1.\" 2.\" Copyright (c) 1992, 1993, 1994 3.\" The Regents of the University of California. All rights reserved. 4.\" 5.\" This code is derived from software donated to Berkeley by 6.\" John Heidemann of the UCLA Ficus project. 7.\" 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.\" $FreeBSD: src/sbin/mount_null/mount_null.8,v 1.11.2.6 2001/12/20 16:40:00 ru Exp $ 39.\" $DragonFly: src/sbin/mount_null/mount_null.8,v 1.8 2008/10/26 00:05:24 swildner Exp $ 40.\" 41.Dd September 28, 2008 42.Dt MOUNT_NULL 8 43.Os 44.Sh NAME 45.Nm mount_null 46.Nd "mount a loopback filesystem sub-tree; demonstrate 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 the 63.Xr getcwd 3 64functions work 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 null 72filesystem differs from a traditional 73loopback file system in two respects: it is implemented using 74a stackable layers techniques, and its 75.Do null-node Dc Ns s 76stack above 77all lower-layer vnodes, not just over directory vnodes. 78.Pp 79The options are as follows: 80.Bl -tag -width indent 81.It Fl o 82Options are specified with a 83.Fl o 84flag followed by a comma separated string of options. 85See the 86.Xr mount 8 87man page for possible options and their meanings. 88.El 89.Pp 90The null layer has three purposes. 91First, it serves as a demonstration of layering by providing a layer 92which does nothing. 93(It actually does everything the loopback file system does, 94which is slightly more than nothing.) 95Second, it is used for NFS exporting 96.Nm HAMMER 97PFSs. 98Third, the null layer can serve as a prototype layer. 99Since it provides all necessary layer framework, 100new file system layers can be created very easily by starting 101with a null layer. 102.Pp 103The remainder of this man page examines the null layer as a basis 104for constructing new layers. 105.\" 106.\" 107.Sh INSTANTIATING NEW NULL LAYERS 108New null layers are created with 109.Nm . 110.Nm Mount_null 111takes two arguments, the pathname 112of the lower vfs (target-pn) and the pathname where the null 113layer will appear in the namespace (mount-point-pn). After 114the null layer is put into place, the contents 115of target-pn subtree will be aliased under mount-point-pn. 116.\" 117.\" 118.Sh OPERATION OF A NULL LAYER 119The null layer is the minimum file system layer, 120simply bypassing all possible operations to the lower layer 121for processing there. The majority of its activity centers 122on the bypass routine, through which nearly all vnode operations 123pass. 124.Pp 125The bypass routine accepts arbitrary vnode operations for 126handling by the lower layer. It begins by examining vnode 127operation arguments and replacing any null-nodes by their 128lower-layer equivalents. It then invokes the operation 129on the lower layer. Finally, it replaces the null-nodes 130in the arguments and, if a vnode is returned by the operation, 131stacks a null-node on top of the returned vnode. 132.Pp 133Although bypass handles most operations, 134.Em vop_getattr , 135.Em vop_inactive , 136.Em vop_reclaim , 137and 138.Em vop_print 139are not bypassed. 140.Em Vop_getattr 141must change the fsid being returned. 142.Em Vop_inactive 143and 144.Em vop_reclaim 145are not bypassed so that 146they can handle freeing null-layer specific data. 147.Em Vop_print 148is not bypassed to avoid excessive debugging 149information. 150.\" 151.\" 152.Sh INSTANTIATING VNODE STACKS 153Mounting associates the null layer with a lower layer, 154in effect stacking two VFSes. Vnode stacks are instead 155created on demand as files are accessed. 156.Pp 157The initial mount creates a single vnode stack for the 158root of the new null layer. All other vnode stacks 159are created as a result of vnode operations on 160this or other null vnode stacks. 161.Pp 162New vnode stacks come into existence as a result of 163an operation which returns a vnode. 164The bypass routine stacks a null-node above the new 165vnode before returning it to the caller. 166.Pp 167For example, imagine mounting a null layer with 168.Bd -literal -offset indent 169mount_null /usr/include /dev/layer/null 170.Ed 171.Pp 172Changing directory to 173.Pa /dev/layer/null 174will assign 175the root null-node (which was created when the null layer was mounted). 176Now consider opening 177.Pa sys . 178A 179.Em vop_lookup 180would be 181done on the root null-node. This operation would bypass through 182to the lower layer which would return a vnode representing 183the 184.Xr UFS 5 185.Pa sys 186(assuming that the lower layer is an 187.Xr UFS 5 188file system). 189Null_bypass then builds a null-node 190aliasing the 191.Xr UFS 5 192.Pa sys 193and returns this to the caller. 194Later operations on the null-node 195.Pa sys 196will repeat this 197process when constructing other vnode stacks. 198.\" 199.\" 200.Sh CREATING OTHER FILE SYSTEM LAYERS 201One of the easiest ways to construct new file system layers is to make 202a copy of the null layer, rename all files and variables, and 203then begin modifying the copy. 204.Xr Sed 1 205can be used to easily rename 206all variables. 207.\" 208.\" 209.Sh INVOKING OPERATIONS ON LOWER LAYERS 210There are two techniques to invoke operations on a lower layer 211when the operation cannot be completely bypassed. Each method 212is appropriate in different situations. In both cases, 213it is the responsibility of the aliasing layer to make 214the operation arguments "correct" for the lower layer 215by mapping a vnode argument to the lower layer. 216.Pp 217The first approach is to call the aliasing layer's bypass routine. 218This method is most suitable when you wish to invoke the operation 219currently being handled on the lower layer. 220It has the advantage that 221the bypass routine already must do argument mapping. 222An example of this is 223.Em null_getattrs 224in the null layer. 225.Pp 226A second approach is to directly invoke vnode operations on 227the lower layer with the 228.Em VOP_OPERATIONNAME 229interface. 230The advantage of this method is that it is easy to invoke 231arbitrary operations on the lower layer. The disadvantage 232is that vnode arguments must be manually mapped. 233.\" 234.\" 235.Sh SEE ALSO 236.Xr HAMMER 5 , 237.Xr mount 8 238.Pp 239UCLA Technical Report CSD-910056, 240.Em "Stackable Layers: an Architecture for File System Development" . 241.Sh HISTORY 242The 243.Nm 244utility first appeared in 245.Bx 4.4 . 246.An Matthew Dillon 247made 248.Nm 249work in 250.Dx 1.7 , 251after it had been broken for some time. 252