Kernel Modules – Introduction CSC/ECE 573, Sections 001 Fall, 2012

Kernel The “Operating System” System of software that – Sits between hardware and application programs – Provides necessary abstractions – Mediated by system calls Process management Memory management File Systems Device drivers Network (drivers and protocols)

Why Kernel? Networking “subsystem” originally implemented as user-space – Code easy to examine and change Later, as part of kernel – Not just device drivers, but protocols as well – Experimentation with common protocols require kernel access – Even if only “probing” or “reading off” Kernel problems – Keeps moving – Device dependent

Communicating with Kernel Kernel programs run in separate (and unique) address space – Cannot be swapped out – Does not overlap user space address spaces All kernel programs share address space – Mistakes typically causes kernel to crash – VMs ideal in experimenting with kernel programming /proc filesystem is an elegant interface in UNIX – Dynamically generated files – User-space communication with kernel programs

Kernel Modules Linux uses kernel modules – Dynamically load and unload – flexibility – Shares address space – still monolithic – Can be completely new code, and “observer” Linux commands insmod, rmmod, lsmod etc. Callback functions – Initialization and exit – Any other functions Device open or close, for device drivers – Register functions for callback conditions

Hooks for Callback Linux provides netfilter that includes “hooks” that callback code can register for Can register kernel modules at hooks – Can pop functionality out to user space, but more efficient in kernel space

Packet Processing Packets injected into kernel (from device or from user space) stored in special structures Copying eliminated inside kernel – various kernel operations all operate on this structure General framework for packet processing – transcends specific protocol stack, specific protocol data formats

Packet Storage

Summary In Linux, particularly easy to investigate and experiment with network protocols, or build new protocols Powerful enough that real advanced application building is possible