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 1.6 2007/01/21 10:07:50 swildner Exp $ 40.\" 41.Dd January 21, 2007 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 72filesystem differs from a traditional 73loopback file system in two respects: it is implemented using 74a stackable layers techniques, and it's 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 two 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, 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 Mount_null 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). After 111the 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 bypassing all possible operations to the lower layer 118for processing there. The majority of its activity centers 119on the bypass routine, through which nearly all vnode operations 120pass. 121.Pp 122The bypass routine accepts arbitrary vnode operations for 123handling by the lower layer. It begins by examining vnode 124operation arguments and replacing any null-nodes by their 125lower-layer equivalents. It then invokes the operation 126on the lower layer. Finally, it replaces the null-nodes 127in the arguments and, if a vnode is returned by the operation, 128stacks a null-node on top of the returned vnode. 129.Pp 130Although bypass handles most operations, 131.Em vop_getattr , 132.Em vop_inactive , 133.Em vop_reclaim , 134and 135.Em vop_print 136are not bypassed. 137.Em Vop_getattr 138must change the fsid being returned. 139.Em Vop_inactive 140and 141.Em vop_reclaim 142are not bypassed so that 143they can handle freeing null-layer specific data. 144.Em Vop_print 145is not bypassed to avoid excessive debugging 146information. 147.\" 148.\" 149.Sh INSTANTIATING VNODE STACKS 150Mounting associates the null layer with a lower layer, 151in effect stacking two VFSes. Vnode 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. All other vnode stacks 156are created as a result of vnode operations on 157this or other null vnode stacks. 158.Pp 159New vnode stacks come into existence as a result of 160an operation which returns a vnode. 161The bypass routine stacks a null-node above the new 162vnode before returning it to the caller. 163.Pp 164For example, imagine mounting a null layer with 165.Bd -literal -offset indent 166mount_null /usr/include /dev/layer/null 167.Ed 168Changing directory to 169.Pa /dev/layer/null 170will assign 171the root null-node (which was created when the null layer was mounted). 172Now consider opening 173.Pa sys . 174A vop_lookup would be 175done on the root null-node. This operation would bypass through 176to the lower layer which would return a vnode representing 177the UFS 178.Pa sys . 179Null_bypass then builds a null-node 180aliasing the UFS 181.Pa sys 182and returns this to the caller. 183Later operations on the null-node 184.Pa sys 185will repeat this 186process when constructing other vnode stacks. 187.\" 188.\" 189.Sh CREATING OTHER FILE SYSTEM LAYERS 190One of the easiest ways to construct new file system layers is to make 191a copy of the null layer, rename all files and variables, and 192then begin modifying the copy. 193.Xr Sed 1 194can be used to easily rename 195all variables. 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. Each method 201is appropriate in different situations. In both cases, 202it is the responsibility of the aliasing layer to make 203the operation arguments "correct" for the lower layer 204by mapping a vnode argument 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 210the bypass routine already must do argument mapping. 211An example of this is 212.Em null_getattrs 213in the null layer. 214.Pp 215A second approach is to directly invoke vnode operations on 216the lower layer with the 217.Em VOP_OPERATIONNAME 218interface. 219The advantage of this method is that it is easy to invoke 220arbitrary operations on the lower layer. The disadvantage 221is that vnode arguments must be manually mapped. 222.\" 223.\" 224.Sh SEE ALSO 225.Xr mount 8 226.Pp 227UCLA Technical Report CSD-910056, 228.Em "Stackable Layers: an Architecture for File System Development" . 229.Sh HISTORY 230The 231.Nm 232utility first appeared in 233.Bx 4.4 . 234.An Matthew Dillon 235made 236.Nm 237work in 238.Dx 1.7 , 239after it had been broken for some time. 240