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.\" $FreeBSD$ 39.\" 40.Dd May 1, 1995 41.Dt MOUNT_NULLFS 8 42.Os 43.Sh NAME 44.Nm mount_nullfs 45.Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer" 46.Sh SYNOPSIS 47.Nm 48.Op Fl o Ar options 49.Ar target 50.Ar mount-point 51.Sh DESCRIPTION 52The 53.Nm 54utility creates a 55null layer, duplicating a sub-tree of the file system 56name space under another part of the global file system namespace. 57This allows existing files and directories to be accessed 58using a different pathname. 59.Pp 60The primary differences between a virtual copy of the file system 61and a symbolic link are that the 62.Xr getcwd 3 63functions work correctly in the virtual copy, and that other file systems 64may be mounted on the virtual copy without affecting the original. 65A different device number for the virtual copy is returned by 66.Xr stat 2 , 67but in other respects it is indistinguishable from the original. 68.Pp 69The 70.Nm 71file system differs from a traditional 72loopback file system in two respects: it is implemented using 73a stackable layers techniques, and it's 74.Do null-node Dc Ns s 75stack above 76all lower-layer vnodes, not just over directory vnodes. 77.Pp 78The options are as follows: 79.Bl -tag -width indent 80.It Fl o 81Options are specified with a 82.Fl o 83flag followed by a comma separated string of options. 84See the 85.Xr mount 8 86man page for possible options and their meanings. 87.El 88.Pp 89The null layer has two purposes. 90First, it serves as a demonstration of layering by providing a layer 91which does nothing. 92(It actually does everything the loopback file system does, 93which is slightly more than nothing.) 94Second, the null layer can serve as a prototype layer. 95Since it provides all necessary layer framework, 96new file system layers can be created very easily by starting 97with a null layer. 98.Pp 99The remainder of this man page examines the null layer as a basis 100for constructing new layers. 101.\" 102.\" 103.Sh INSTANTIATING NEW NULL LAYERS 104New null layers are created with 105.Nm . 106The 107.Nm 108utility takes 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_nullfs /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.Pp 197The umap layer is an example of a layer descended from the 198null layer. 199.\" 200.\" 201.Sh INVOKING OPERATIONS ON LOWER LAYERS 202There are two techniques to invoke operations on a lower layer 203when the operation cannot be completely bypassed. Each method 204is appropriate in different situations. In both cases, 205it is the responsibility of the aliasing layer to make 206the operation arguments "correct" for the lower layer 207by mapping a vnode argument to the lower layer. 208.Pp 209The first approach is to call the aliasing layer's bypass routine. 210This method is most suitable when you wish to invoke the operation 211currently being handled on the lower layer. 212It has the advantage that 213the bypass routine already must do argument mapping. 214An example of this is 215.Em null_getattrs 216in the null layer. 217.Pp 218A second approach is to directly invoke vnode operations on 219the lower layer with the 220.Em VOP_OPERATIONNAME 221interface. 222The advantage of this method is that it is easy to invoke 223arbitrary operations on the lower layer. The disadvantage 224is that vnode arguments must be manually mapped. 225.\" 226.\" 227.Sh SEE ALSO 228.Xr mount 8 229.Pp 230UCLA Technical Report CSD-910056, 231.Em "Stackable Layers: an Architecture for File System Development" . 232.Sh BUGS 233THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK) 234AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR 235OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET. 236.Pp 237This code also needs an owner in order to be less dangerous - serious 238hackers can apply by sending mail to 239.Aq hackers@FreeBSD.org 240and announcing 241their intent to take it over. 242.Sh HISTORY 243The 244.Nm 245utility first appeared in 246.Bx 4.4 . 247