Project 4: Add a Custom System Call

A new system call, uname_set_hostname, that allows a process (like a user-space utility) to change the system’s hostname. You will implement the syscall in the kernel and a utility to call it.

Quick Reference

Attribute Value
Primary Language C++
Alternative Languages N/A
Difficulty Level 3: Advanced
Time Estimate 1-2 weeks
Knowledge Area Kernel Development / System Calls
Tooling SerenityOS Kernel
Prerequisites Projects 1 & 3, solid C++ understanding, conceptual knowledge of what an OS kernel is.

What You Will Build

A new system call, uname_set_hostname, that allows a process (like a user-space utility) to change the system’s hostname. You will implement the syscall in the kernel and a utility to call it.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • Defining a new syscall → maps to modifying the Syscall.tbl file and kernel headers
  • Implementing the syscall handler → maps to writing C++ code inside the Kernel directory
  • Safely copying data from user-space → maps to understanding pointers and memory protection
  • Calling the syscall from a user-space program → maps to using the syscall() wrapper

Key Concepts

  • Syscall Dispatch: Kernel/Syscall/handlers.cpp
  • User/Kernel Boundary: “Operating Systems: Three Easy Pieces” Ch. 6
  • Kernel Data Structures: Kernel/utsname.h for the hostname storage.

Real-World Outcome

Deliver a working demo with observable output that proves the feature is correct.

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: LEARN_SERENITYOS_DEEP_DIVE.md
  • “Operating Systems: Three Easy Pieces” by Arpaci-Dusseau