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Linux Loadable Kernel Module HOWTO

Bryan Henderson

January 12, 2005

Revision History
Revision v1.062005-01-12Revised by: bjh
Cover Linux 2.6 briefly. Update hello.c and reference to Lkmpg. Add information about perils of unloading. Mention dmesg as way to see kernel messages.
Revision v1.052004-01-05Revised by: bjh
Add information on module.h and -DMODULE. Fix tldb.org to tldp.org. Add information on kallsyms.
Revision v1.042003-10-10Revised by: bjh
Fix typo: AHA154x should be AHA152x Add information on what module names the kernel module loader calls for. Add information on what an LKM does when you first load it. Add information on loop module. Change linuxdoc.org to tldp.org.
Revision v1.032003-07-03Revised by: bjh
Update on kernels that don't load into vmalloc space. Add explanation of "deleted" state of an LKM. Explain GPLONLY.
Revision v1.022002-05-21Revised by: bjh
Correct explanation of symbol versioning. Correct author of Linux Device Drivers. Add info about memory allocation penalty of LKM vs bound-in. Add LKM-to-LKM symbol matching requirement. Add open source licensing issue in LKM symbol resolution. Add SMP symbol versioning info.
Revision v1.012001-08-18Revised by: bjh
Add material on various features created in the last few years: kernel module loader, ksymoops symbols, kernel-version-dependent LKM file location.
Revision v1.002001-06-14Revised by: bjh
Initial release.

This is the HOWTO for Linux loadable kernel modules (LKMs). It explains what they are and how to use and create them. It also includes documentation of parameters and other details of use of some particular modules.

Table of Contents
1. Preface
2. Introduction to Linux Loadable Kernel Modules
2.1. Terminology
2.2. History of Loadable Kernel Modules
2.3. The Case For Loadable Kernel Modules
2.4. What LKMs Can't Do
2.5. What LKMs Are Used For
3. Making Loadable Kernel Modules
4. LKM Utilities
5. How To Insert And Remove LKMs
5.1. Could Not Find Kernel Version...
5.2. What Happens When An LKM Loads
5.3. Intelligent Loading Of LKMs - Modprobe
5.4. Automatic LKM Loading and Unloading
5.5. /proc/modules
5.6. Where Are My LKM Files On My System?
6. Unresolved Symbols
6.1. Some LKMs Prerequire Other LKMs
6.2. An LKM Must Match The Base Kernel
6.3. If You Run Multiple Kernels
6.4. SMP symbols
6.5. You Are Not Licensed To Access The Symbol
6.6. An LKM Must Match Prerequisite LKMs
7. How To Boot Without A Disk Device Driver
8. About Module Parameters
9. Persistent Data
10. Technical Details
10.1. How They Work
10.2. The .modinfo Section
10.3. The __ksymtab And .kstrtab Sections
10.4. Ksymoops Symbols
10.5. Other Symbols
10.6. Debugging Symbols
10.7. Memory Allocation For Loading
10.8. Linux internals
11. Writing Your Own Loadable Kernel Module
11.1. Simpler hello.c
11.2. Using the Kernel Build System
11.3. Rubini & Corbet: Linux Device Drivers
11.4. Improving On Use Counts
12. Differences Between Versions Of Linux
12.1. Linux 2.4 - Linux 2.6
13. Related Documentation
14. Individual Modules
14.1. Executable Interpreters
14.2. Block Device Drivers
14.3. SCSI Drivers
14.4. Network Device Drivers
14.5. CDROM Device Drivers
14.6. Filesystem Drivers
14.7. Miscellaneous Device Driver
14.8. Serial Device Drivers
14.9. Parallel Device Drivers
14.10. Bus Mouse Device Drivers
14.11. Tape Device Drivers
14.12. Watchdog Timers
14.13. Sound Device Drivers
15. Maintenance Of This Document
16. History
17. Copyright