.\" $NetBSD: lkm.4,v 1.21 2004/11/09 12:09:58 wiz Exp $ .\" .\" Copyright (c) 1993 Christopher G. Demetriou .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed for the .\" NetBSD Project. See http://www.NetBSD.org/ for .\" information about NetBSD. .\" 4. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" <> .\" .Dd September 4, 1993 .Dt LKM 4 .Os .Sh NAME .Nm lkm .Nd Loadable Kernel Modules interface .Sh SYNOPSIS .Cd "options LKM" .Sh 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. .Pp 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. .Pp The .Nm interface is used by performing .Xr ioctl 2 calls on the .Pa /dev/lkm device. Normally all operations involving Loadable Kernel Modules are handled by the .Xr modload 8 , .Xr modunload 8 , and .Xr modstat 8 programs. Users should never have to interact with .Pa /dev/lkm directly. .Sh MODULE TYPES .Ss System Call modules System calls may be replaced by loading new ones via the .Nm interface. All system calls may be replaced, but special care should be taken with the .Xr ioctl 2 system call, as it is used to load and unload modules. .Pp 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. .Ss Virtual File System modules Virtual file systems may be added via the .Nm interface. .Ss Device Driver modules New block and character device drivers may be loaded into the system with .Li "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 .Xr modload 8 to run an appropriate program when the driver has been successfully loaded. .Ss Emulation modules Emulation modules allow to load an emulation support for foreign operating systems. .Ss 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. .Ss 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. .Sh NOTES .Ss 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 .Pa /dev/lkm can do anything they want with the system. .Pp To avoid associated security risks, new LKMs cannot be loaded when .Pa securelevel is higher than zero. .Ss 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. .Sh FILES .Bl -tag -width /usr/include/sys/lkm.h -compact .It Pa /dev/lkm .Nm interface device .It Pa /usr/include/sys/lkm.h file containing definitions of module types .It Pa lkm/* subdirectory .Pa lkm within kernel source tree contains many LKMs which are suitable as a base for new ones .El .Sh SEE ALSO .Xr modload 8 , .Xr modstat 8 , .Xr modunload 8 .Sh HISTORY The .Nm facility was designed to be similar in functionality to the loadable kernel modules facility provided by .Tn "SunOS 4.1.3" . .Sh AUTHORS .An Terrence R. Lambert .Aq terry@cs.weber.edu