168 lines
5.2 KiB
Groff
168 lines
5.2 KiB
Groff
.\" Copyright (c) 1993 Christopher G. Demetriou
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $Id: lkm.4,v 1.9 1997/02/22 13:24:36 peter Exp $
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.\"
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.Dd January 17, 1996
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.Dt LKM 4
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.Os FreeBSD 2.0
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.Sh NAME
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.Nm lkm
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.Nd loadable kernel module facility
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.Sh DESCRIPTION
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Loadable kernel modules allow the system administrator to
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dynamically add and remove functionality from a running system.
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This ability also helps software developers to develop
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new parts of the kernel without constantly rebooting to
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test their changes.
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.Pp
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Various types of modules can be loaded into the system.
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There are several defined module types, listed below, which can
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be added to the system in a predefined way. In addition, there
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is a generic type, for which the module itself handles loading and
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unloading.
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.Pp
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The
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.Tn FreeBSD
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system makes extensive use of loadable kernel modules, and provides
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loadable versions of most filesystems, the
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.Tn NFS
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client and server, all the screen-savers, and the
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.Tn iBCS2
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and
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.Tn Linux
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emulators. Modules distributed with the system are found in the
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.Pa /lkm
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directory.
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.Pp
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The
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.Nm
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interface is used by performing
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.Xr ioctl 2
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calls on the
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.Pa /dev/lkm
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device. Normally all operations involving
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Loadable Kernel Modules are handled by the
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.Xr modload 8 ,
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.Xr modunload 8 ,
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and
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.Xr modstat 8
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programs. Users should never have to interact with
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.Pa /dev/lkm
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directly.
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.Sh "MODULE TYPES"
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.Bl -ohang
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.It Em "System Call modules"
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System calls may be replaced by loading
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new ones via the
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.Nm
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interface. All system calls may be
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replaced, but special care should
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be taken with the
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.Xr ioctl 2
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system call, as it is used to load and
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unload modules.
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.Pp
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When a system call module is unloaded,
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the system call which
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was replaced by the loadable module
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is returned to its rightful place
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in the system call table.
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.It Em "Virtual File System modules"
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Virtual file systems may be added
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via the
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.Nm
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interface.
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.It Em "Device Driver modules"
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New block and character device
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drivers may be loaded into the system with
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.Nm LKM .
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The major problem with loading
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a device driver is that the driver's
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device nodes must be exist for the
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devices to be accessed. They are usually
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created by instructing
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.Xr modload 8
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to run an appropriate program when
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the driver has been successfully loaded.
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.It Em "Execution classes"
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Also known as image activators, execution classes are the mechanisms
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by which the
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.Xr execve 2
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system call is able to recognize an executable file's format and load it
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into memory.
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.It Em "Miscellaneous modules"
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Miscellaneous modules are modules
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for which there are not currently
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well-defined or well-used interfaces
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for extension. Users are expected
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to write their own loaders to manipulate
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whatever kernel data structures necessary
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to enable and disable the new module
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when it is loaded and unloaded.
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.El
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.Sh FILES
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.Bl -tag -width /usr/share/examples/lkm -compact
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.It Pa /dev/lkm
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.Nm
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interface device
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.It Pa /lkm
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directory containing module binaries shipped with the system
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.It Pa /usr/include/sys/lkm.h
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file containing definitions of module types
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.It Pa /usr/share/examples/lkm
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example source code implementing two of the module types
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.Sh SEE ALSO
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.Xr modload 8 ,
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.Xr modstat 8 ,
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.Xr modunload 8
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.Sh BUGS
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If a module fails to initialize itself correctly, and the system is
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able to detect this failure, it will panic immediately.
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.Pp
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When system internal interfaces change, old modules often cannot
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detect this, and such modules when loaded will often cause crashes or
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mysterious failures.
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.Sh AUTHORS
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The
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.Nm
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facility was originally implemented by
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.An Terrence R. Lambert .
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Loadable filesystems were implemented by
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.An Garrett Wollman ,
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and loadable
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execution classes were implemented by
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.An David Greenman ,
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.An Soren Schmidt ,
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and
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.An Garrett Wollman .
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.Sh HISTORY
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The
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.Nm
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facility was designed to be similar in functionality
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to the loadable kernel modules facility provided by
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.Tn SunOS
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4.1.3.
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