221 lines
8.5 KiB
Plaintext
221 lines
8.5 KiB
Plaintext
#
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# Copyright (c) 1993 Terrence R. Lambert.
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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. All advertising materials mentioning features or use of this software
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# must display the following acknowledgement:
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# This product includes software developed by Terrence R. Lambert.
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# 4. The name Terrence R. Lambert may not be used to endorse or promote
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# products derived from this software without specific prior written
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# permission.
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#
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# THIS SOFTWARE IS PROVIDED BY TERRENCE R. LAMBERT ``AS IS'' AND ANY
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# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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# ARE DISCLAIMED. IN NO EVENT SHALL THE TERRENCE R. LAMBERT BE LIABLE
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# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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# SUCH DAMAGE.
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#
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# $NetBSD: README,v 1.4 1997/10/13 11:20:02 lukem Exp $
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#
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0.0 README
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README file for the loadable kernel modules interface.
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Direct questions and comments to:
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Terry Lambert
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terry@cs.weber.edu
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Please do *not* mail me at Novell.
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1.0 About this build heirarchy
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This is the build heirarchy for the loadable kernel modules
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(lkm) command line interface and test suite (including a
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set of sample code for each possible module type).
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The procedures in this file assume you have installed the
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kernel portions of the lkm system and have rebooted your
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machine so that they are ready for use.
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If you have not done this, then there is no reason for you to
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continue; please take the time to install the lkm system into
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your kernel at this time.
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2.0 Compiler warnings
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Some compiler warnings will occur due to inclusion of kernel
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and non-kernel header files in the same program that have had
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the same function names ANSIfied and the prototypes for the
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kernel and libc functions conflict. This needs to be resolved
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by fixing the header files, which I haven't bothered to do (the
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main conflict was "printf", and I made a dirty hack to get
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around it until the header files have been fixed).
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3.0 Usage warnings
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Loading a bogus module will kill your machine, but if you are
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doing developement, this will end up happening (hopefully)
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less frequently than changing, recompiling, installing, and
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rebooting would normally occur. This should speed developement
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considerably for a lot of the in-kernel work that is currently
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taking place.
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4.0 Loadable module types supported
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There are 6 loadable modules types supported; 5 of these are
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specific module types; the sixth is to allow the user to make
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their own loader as part of the module and allow them to replace
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or extend apropriate tables in the kernel.
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4.1 System call modules
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System calls as loadable modules use one of two approaches.
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If the system call slot is unspecified (-1), it will attempt
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to locate (and allocate) the next free call slot that points
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to the address of the "lkmnosys" function (an alias for the
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"nosys" function). It replaces this with the user's call;
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the user can tell which slot was allocated using the "modstat"
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command (the call slot is indicated by the value of "Off").
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If the system call slot is specified, it will replace that
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specific call (assuming it is in range of the entries in the
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sysent[] table). Care should be taken when replacing system
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calls. Good candiates are calls which the user is attempting
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to repair or make POSIX compliant. It is possible to replace
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all calls, although care should be taken with the "ioctl()"
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call, as it is the interface for the lkm loader.
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When unloaded, the system call module replaces the previous
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contents of the call slot it was loaded in. If this was an
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allocable slot, it is now reallocable; if it was a particular
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call slot, the previous function is restored.
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The directory ./sample/syscall contains a sample implementation
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of a loadable system call.
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4.2 Virtual file system modules
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A virtual file system can be loaded as a module. The example
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provided is for the "kernfs" file system; this is the code in
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NetBSD's /sys/kernfs combined in a single object with another
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piece of code giving a module entry point for the file system;
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with very little effort, any file system can be set up this way
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(although I suggest you leave "ufs" statically linked, since
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it is necessary for booting).
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The critical section of loading a VFS is to get the entry in
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the right slot and mounted.
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Because of the dependency on the vfssw[] table index during
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the mount, we can't simply mix and match file systems except
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in their predefined locations with regard to mount. This
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means that there are changes to vfssw[] and mount coming
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down the road (which will end up incrementing the lkm version
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and introducing an incompatability as far as file system modules
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are converned).
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The directory ./sample/vfs contains the sample implementation
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of the loadable kernfs vfs.
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4.3 Device driver modules
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The major issue to deal with when creating device drivers is
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insuring the creation of the device node. The current approach
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to this is executing a module specific shell script upon a
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successful load.
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A potentially better soloution is encoding the device name in
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the device switch, or, better, providing a functional interface
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to the init routine, and then using a "/devices" file system
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to export devices to the file system name space. Of course,
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the default "/dev" directory would have to be maintained for
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compatability (probably using symbolic links).
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This distribution does not contain a loadable device driver
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example. A potentially beneficial example could be made of
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the "lpa" interruptless printer driver.
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4.4 Streams modules
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Streams module support has been removed from this release; when
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the streams implementation is ready, it wil be restored as a
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patch.
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Please do not ask me for early availability on my streams
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implementation; until I have some non-proprietary modules
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to distribute, I'm putting work on it on the back burner
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while I finish shared libraries.
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4.5 Execution interpreters
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Execution interpreters allow loading of programs with magic
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numbers other than the default numbers supported by NetBSD.
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The reasoning behind this is to effectively allow user space
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developement of changes in exec format to support, among
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other things, shared libraries.
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Another portential use requires changing the references to
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the "sysent[]" system call table from direct references to
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indirect through a pointer in the proc struct. This allows
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the execution interpreter to, among other things, support
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(statically linked) executables from other environments,
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like Xenix, SVR3, SVR4, and Linux.
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There is no example of a loadable execution interpreter
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provided with this distribution.
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4.6 Miscellaneous modules
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Miscellaneous modules are modules for which there is not a
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current, well defined, or well used interface for extension.
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They are provided for extension, and the user is expected to
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write their own loader to handle the kernel pointer/table
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manipulation to "wire in" their loaded module (and "unwire"
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it on uload).
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One example of a "miscellaneous module" might be a loader for
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card-specific VGA drivers or alternate terminal emualtions in
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an appropriately layered console driver.
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The table manipulations required are specific to the console
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interface, yet a loadable module may be used if code is written
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to tell it how to manipulate the interfaces within the internal
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console interfaces.
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An example of a "miscellaneous module" is provided to show how
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to write "miscellaneous modules"; it duplicates the functionality
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of the "system call" module type, and is not intended to be
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seriously used, as it could interfere with the "system call"
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module type. The sample code is located in ./sample/misc.
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5.0 END OF DOCUMENT
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