1.\" $NetBSD: mount_null.8,v 1.17 2002/10/01 13:40:42 wiz Exp $ 2.\" 3.\" Copyright (c) 1992, 1993, 1994 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software donated to Berkeley by 7.\" John Heidemann of the UCLA Ficus project. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the University of 20.\" California, Berkeley and its contributors. 21.\" 4. Neither the name of the University nor the names of its contributors 22.\" may be used to endorse or promote products derived from this software 23.\" without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 26.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 27.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 28.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 29.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 30.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 31.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 32.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 33.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 34.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 35.\" SUCH DAMAGE. 36.\" 37.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95 38.\" 39.\" 40.Dd May 1, 1995 41.Dt MOUNT_NULL 8 42.Os 43.Sh NAME 44.Nm mount_null 45.Nd mount a loopback filesystem sub-tree; 46demonstrate the use of a null file system layer 47.Sh SYNOPSIS 48.Nm 49.Op Fl o Ar options 50.Ar target 51.Ar mount-point 52.Sh DESCRIPTION 53The 54.Nm 55command creates a 56null layer, duplicating a sub-tree of the file system 57name space under another part of the global file system namespace. 58This allows existing files and directories to be accessed 59using a different pathname. 60.Pp 61The primary differences between a virtual copy of the filesystem 62and a symbolic link are that 63.Xr getcwd 3 64functions correctly in the virtual copy, and that other filesystems 65may be mounted on the virtual copy without affecting the original. 66A different device number for the virtual copy is returned by 67.Xr stat 2 , 68but in other respects it is indistinguishable from the original. 69.Pp 70The 71.Nm 72filesystem differs from a traditional 73loopback file system in two respects: it is implemented using 74a stackable layers technique, and its 75.Do 76null-nodes 77.Dc 78stack above 79all lower-layer vnodes (not just above directory vnodes). 80.Pp 81The options are as follows: 82.Bl -tag -width indent 83.It Fl o 84Options are specified with a 85.Fl o 86flag followed by a comma separated string of options. 87See the 88.Xr mount 8 89man page for possible options and their meanings. 90.El 91.Pp 92The null layer has two purposes. 93First, it serves as a demonstration of layering by providing a layer 94which does nothing. 95Second, the null layer can serve as a prototype layer. 96Since it provides all necessary layer framework, 97new file system layers can be created very easily by starting 98with a null layer. 99.Pp 100The remainder of this man page examines the null layer as a basis 101for constructing new layers. 102.\" 103.\" 104.Sh INSTANTIATING NEW NULL LAYERS 105New null layers are created with 106.Nm "" . 107.Nm 108takes two arguments, the pathname 109of the lower vfs (target-pn) and the pathname where the null 110layer will appear in the namespace (mount-point-pn). 111After the null layer is put into place, the contents 112of target-pn subtree will be aliased under mount-point-pn. 113.\" 114.\" 115.Sh OPERATION OF A NULL LAYER 116The null layer is the minimum file system layer, 117simply passing all possible operations to the lower layer 118for processing there. 119The majority of its activity centers on the bypass routine, 120through which nearly all vnode operations pass. 121.Pp 122The bypass routine accepts arbitrary vnode operations for 123handling by the lower layer. 124It begins by examining vnode operation arguments and replacing 125any null-nodes by their lower-layer equivalents. 126It then invokes the operation on the lower layer. 127Finally, it replaces the null-nodes 128in the arguments and, if a vnode is returned by the operation, 129stacks a null-node on top of the returned vnode. 130.Pp 131Although bypass handles most operations, 132.Em vop_getattr , 133.Em vop_inactive , 134.Em vop_reclaim , 135and 136.Em vop_print 137are not bypassed. 138.Em vop_getattr 139must change the fsid being returned. 140.Em vop_inactive 141and vop_reclaim are not bypassed so that 142they can handle freeing null-layer specific data. 143.Em vop_print 144is not bypassed to avoid excessive debugging 145information. 146.\" 147.\" 148.Sh INSTANTIATING VNODE STACKS 149Mounting associates the null layer with a lower layer, 150in effect stacking two VFSes. 151Vnode stacks are instead 152created on demand as files are accessed. 153.Pp 154The initial mount creates a single vnode stack for the 155root of the new null layer. 156All other vnode stacks 157are created as a result of vnode operations on 158this or other null vnode stacks. 159.Pp 160New vnode stacks come into existence as a result of 161an operation which returns a vnode. 162The bypass routine stacks a null-node above the new 163vnode before returning it to the caller. 164.Pp 165For example, imagine mounting a null layer with 166.Bd -literal -offset indent 167mount_null /usr/include /dev/layer/null 168.Ed 169Changing directory to 170.Pa /dev/layer/null 171will assign 172the root null-node (which was created when the null layer was mounted). 173Now consider opening 174.Pa sys . 175A vop_lookup would be done on the root null-node. 176This operation would bypass through to the lower layer 177which would return a vnode representing the UFS 178.Pa sys . 179null_bypass then builds a null-node aliasing the UFS 180.Pa sys 181and returns this to the caller. 182Later operations on the null-node 183.Pa sys 184will repeat this process when constructing other vnode stacks. 185.\" 186.\" 187.Sh CREATING OTHER FILE SYSTEM LAYERS 188One of the easiest ways to construct new file system layers is to make 189a copy of the null layer, rename all files and variables, and 190then begin modifying the copy. 191.Xr sed 1 192can be used to easily rename all variables. 193.Pp 194The umap layer is an example of a layer descended from the 195null layer. 196.\" 197.\" 198.Sh INVOKING OPERATIONS ON LOWER LAYERS 199There are two techniques to invoke operations on a lower layer 200when the operation cannot be completely bypassed. 201Each method is appropriate in different situations. 202In both cases, it is the responsibility of the aliasing layer to make 203the operation arguments "correct" for the lower layer 204by mapping any vnode arguments to the lower layer. 205.Pp 206The first approach is to call the aliasing layer's bypass routine. 207This method is most suitable when you wish to invoke the operation 208currently being handled on the lower layer. 209It has the advantage that the bypass routine already must do argument mapping. 210An example of this is 211.Em null_getattrs 212in the null layer. 213.Pp 214A second approach is to directly invoke vnode operations on 215the lower layer with the 216.Em VOP_OPERATIONNAME 217interface. 218The advantage of this method is that it is easy to invoke 219arbitrary operations on the lower layer. 220The disadvantage is that vnode arguments must be manually mapped. 221.\" 222.\" 223.Sh SEE ALSO 224.Xr mount 8 225.sp 226UCLA Technical Report CSD-910056, 227.Em "Stackable Layers: an Architecture for File System Development" . 228.Sh HISTORY 229The 230.Nm 231utility first appeared in 232.Bx 4.4 . 233