LKM(4)                  NetBSD Kernel Interfaces 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 sev-
     eral 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 exist for the devices to be accessed.  They are usually
     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
     support for executing foreign system binaries.  Execution Interpreters
     normally depend on Emulation modules, in that appropriate Emulation mod-
     ule 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
     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.

   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 /dev/lkm can do anything they want with
     the system.

     To avoid associated security risks, new LKMs cannot be loaded when
     securelevel is 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 cur-
     rently 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 5.0.1                   September 4, 1993                  NetBSD 5.0.1

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