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.
The attached code hw3.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[24 points] Read above three source file to understand how a simple network client/server works. Once you completely understand each file, add brief comments of each line (M1-M10, S1-S8, and C1-C6) explaining what the line means. Turn in a gzip-ed tarball.
$ tar czvf hw3.tar.gz code-socket/
code-socket/
code-socket/client.c
code-socket/server.c
code-socket/Makefile[6 points] 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 the system call implementation of accept() in Linux kernel v6.8. Check if the modified kernel prints out messages you added when you run the network client/server. Then create a patch against kernel v6.8 using git diff command and turn in the patch named hw3_firstname_printk.patch.
[2 points] Test your kernel and upload a screenshot of your kernel debug message using dmesg while running the network client/server. Run server, client and dmesg in one tmux session and turn in a screenshot named hw3_firstname_printk.jpg | png.
Add a new super simple 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 (i.e., encryption key) to each character of the string. 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 printk(). The system call returns 0 when everything is okay. If the encryption key is out of bounds, it returns -EINVAL.
[9 points] You should implement the system call in a separate file under the kernel/ directory. Then create a patch against kernel v6.8 using git diff command and turn in the patch named hw3_firstname_syscall.patch.
[7 points] Write a simple test program takes two options, -s string -k key 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 make clean. The program prints out the return value of sys_s2_encrypt(). Turn in the gzip-ed tarball of the source code and Makefile.
[2 points] Upload a screenshot of your kernel debug message using dmesg while running your test program. Run your test program and dmesg in one tmux session and turn in a screenshot named hw3_firstname_syscall.jpg | png.