LKM(4)                    NetBSD Programmer's Manual                    LKM(4)

     lkm - Loadable Kernel Modules interface

     options 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 sever-
     al 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.

   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 by LKM code.

   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
     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 usu-
     ally created by instructing modload(8) to run an appropriate program when
     the driver has been successfully loaded.

   Emulation modules
     Emulation modules allow to load an emulation support for foreign operat-
     ing systems.

   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 sup-
     port for executing foreign system binaries.  Execution Interpreters nor-
     mally depend on Emulation modules, in that appropriate Emulation module
     has to be loaded before Execution Interpreter can be.

   Miscellaneous modules
     Miscellaneous modules are modules for which there are not currently well-
     defined or well-used interfaces for extension.  They are provided for ex-
     tension, 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.

   Security considerations
     Loaded kernel module can do anything with kernel structures. There is no
     memory protection between module and rest of kernel. Hence, potential at-
     tacker controlling /dev/lkm can do anything they want with the system.

     To avoid associated security risk, new LKMs cannot be loaded on
     securelevel higher than zero.

   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, recompil-
     ing, installing, and rebooting would normally occur.  This should speed
     development considerably for a lot of the in-kernel work that is current-
     ly taking place.

     /dev/lkm                lkm interface device
     /usr/include/sys/lkm.h  file containing definitions of module types
     lkm/*                   subdirectory lkm within kernel source tree con-
                             tains many LKMs which are suitable as a base for
                             new ones

     modload(8), modstat(8), modunload(8)

     The lkm facility was designed to be similar in functionality to the load-
     able kernel modules facility provided by SunOS 4.1.3.

     Terrence R. Lambert,

NetBSD 1.6                     September 4, 1993                             2

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