LKM(4)

NAME
lkm - loadable kernel module facility

DESCRIPTION
Loadable kernel modules allow the system administrator to dynamically add and remove functionality from a running system. This ability also helps software developers to develop new parts of the kernel without constantly rebooting to test their changes.

Various types of modules can be loaded into the system. There are several defined module types, listed below, which can be added to the system in a predefined way. In addition, there is a generic type, for which the module itself handles loading and unloading.

The FreeBSD system makes extensive use of loadable kernel modules, and provides loadable versions of most filesystems, the NFS client and server, all the screen-savers, and the iBCS2 and Linux emulators. Modules distributed with the system are found in the /lkm directory.

The lkm interface is used by performing ioctl(2) calls on the /dev/lkm device. Normally all operations involving Loadable Kernel Modules are handled by the modload(8), modunload(8), and modstat(8) programs. Users should never have to interact with /dev/lkm directly.

MODULE TYPES

System Call modules
System calls may be replaced by loading new ones via the lkm interface. All system calls may be replaced, but special care should be taken with the ioctl(2) system call, as it is used to load and unload modules.

When a system call module is unloaded, the system call which was replaced by the loadable module is returned to its rightful place in the system call table.

Virtual File System modules
Virtual file systems may be added via the lkm interface.

Device Driver modules
New block and character device drivers may be loaded into the system with LKM. The major problem with loading a device driver is that the driver's device nodes must be exist for the devices to be accessed. They are usually created by instructing modload(8) to run an appropriate program when the driver has been successfully loaded.

Execution classes
Also known as image activators, execution classes are the mechanisms by which the execve(2) system call is able to recognize an executable file's format and load it into memory.

Miscellaneous modules
Miscellaneous modules are modules for which there are not currently well-defined or well-used interfaces for extension. Users are expected to write their own loaders to manipulate whatever kernel data structures necessary to enable and disable the new module when it is loaded and unloaded.

FILES
/dev/lkm
    lkm interface device
/lkm
    directory containing module binaries shipped with the system
/usr/include/sys/lkm.h
    file containing definitions of module types
/usr/share/examples/lkm
    example source code implementing two of the module types

SEE ALSO
modload(8), modunload(8), modstat(8)

BUGS
If a module fails to initialize itself correctly, and the system is able to detect this failure, it will panic immediately.

When system internal interfaces change, old modules often cannot detect this, and such modules when loaded will often cause crashes or mysterious failures.

AUTHOR
The lkm facility was originally implemented by Terrence R. Lambert. Loadable filesystems were implemented by Garrett Wollman, and loadable execution classes were implemented by David Greenman, Soren Schmidt, and Garrett Wollman.

HISTORY
The lkm facility was designed to be similar in functionality to the loadable kernel modules facility provided by SunOS 4.1.3.