NetBSD/share/man/man4/lkm.4

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.\" Copyright (c) 1993 Christopher G. Demetriou
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.\" <<Id: LICENSE,v 1.2 2000/06/14 15:57:33 cgd Exp>>
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.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 be 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 uload).
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 module can do anything with kernel structures. There is
no memory protection between module and rest of kernel. Hence, potential
attacker controlling
.Pa /dev/lkm
can do anything they want with the system.
.Pp
To avoid associated security risk, new LKMs cannot be loaded on
.Pa securelevel
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
.Bl -tag
Terrence R. Lambert, terry@cs.weber.edu
.El