Transcript CS 635 Advanced Systems Programming Spring 2003 Professor Allan B. Cruse

CS 635
Advanced Systems Programming
Spring 2003
Professor Allan B. Cruse
University of San Francisco
Instructor Contact Information
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Office: Harney Science Center – 212
Hours: Mon-Wed 2:30pm-4:00pm
Phone: (415) 422-6562
Email: [email protected]
• Webpage: nexus.cs.usfca.edu/~cruse/
Course Textbooks
Alessandro Rubini and Jonathan Corbet,
Linux Device Drivers (Second Edition),
O’Reilly & Associates, Incorporated (2001)
M. Beck et al, Linux Kernel Programming
(Third Edition), Addison-Wesley (2002)
Linux kernel modules
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Great mechanism for kernel ‘extensibility’
Neat tool for studying how kernel works
Kernel can be modified while it’s running
Unnecessary to recompile and then reboot
But inherently unsafe: bugs cause system
crashes!
‘Extensibility’
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Modern OS needs ability to evolve
Will need to support new devices
Will need to allow ‘bugs’ to be fixed
Will need to permit performance gains
• Otherwise: suffer early obsolescence!
Two Extensibility Mechanisms
• ‘Open Source’ programming
• ‘Installable’ kernel modules
‘Superuser’ privileges
• Modifying a running kernel is ‘risky’
• Only authorized ‘system administrators’
are allowed to install kernel modules
A few ‘/proc’ examples
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$ cat /proc/version
$ cat /proc/cpuinfo
$ cat /proc/modules
$ cat /proc/iomem
$ cat /proc/self/maps
Module structure
• Two ‘module administration’ functions
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• Appropriate ‘module service’ functions
Required module functions
• int init_module( void );
• // gets called during module installation
• void cleanup_module( void );
• // gets called during module removal
How to compile a module
• gcc –c –O mod.c
Using ‘insmod’ and ‘rmmod’
root# /sbin/insmod jiffies.o
root# /sbin/rmmod jiffies
Using the ‘sudo’ command
user$ sudo /sbin/insmod jiffies.o
user$ sudo /sbin/rmmod jiffies
jiffies
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unsigned long volatile jiffies;
global kernel variable (used by scheduler)
Initialized to zero when system reboots
Gets incremented when timer interrupts
So it counts ‘clock-ticks’ since cpu restart
‘tick-frequency’ is architecture dependent
jiffies overflow
• Won’t overflow for at least 16 months
• Linux recently modified to ‘fix’ overflow
• New declaration in ‘linux/sched.h’:
unsigned long long
jiffies_64;
and a new instruction in ‘do_timer()’
(*(u64*)&jiffies_64)++;
which compiles to assembly language as
add $1, jiffies+0
adc $0, jiffies+4