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LKM(4) Device Drivers Manual LKM(4)

LKMLoadable Kernel Modules interface

option LKM

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. The loading and unloading of modules is also dependent on the system securelevel(7).

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 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 Interpreters
Execution interpreters allow the loading and execution of binaries which are normally not usable by the operating system.
Miscellaneous modules
Miscellaneous modules are modules for which there are not currently well-defined or well-used interfaces for extension. The user is expected to write their own loader to manipulate whatever kernel data structures necessary to enable and disable the new module when it is loaded and unloaded.

LKM interface device.
File containing definitions of module types.
Example source code implementing several of the modules types.

securelevel(7), modload(8), modstat(8), modunload(8)

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

Terrence R. Lambert ⟨⟩

Loading a bogus module is likely to kill your machine.

August 19, 2011 OpenBSD-5.1