CS134, Spring 2002
Homework Assignment #2
Due 9 P.M. Sunday, April 21, 2002

Summary

This assignment consists of doing the "Modules" project in Chapter 4 of the handout selected from Gary Nutt, Linux Kernel Projects.

You may do the work on your own Linux box or on Emulab. However, if you use your own box and it's not running the 2.4 kernel, you might find that the documentation here differs from how your kernel works. That's your problem, not mine.

Please note that getting an Emulab account takes time, since I must approve you before you can begin work. Don't wait until the last minute; I don't promise to approve accounts promptly at any time, but especially on the weekend.

Submission

Please use cs134submit to submit the following files:

• The Makefile for your project,
• The source code for your module,
• The source for your clock test program, and
• An output log, named script.txt, containing:
  • The output from a sample run showing the results of running your clock test program long enough to show 5-10 different times, and
  • The output of "date; cat /proc/clock".

Resources

Handouts

There were two class sessions related to this project. You should have gotten handouts there. However, to save myself from requests from people who should know better, here are links to the handouts:

• The handout from the "Linux Introduction" lecture (Postscript).
• The handout from the "Emulab Introduction" lecture (Postscript).

Web Sites

Corrections and Additions

Some of the information in the book and the handout is incorrect or outdated. In particular:

• You can find out how the kernel maintains time by looking at the function sys_gettimeofday. You will have to find that function yourself. You'll also discover that you need to dig further than simply looking at that function.
• You don't need a fancy Makefile to make your module compile. However, your compilation flags must include:

      -O2 -D__KERNEL__ -DMODULE
  

  at a minimum. The -O2 is necessary or you'll get "undefined symbol" errors when you try to insert the module.
• If you include the following flags

      -I/proj/HMC_CS134/usr/src/linux/include
  

  at the end of your CFLAGS, you'll be technically more correct, but you'll have some minor problems when you do the insmod (see below).
• You need to include <linux/proc_fs.h> in the module.
• Registering a device in /proc is much easier than it used to be. I used the following code:

      #include <linux/proc_fs.h>
  
      static int clock_read_proc(char *page, char **start, off_t off,
  			      int count, int *eof, void *data);
      ...
  	if (create_proc_read_entry("clock", 0, NULL, clock_read_proc, NULL)
  	  == NULL)
  	    return -ENOMEM;
  

• To unregister a /proc device, use:

      remove_proc_entry("clock", NULL);
  

• Since insmod is privileged, you'll need to use sudo to execute it:

      sudo insmod proc_clock.o
  

  If it complains about a version mismatch, specify the -f switch to insmod.
• There is a copy of the Linux 2.4.2 kernel in /proj/HMC_CS134/usr/src/linux. There are tags files for both vi and emacs. You will need to learn to find your way around the kernel. Note that stuff that is specific to the Pentium architecture is in the subtree arch/i386.
• The Emulab computers are running Linux 2.4.2-2, which is slightly different from the 2.4.2 version that I installed. For our purposes, the differences don't matter (except for the insmod complaint, which is just a nuisance). I'll patch the sources up to 2.4.2-2 when I get a chance.
• If you get a warning like this from insmod:

      Warning: loading foo.o will taint the kernel: no license
  

  you safely ignore it, it's just the GNU people trying to control the world. If you really want to suppress it, add the following to your module source (after the #include stuff):

      MODULE_LICENSE("GPL");