MODULE(7)           NetBSD Miscellaneous Information Manual          MODULE(7)

     module -- Kernel Modules interface

     options MODULAR

     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 dependent
     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 inter-
     act with modctl(2) directly.

   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 cre-
     ated 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 sup-
     port 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 operating
     system.  An example of a "miscellaneous module" might be a loader for
     card-specific VGA drivers or alternate terminal emulations in an appro-
     priately layered console driver.

   Security-Model modules
     Alternate system security models may loaded using the module facility.

     The common build tool of NetBSD, ``'', automatically compiles and
     installs all modules during a full system build and install.  Sometimes
     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 install

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

     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.

     The module subsystem was implemented by Andrew Doran <>.

     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 ver-
            sion, 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

       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

     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.

     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 pro-
     tection between modules and the rest of the kernel.  Hence, a potential
     attacker with access to the modctl(2) system call can acquire complete
     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.

NetBSD 7.1.2                   December 14, 2010                  NetBSD 7.1.2

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