MODULE(7)					       BSD Miscellaneous Information Manual						 MODULE(7)

NAME

     module -- Kernel Modules interface

SYNOPSIS

     options MODULAR

DESCRIPTION

     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.

     Additionally, the kernel may automatically load software modules as needed to perform requested operations.  For example, an ``xyzfs'' module
     can be loaded automatically when an attempt is made to mount an ``xyzfs'' file system.  Modules can also depend on other modules, and depen-
     dent modules are automatically loaded.  When a module is no longer needed, it can be automatically unloaded.

     An in-kernel linker resolves symbol references between the module and the rest of the kernel.

     The module interface is accessed with the modctl(2) system call.  All common operations involving kernel modules are handled by the
     modload(8), modunload(8), and modstat(8) programs.  Users should never have to interact with modctl(2) directly.

MODULE CLASSES

   Virtual File System modules
     Virtual file systems may be added via the module interface.

   Device Driver modules
     Many device drivers can be loaded as a kernel module.  One potential problem specific to block and character device drivers is that the
     device nodes must exist for the devices to be accessed.  These need to be created manually, after the driver module has been successfully
     loaded.  The majority of the device driver modules however does not need any manual intervention to function properly.

   Execution Interpreters
     Execution Interpreters can be loaded to provide support for executing binaries not normally supported by kernel.  This also allows loading
     support for executing foreign system binaries.  Execution Interpreters may require that an appropriate emulation module also be loaded.

   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-provided module initialization routine is expected to install the necessary "hooks" into the rest of the operat-
     ing system.  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-Model modules
     Alternate system security models may loaded using the module facility.

EXAMPLES

     The common build tool of NetBSD, ``build.sh'', automatically compiles and installs all modules during a full system build and install.  Some-
     times it is however useful to update only modules.  The following example demonstrates one way to do this.  It is assumed that the source
     code is under /usr/src, while the object and toolchain directories are under /usr/obj and /usr/tools, respectively.

	   cd /usr/src/sys/modules

	   export OBJDIR=/usr/obj
	   export TOOLDIR=/usr/tools

	   make clean
	   make
	   make install

SEE ALSO

     modctl(2), modload(8), modstat(8), modunload(8), module(9)

HISTORY

     The module facility was designed to be similar in functionality to the loadable kernel modules facility provided by SunOS 4.1.3.  The old LKM
     interface was replaced by module in NetBSD 5.0.

AUTHORS

     The module subsystem was implemented by Andrew Doran <ad@netbsd.org>.

CAVEATS

     The module framework is still under active development.  At least two potential caveats can be mentioned.

       1.   Kernel modules are built to operate only with a specific version of the NetBSD kernel.  When the kernel is updated to a new version,
	    the contents of the /stand/${ARCH}/${VERSION}/modules/ directory should be updated as well.  (This location has been the subject of
	    much discussion, and may change in future versions of NetBSD.)

       2.   If an attempt is made to boot the operating system from a file system for which the module is not built into the kernel, the boot may
	    fail with the message ``Cannot mount root, error 79''.  On certain architectures (currently, i386 and amd64), one may be able to
	    recover from this error by using the ``load xxxfs'' command before trying to boot.	This command is only available on newer bootload-
	    ers.

     The absence of required modules or the inability of the bootloader to load the modules are common reasons for failures to boot a MODULAR ker-
     nel.  It may be a good practice to maintain a non-MODULAR kernel in the root file system for recovery purposes.

SECURITY CONSIDERATIONS

     A module becomes part of the kernel once loaded.  Compared to userland programs, all errors in the code can be fatal.  There is no memory
     protection between modules and the rest of the kernel.  Hence, a potential attacker with access to the modctl(2) system call can acquire com-
     plete and total control over the system.

     To avoid associated security risks, new modules can only be loaded when securelevel is less than or equal to zero, or if the kernel was built
     with options INSECURE.  Refer to secmodel_securelevel(9) for additional details on the securelevel.  Only use modules from trusted sources.

BSD
								 December 14, 2010							       BSD