Linux Kernel Structure

Today's Lecture

• Kernel Responsibility
• Kernel Organization
• Kernel Modules
• Project 2

Q&A

• Have everybody turned in their project 1?
• Any problem with project 1?

Running User-Mode-Linux

• Kill Your Processes if your UML hangs
  • Check with ps -x | grep ./linux
  • Kill each process by kill pid
  • Sometimes you need to kill -9 pid

Kernel Responsibility

• Kernel Mode and User Mode
  • Modern CPU architecture:
  • Supervisor mode: execute any instructions
  • User mode: execute non-privileged instructions, in bounded address space
• Kernel's Responsibilities
  • Managing machine's resources (hardware, data structures)

Kernel/User Space Partition

• Strict separation of responsibility
• Strictly-defined interface between kernel and user space
• Next lecture: kernel/user space interaction

Kernel Organization

• Monolithic kernel
  • Everything in a single executable piece of software
  • Every data structure/function is available to every other function
  • But it does not imply that Linux kernel has no structure
• Q: What is the name of the other type of kernel structure (that you have learned in CS372)?

Monolithic vs Microkernel

• Problem with Monolithic Kernel
  • Kernel Getting Bigger and Bigger
  • New functionality
  • New hardware
• Linux Solution: Module
  • Unessential kernel functions implemented as a dynamic loadable modules

Basic Concept

• Modular Programming
  • Compiling and Linking
  • Dynamic loading vs static linking
  • Symbol relocation
  • Q: what is symbol?
    • Think examples of user space
• Shared library (libxxx.so vs libxxx.a)
• Dynamic linker/loader (ld.so)

Linux Modules

• How Module Works
  • Written as a .c file
• Follow some conventions (i.e. special macros)
  • Compiled into an .o file
  • With special compiler option
• Loaded into a running kernel (if so demanded)
  • With special commands
• Dynamic loading (symbol relocation)
• Unloaded it later (if unused)

Symbols

• Kernel Symbol Tables
  • The list of symbols that are made available for reference by modules
  • To include a symbol in the list, call
    EXPORT_SYMBOL(var)
• Module Symbol Tables
  • All global symbols are exported
  • To find out the list of symbols
    cat /proc/ksyms

Module Dependency

• If A references symbols in B, B must be loaded first
• Tools for helping dependency
  /sbin/depmod -- find out the dependency for all modules under /lib/modules/version/

Module Commands

• /sbin/insmod - load a module into the kernel
• /sbin/rmmod - unload a module
• /sbin/lsmod - list all the loaded module
• /sbin/modprobe - automatically load required module according to the dependency

Writing Module

• Includes
  #include <linux/kernel.h>
  #include <linux/module.h>
• Compile option:
  gcc -c -Ilinux/include -Wall -D__KERNEL__ -DMODULE foo.c
• Functions:
  #ifdef MODULE
  int init_module() { .... }
  void cleanup_module() { .... }
  #endif

More about Modules

• Kernel must be configured to support kernel module
• Kernel must be compiled first!
  • You will need the linux/include directory in your kernel source tree
  • Symbols, version number
• Many kernel modules can be statically linked
• You can configure a module to be dynamic loadable

Homework

• Read Appendix B (Understanding Linux Kernel)
• Explore module configuration
• Do project 2

Project 2

• Do Exercise 4 in "Kernel Projects for Linux", Page 97-106
• Follow the book by yourself
• Write the code and test it out
• Due: Feb 1, Fri (one week from today)