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Some more corrections by Michael K. Sanders.

/*	$NetBSD: null_vnops.c,v 1.20 2001/06/09 11:02:08 wiz Exp $	*/

/*
 * Copyright (c) 1999 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. 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: null_vnops.c,v 1.20 2001/06/09 11:02:08 wiz Exp $
 *	...and...
 *	@(#)null_vnodeops.c 1.20 92/07/07 UCLA Ficus project
 */

/*
 * Null Layer
 *
 * (See mount_null(8) for more information.)
 *
 * The null layer duplicates a portion of the file system
 * name space under a new name.  In this respect, it is
 * similar to the loopback file system.  It differs from
 * the loopback fs in two respects:  it is implemented using
 * a stackable layers technique, and its "null-nodes" stack above
 * all lower-layer vnodes, not just over directory vnodes.
 *
 * The null layer has two purposes.  First, it serves as a demonstration
 * of layering by providing a layer which does nothing (it actually
 * does everything the loopback file system does, which is slightly
 * more than nothing).  Second, the null 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 a null layer.
 *
 * The remainder of this comment examines the null layer as a basis
 * for constructing new layers.
 *
 *
 * INSTANTIATING NEW NULL LAYERS
 *
 * New null layers are created with mount_null(8).
 * mount_null(8) takes two arguments, the pathname
 * of the lower vfs (target-pn) and the pathname where the null
 * layer will appear in the namespace (alias-pn).  After
 * the null layer is put into place, the contents
 * of target-pn subtree will be aliased under alias-pn.
 *
 *
 * OPERATION OF A NULL LAYER
 *
 * The null layer is the minimum file system layer,
 * simply bypassing all possible operations to the lower layer
 * for processing there.  The majority of its activity centers
 * on the bypass routine, through which nearly all vnode operations
 * pass.
 *
 * The bypass routine accepts arbitrary vnode operations for
 * handling by the lower layer.  It begins by examining vnode
 * operation arguments and replacing any null-nodes by their
 * lower-layer equivalents.  It then invokes the operation
 * on the lower layer.  Finally, it replaces the null-nodes
 * in the arguments and, if a vnode is returned by the operation,
 * stacks a null-node on top of the returned vnode.
 *
 * Although bypass handles most operations, vop_getattr, vop_lock,
 * vop_unlock, vop_inactive, vop_reclaim, and vop_print are not
 * bypassed. vop_getattr must change the fsid being returned.
 * vop_lock and vop_unlock must handle any locking for the
 * current vnode as well as pass the lock request down.
 * vop_inactive and vop_reclaim are not bypassed so that
 * they can handle freeing null-layer specific data. vop_print
 * is not bypassed to avoid excessive debugging information.
 * Also, certain vnode operations change the locking state within
 * the operation (create, mknod, remove, link, rename, mkdir, rmdir,
 * and symlink). Ideally these operations should not change the
 * lock state, but should be changed to let the caller of the
 * function unlock them. Otherwise all intermediate vnode layers
 * (such as union, umapfs, etc) must catch these functions to do
 * the necessary locking at their layer.
 *
 *
 * INSTANTIATING VNODE STACKS
 *
 * Mounting associates the null layer with a lower layer,
 * in effect stacking two VFSes.  Vnode stacks are instead
 * created on demand as files are accessed.
 *
 * The initial mount creates a single vnode stack for the
 * root of the new null layer.  All other vnode stacks
 * are created as a result of vnode operations on
 * this or other null vnode stacks.
 *
 * New vnode stacks come into existence as a result of
 * an operation which returns a vnode.  
 * The bypass routine stacks a null-node above the new
 * vnode before returning it to the caller.
 *
 * For example, imagine mounting a null layer with
 * "mount_null /usr/include /dev/layer/null".
 * Changing directory to /dev/layer/null will assign
 * the root null-node (which was created when the null layer was mounted).
 * Now consider opening "sys".  A vop_lookup would be
 * done on the root null-node.  This operation would bypass through
 * to the lower layer which would return a vnode representing 
 * the UFS "sys".  null_bypass then builds a null-node
 * aliasing the UFS "sys" and returns this to the caller.
 * Later operations on the null-node "sys" will repeat this
 * process when constructing other vnode stacks.
 *
 *
 * CREATING OTHER FILE SYSTEM LAYERS
 *
 * One of the easiest ways to construct new file system layers is to make
 * a copy of the null layer, rename all files and variables, and
 * then begin modifying the copy.  sed(1) can be used to easily rename
 * all variables.
 *
 * The umap layer is an example of a layer descended from the 
 * null layer.
 *
 *
 * INVOKING OPERATIONS ON LOWER LAYERS
 *
 * There are two techniques to invoke operations on a lower layer 
 * when the operation cannot be completely bypassed.  Each method
 * is appropriate in different situations.  In both cases,
 * it is the responsibility of the aliasing layer to make
 * the operation arguments "correct" for the lower layer
 * by mapping any vnode arguments to the lower layer.
 *
 * The first approach is to call the aliasing layer's bypass routine.
 * This method is most suitable when you wish to invoke the operation
 * currently being hanldled on the lower layer.  It has the advantage
 * that the bypass routine already must do argument mapping.
 * An example of this is null_getattrs in the null layer.
 *
 * A second approach is to directly invoke vnode operations on
 * the lower layer with the VOP_OPERATIONNAME interface.
 * The advantage of this method is that it is easy to invoke
 * arbitrary operations on the lower layer.  The disadvantage
 * is that vnode arguments must be manually mapped.
 *
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/namei.h>
#include <sys/malloc.h>
#include <sys/buf.h>
#include <miscfs/genfs/genfs.h>
#include <miscfs/nullfs/null.h>
#include <miscfs/genfs/layer_extern.h>

/*
 * Global vfs data structures
 */
int (**null_vnodeop_p) __P((void *));
const struct vnodeopv_entry_desc null_vnodeop_entries[] = {
	{ &vop_default_desc,  layer_bypass },

	{ &vop_lookup_desc,   layer_lookup },
	{ &vop_setattr_desc,  layer_setattr },
	{ &vop_getattr_desc,  layer_getattr },
	{ &vop_access_desc,   layer_access },
	{ &vop_lock_desc,     layer_lock },
	{ &vop_unlock_desc,   layer_unlock },
	{ &vop_islocked_desc, layer_islocked },
	{ &vop_fsync_desc,    layer_fsync },
	{ &vop_inactive_desc, layer_inactive },
	{ &vop_reclaim_desc,  layer_reclaim },
	{ &vop_print_desc,    layer_print },

	{ &vop_open_desc,     layer_open },	/* mount option handling */

	{ &vop_strategy_desc, layer_strategy },
	{ &vop_bwrite_desc,   layer_bwrite },
	{ &vop_bmap_desc,     layer_bmap },

	{ (struct vnodeop_desc*)NULL, (int(*)__P((void *)))NULL }
};
const struct vnodeopv_desc null_vnodeop_opv_desc =
	{ &null_vnodeop_p, null_vnodeop_entries };