Publisher-Subscriber IPC Kernel Module

This project consists in the development of a kernel module for Linux systems that implements the publisher-subscriber design pattern for inter-processes communication.

The module is the final result of Advanced Operating Systems course in Computer Science & Engineering Master Degree at Politecnico di Milano, Italy.

• Project supervisor: Federico Reghenzani
• Course head professor: Vittorio Zaccaria
• Student developers: Ottavia Belotti and Riccardo Izzo

Module Injection

From terminal, run the given Makefile in the project directory to compile the C source code into the psipc_kmodule.ko kernel object file.

make

After compilation, to insert the module in the current system:

#optional: clean the kernel message buffer before insertion
sudo dmesg -C
sudo insmod psipc_kmodule.ko

Once it is not needed anymore, remove it with:

sudo rmmod psipc_kmodule

Purpose of the project & How to use it

The module creates a tree-structure in the /dev directory that, once the module has been loaded succesfully, will resemble the one proposed below.

/dev/psipc/
        |   
        |__ /new_topic
        |__ /topics
            |
            |__ /my_topic_1
            |   |
            |   |__ /subscribe
            |   |__ /subscribers_list
            |   |__ /signal_nr
            |   |__ /endpoint
            |
            |__ /my_topic_2
            .   |
            .   .
            .   .

A user process that acts as a publisher has to request the creation of the desired topic by writing its name on the new_topic file. The module creates a new directory in /dev/psipc/topics specific to that topic with all the necessary files related to it: subscribe, subscribers_list, signal_nr and endpoint. Once this has been set up, other user processes running under the same user space as the creator of the topic can:

• Subscribe to the topic: write their PID in /subscribe. When a message is published for that topic, the subscriber will receive a signal (following POSIX standard), chosen by the publisher
• Retrieve a list of all the subscribers: read the /subscribers_list file to know the PIDs of all the processes currently subscribed to that topic
• Read the published message: read the last published message from /endpoint. This is allowed just for subscribed processes.

The publisher can:

• Choose a signal: choose a POSIX standard message by writing its number on /signal_nr file. Once the publisher write a new message, the signal will be sent to all the current subscribers of the topic. Writing more than once on /signal_nr overwrites the signal. If no signal is set, the module won't send anything to the subscribers.

Note: not setting a signal doesn't mean that the message can't be written. It just won't notify the subscribers.

• Publish a message: add a new message by writing it in /endpoint. If a signal has been set, the act of writing in here will notify the subscribers.

Note: A publisher can't write a new message if all the present notified subscribers have not read the previous one yet.

Development Environment

The module has been developed in Ubuntu 20.04 LTS distro with development tools on a Virtual Machine.

Kernel version: v5.11

Tools & Reference Material

• The Linux Kernel Module Programming Guide - by Peter Jay Salzman, Michael Burian, Ori Pomerantz, Bob Mottram, Jim Huang
• Linux - Linus Torvalds official repository on GitHub
• Bootlin Elixir Cross Referencer - to explore in an easier way the Linux source code
• Oracle VM VirtualBox