CS 635 Advanced Systems Programming

1
CS 635Advanced Systems Programming

• Fall 2007
• Professor Allan B. Cruse
• University of San Francisco

2
Instructor Contact Information

• Office Harney Science Center 212
• Hours Mon-Wed-Fri 1245pm-130pm
• Tues-Thurs 630pm-715pm
• Phone [phone]
• Email cruse_at_usfca.edu
• Webpage http//cs.usfca.edu/cruse/

3
The class website

• URL http//cs.usfca.edu/cruse/cs635/
• General description of the course
• Links to some useful online resources
• Lecture-slides and demo-programs
• System software for use with this course
• Class announcements (e.g., exam dates)
• A link to our CS635 discussion-list
• Our textbook reading assignments

4
Course Textbooks

• Jonathan Corbet, Alessandro Rubini and Greg
  Kroah-Hartman,
• Linux Device Drivers (3rd Ed),
• OReilly Media, Inc (2005)
• Daniel Bovet and Marco Cesati, Understanding the
  Linux Kernel (3rd Ed), OReilly Media, Inc (2006)

5
Some important prerequisites

• You are acquainted with x86 architecture
• You can execute Linux/UNIX commands
• You know how to use a text-editing tool
• You can write programs in the C language
• You can print out a programs source-file

6
Typical C layout

• Basic structure of a C program
• Comment-banner (showing title and abstract)
• Preprocessor directives (e.g., for header-files)
• Global data-declarations (if they are needed)
• Required main() function (as the entry-point)
• Can invoke printf() (for formatted output)
• Optionally may define some other functions

7
Example program in C
8
Extensibility

• A modern OS needs the ability to evolve
• Will need to support new devices
• Will need to allow bugs to be fixed
• Will need to permit performance gains
• Else OS may suffer early obsolescence!

9
Extensibility with Linux

• Two mechanisms for extensibility
• Open Source development
• Loadable kernel modules (LKMs)

10
Loadable Kernel Modules

• Convenient technique for OS extensibility
• Also allows us to study how kernel works
• Kernel can be modified while its running
• No need to recompile and then reboot
• But inherently unsafe any bug can cause a
  system malfunction -- or complete crash!

11
Superuser privileges

• Modifying a running kernel is risky
• Only authorized system administrators
• are allowed to install kernel modules
• But our classroom workstations will allow us some
  (limited) administrator privileges

12
insmod and rmmod

• Were allowed to install kernel objects
• /sbin/insmod myLKM.ko
• Were allowed to remove kernel objects
• /sbin/rmmod myLKM
• Anyone is allowed to list kernel objects
• /sbin/lsmod

13
Creating a new LKM

• You can use any text-editor (e.g., vi or
  emacs) to create source-code (in the C
  language) for a Linux kernel module (i.e., an
  LKM)
• But a kernel module differs from a normal C
  application program (e.g., no main() function)
• A kernel module cannot call any of the familiar
  functions from the standard C runtime libraries
• For any LKM, two entry-points are mandatory (one
  for initialization, and one for cleanup)

14
Normal LKM structure

• Resembles normal layout of C programs
• but
• Two module administration functions
  these are required
• plus
• Appropriate module service functions
  these are optional

15
Other LKM differences

• Module uses printk() instead of printf()
• Includes the ltlinux/module.hgt header-file
• Specifies a legal software license (GPL)
• Compilation requires a special Makefile
• Execution is passive (its a side-effect)
• Module has no restriction on privileges

16
Required module functions

• int init_module( void )
• // this gets called during module installation
• void cleanup_module( void )
• // this gets called during module removal
• A newer syntax allows memory-efficiency
• module_init(my_init)
• module_exit(my_exit)

17
Kernel module written in C
18
Format of the Makefile

• ifneq ((KERNELRELEASE),)
• obj-m mymod.o
• else
• KERNELDIR /lib/modules/(shell uname
  r)/build
• PWD (shell pwd)
• default
• (MAKE) -C (KERNELDIR) M(PWD) modules
• endif

19
Inconveniences

• That Makefile has to be edited every time you
  create another new module! ?
• Then, when you compile the new module, like this
  make
• there are more than a half-dozen files that get
  created (some of them are hidden) in your
  current directory, but just one is the .ko
  (kernel object) that you really wanted

20
Our mmake tool

• Since we will be writing and compiling lots of
  modules during our course, we wrote a tool that
  conveniently automates the steps
• You can simply type ./mmake
• It creates the Makefile you need, in your
  current directory, to compile all modules that
  currently reside in that directory
• Afterward it erases all the unneeded files!

21
Improvement to mmake

• After watching past students use mmake we
  realized that it would be better to allow
  compiling just one module at a time
• We kept the former behavior as an option
• But now we allow the user to specify with a
  command-line parameter which module (or modules)
  they wish to re-compile
• ./mmake mymod

22
In-class exercise 1

• Download mmake.cpp from class website and
  compile it with make (or alternatively use
  g mmake.cpp o mmake )
• Download the kello.c source-file from the
  website, and compile it using mmake
• Add the kello.ko kernel-object to Linux using
  the Linux /sbin/insmod command
• Use dmesg to view the kernels log-file
• Remove kello (with /sbin/rmmod kello)

23
Showing kernel messages

• You can modify the printk() text-string so its
  message will be sure to be displayed -- it will
  be output to the graphical desktop
• Heres how you can do it
• printk( lt0gt Hello, everybody! \n )

This log-level indicates a kernel emergency
24
In-class exercise 2

• Modify the kello.c source-file so that the
  messages will be visible in a window on the
  graphical desktop (in addition to being written
  to the kernels log-file)
• You can switch from graphics-mode to a text-mode
  console with ltCTRLgtltALTgtF1
• You can switch back to graphics mode by typing
  ltCTRLgtltALTgtF7

25
Summary

• Download mmake.cpp and kello.c
• Compile mmake.cpp using make
• Then compile kello.c using mmake
• Install kello.ko (and see printk-message)
• Remove kello (to see another message)
• Modify the printk() statements in kello.c
• Recompile and reinstall to view new info