Kernel Modules

Giving your Linux more pop since 1995

History

• Introduced in Linux version 1.2 (1995)

• At the time, drivers were already designed to be somewhat modular, so not a lot of work was needed to revamp one driver for use as a LKM. There are a lot of drivers, though...

• By 2000, most drivers and “module-able” things are converted.

What are Modules

• A Module is a object file that contains code to extend the functionality of the base kernel.

• Modules are used to add support for new hardware and file systems.

• Also used to add new system calls and executable interpreters.

Why are they Important?

• If modules did not exist then adding new features to the kernel would be nearly impossible.

• Remember the systemCall() project???

Pros and Cons

• Pros: – Time savings – Portability– Memory savings– Security (PAM)

• Cons– No safeguards– Security (rootkits)

Modules vs. Applications

• Modules are really a part of the kernel and so run in kernel space.

• Apps just do their tasks and end. Modules sit in the kernel space waiting to be run.

• Apps can be linked to outside libraries; modules can only "see" in the kernel and other loaded modules.

Module Loading

• Kernel Symbol Table: references kernel functions that modules can use.

• When loaded, unresolved symbols in module are linked to system table

• EXPORT_SYMBOL() allows specified symbol to be used by kernel and other modules.

• Use cat /proc/kallsyms to get a list of all exported kernel symbols.

Image from: http://www.xml.com/ldd/chapter/book/ch02.html

Module HOW-TO

• Some useful commands– insmod insert module into kernel

– rmmod remove kernel module

– lsmod show currently loaded modules

• The above work, but are dumb. Instead, use modprobe [-r] which is aware of module dependencies and config settings.

• Use depmod to find module dependencies. File location: /lib/modules/version/modules.dep

Building the Module

• The steps taken depend on whether you want your module in the kernel tree or not

• Living in the tree– The LKD book explains this well with the Fish

Master example (pg 281)

– Quickly: put module in drivers/ directory, edit/create Makefile so module is built, make Kconfig file if you have config options for module.

Example Files

config SOME_OPTIONtristate “Enable some option”default nhelp A file like this must be with your module.

Go to Kconfig above this and add: source “this/directory/Kconfig”

Kconfig:

Makefile:#in module directoryobj-$(CONFIG_SOME_OPTION) += myModule.omyModule-objs := leroy.o jenkins.o

#edit makefile above module directoryobj-m += directory/

• Makefile goes with module source.

• The difference is in the compilation:– make -C /kernel/source/location SUBDIRS=$PWD modules

– /kernel/source/location is the location of your kernel source tree

– Be sure that your kernel source is configured and built

And now for an example...

Building Outside Tree

#similar to before#Note the := rather than += on next lineobj-m := hellomod.ohellomod-objs := goodbye.o hi.o

References

• The Linux Kernel Module Programming Guide

http://www.faqs.org/docs/kernel/

• Wikipedia: Linux Kernel

http://en.wikipedia.org/wiki/Linux_kernel

• Wikipedia: Linux Modules

http://en.wikipedia.org/wiki/Module_(Linux)

• Linux Device Drivers: Building and Running Modules

http://www.xml.com/ldd/chapter/book/ch02.html

• Linux Loadable Kernel Module HOWTO

http://www.tldp.org/HOWTO/Module-HOWTO/

• Linux PAM page

http://www.kernel.org/pub/linux/libs/pam/whatispam.html

• Linux Kernel Development. Ch. 16: Modules. pp.279-289

• Linux Kernel Cross-Reference

http://lxr.linux.no/