/*
* Copyright (c) 1999, 2000 National Aeronautics & Space Administration
* All rights reserved.
*
* This software was written by William Studenmund of the
* Numerical Aerospace Simulation Facility, NASA Ames Research Center.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the National Aeronautics & Space Administration
* nor the names of its contributors may be used to endorse or promote
* products derived from this software without specific prior written
* permission.
*
* THIS SOFTWARE IS PROVIDED BY THE NATIONAL AERONAUTICS & SPACE ADMINISTRATION
* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE ADMINISTRATION OR CONTRIB-
* UTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
* OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
/*
* Copyright (c) 1992, 1993
* The Regents of the University of California. All rights reserved.
*
* This code is derived from software contributed to Berkeley by
* John Heidemann of the UCLA Ficus project.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* @(#)null_vnops.c 8.6 (Berkeley) 5/27/95
*
* Ancestors:
* @(#)lofs_vnops.c 1.2 (Berkeley) 6/18/92
* $Id: overlay_vnops.c,v 1.25 2020/05/16 18:31:51 christos Exp $
* ...and...
* @(#)null_vnodeops.c 1.20 92/07/07 UCLA Ficus project
*/
/*
* Overlay Layer
*
* (See mount_overlay(8) for more information.)
*
* The overlay layer has two purposes. First, it serves as a demonstration
* of layering by providing a layer which really does nothing (the null
* layer makes the underlying files appear elsewhere in the file hierarchy).
* Second, the overlay layer can serve as a prototype layer. Since it
* provides all necessary layer framework, new file system layers can be
* created very easily by starting with an overlay layer.
*
* The remainder of this comment examines the overlay layer as a basis
* for constructing new layers.
*
*
* INSTANTIATING NEW OVERLAY LAYERS
*
* New overlay layers are created with mount_overlay(8).
* mount_overlay(8) takes two arguments, an ignored string
* and the pathname which the overlay will mount over. After
* the overlay layer is put into place, all access to the mount
* point path will proceed through the overlay layer.
*
*
* OPERATION OF AN OVERLAY LAYER
*
* The operation of an overlay layer is identical to that of a null
* layer. See the null layer (and layerfs) documentation for more info.
*
*
* CREATING OTHER FILE SYSTEM LAYERS
*
* One of the easiest ways to construct new file system layers is to make
* a copy of either the null layer or the overlay layer, rename all files
* and variables, and then begin modifying the copy. sed(1) can be used to
* easily rename all variables.
*
* The choice between using a null and an overlay layer depends on
* the desirability of retaining access to the underlying filestore.
* For instance, the umap filesystem presents both a uid-translated and an
* untranslated view of the underlying files, and so it is based off of
* the null layer. However a layer implementing Access Control Lists
* might prefer to block access to the underlying filestore, for which
* the overlay layer is a better basis.
*
*
* INVOKING OPERATIONS ON LOWER LAYERS
*
* See the null layer documentation.
*
*/
