/client.c code-socket/server.c code-socket/Makefile

P1.2: Adding printk()

Now you understand how the user-space application works. To understand how a kernel system call is called, print any message at the very beginning of system call implementations of accept() and connect() in Linux kernel v5.8. Check if the modified kernel prints out messages you added when you run the network client/server. Then create a patch against kernel v5.8 using command and turn in the patch named printk.patch.

P1.3: Test your kernel

Take the screenshot of your kernel debug message using dmesg while running the network client/server. Run server, client, and dmesg in one ssh session using tmux. Turn in the screenshot named printk.png.

Part 2: Adding a new system call

Add a new system call named sys_s2_encrypt() that takes two arguments, a NULL-terminated string and an encryption key, which is a positive integer between 1 and 5. The system call encrypts the given string by simply adding the given integer number. For example, it encrypts hello to ifmmp by adding 1 to each character of the string. After encryption, it prints out the encrypted string using . The system call returns 0 when everything is okay. If the encryption key is out of bound, it returns L.

P2.1: Implementing the system call

You should implement the system call in a separate file under linux/kernel directory. Then create a patch against kernel v5.8 using command and turn in the patch named syscall.patch.

P2.2: Writing a test program

Write a simple test program takes two options, and calls sys_s2_encrypt() with the string and key from the command line. You should implement command line argument processing using GETOPT(3) and the code should be able to build using make and clean using . The program prints out the return value of sys_s2_encrypt(). Turn in gzip-ed tarball of the source code and Makefile, named syscall.tar.gz.

P2.3: Test your system call

Take a screenshot of your kernel debug message using dmesg while running your test program. Run your test program and dmesg in one ssh session using tmux. Turn in the screenshot named syscall.png.