LKM(4)

NAME
lkm - Loadable Kernel Modules interface

SYNOPSIS
options LKM

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 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 by LKM code.

  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 options LKM. A problem with loading a device driver is that the driver's device nodes must 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.

  Emulation modules
Emulation modules allow to load an emulation support for foreign operating systems.

  Execution Interpreters
Execution Interpreters allow to load support for execution of new type of binaries, not normally supported by kernel. This also allows to load support for executing foreign system binaries. Execution Interpreters normally depend on Emulation modules, in that appropriate Emulation module has to be loaded before Execution Interpreter can be.

  Miscellaneous modules
Miscellaneous modules are modules for which there are not currently well-defined or well-used interfaces for extension. They are provided for extension, and the user is expected to write their own loader to handle the kernel pointer/table manipulation to "wire in" their loaded module (and "unwire" it on unload). An example of a "miscellaneous module" might be a loader for card-specific VGA drivers or alternate terminal emulations in an appropriately layered console driver.

NOTES

  Security considerations
Loaded kernel modules can do anything with kernel structures. There is no memory protection between modules and the rest of the kernel. Hence, a potential attacker controlling /dev/lkm can do anything they want with the system.

To avoid associated security risks, new LKMs cannot be loaded when securelevel is higher than zero.

  Module might crash system
Loading and using a buggy module is likely to crash operating system - since the module becomes part of kernel, a code error is much more fatal than for userland programs. If you are doing kernel development, this would hopefully end up happening less frequently than changing, recompiling, installing, and rebooting would normally occur. This should speed development considerably for a lot of the in-kernel work that is currently taking place.

FILES
/dev/lkm
    lkm interface device
/usr/include/sys/lkm.h
    file containing definitions of module types
lkm/*
    subdirectory lkm within kernel source tree contains many LKMs which are suitable as a base for new ones

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

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

AUTHORS
Terrence R. Lambert <terry@cs.weber.edu>