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.\" 4. Neither the name of the University nor the names of its contributors 18.\" may be used to endorse or promote products derived from this software 19.\" without specific prior written permission. 20.\" 21.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 22.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 23.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 24.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 25.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 26.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 27.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 28.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 29.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 30.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95 34.\" $FreeBSD$ 35.\" 36.Dd May 1, 1995 37.Dt MOUNT_NULLFS 8 38.Os 39.Sh NAME 40.Nm mount_nullfs 41.Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer" 42.Sh SYNOPSIS 43.Nm 44.Op Fl o Ar options 45.Ar target 46.Ar mount-point 47.Sh DESCRIPTION 48The 49.Nm 50utility creates a 51null layer, duplicating a sub-tree of the file system 52name space under another part of the global file system namespace. 53This allows existing files and directories to be accessed 54using a different pathname. 55.Pp 56The primary differences between a virtual copy of the file system 57and a symbolic link are that the 58.Xr getcwd 3 59functions work correctly in the virtual copy, and that other file systems 60may be mounted on the virtual copy without affecting the original. 61A different device number for the virtual copy is returned by 62.Xr stat 2 , 63but in other respects it is indistinguishable from the original. 64.Pp 65The 66.Nm 67file system differs from a traditional 68loopback file system in two respects: it is implemented using 69a stackable layers techniques, and its 70.Do null-node Dc Ns s 71stack above 72all lower-layer vnodes, not just over directory vnodes. 73.Pp 74The options are as follows: 75.Bl -tag -width indent 76.It Fl o 77Options are specified with a 78.Fl o 79flag followed by a comma separated string of options. 80See the 81.Xr mount 8 82man page for possible options and their meanings. 83.El 84.Pp 85The null layer has two purposes. 86First, it serves as a demonstration of layering by providing a layer 87which does nothing. 88(It actually does everything the loopback file system does, 89which is slightly more than nothing.) 90Second, the null layer can serve as a prototype layer. 91Since it provides all necessary layer framework, 92new file system layers can be created very easily by starting 93with a null layer. 94.Pp 95The remainder of this man page examines the null layer as a basis 96for constructing new layers. 97.\" 98.\" 99.Sh INSTANTIATING NEW NULL LAYERS 100New null layers are created with 101.Nm . 102The 103.Nm 104utility takes two arguments, the pathname 105of the lower vfs (target-pn) and the pathname where the null 106layer will appear in the namespace (mount-point-pn). After 107the null layer is put into place, the contents 108of target-pn subtree will be aliased under mount-point-pn. 109.\" 110.\" 111.Sh OPERATION OF A NULL LAYER 112The null layer is the minimum file system layer, 113simply bypassing all possible operations to the lower layer 114for processing there. The majority of its activity centers 115on the bypass routine, through which nearly all vnode operations 116pass. 117.Pp 118The bypass routine accepts arbitrary vnode operations for 119handling by the lower layer. It begins by examining vnode 120operation arguments and replacing any null-nodes by their 121lower-layer equivalents. It then invokes the operation 122on the lower layer. Finally, it replaces the null-nodes 123in the arguments and, if a vnode is returned by the operation, 124stacks a null-node on top of the returned vnode. 125.Pp 126Although bypass handles most operations, 127.Em vop_getattr , 128.Em vop_inactive , 129.Em vop_reclaim , 130and 131.Em vop_print 132are not bypassed. 133.Em Vop_getattr 134must change the fsid being returned. 135.Em Vop_inactive 136and 137.Em vop_reclaim 138are not bypassed so that 139they can handle freeing null-layer specific data. 140.Em Vop_print 141is not bypassed to avoid excessive debugging 142information. 143.\" 144.\" 145.Sh INSTANTIATING VNODE STACKS 146Mounting associates the null layer with a lower layer, 147in effect stacking two VFSes. Vnode stacks are instead 148created on demand as files are accessed. 149.Pp 150The initial mount creates a single vnode stack for the 151root of the new null layer. All other vnode stacks 152are created as a result of vnode operations on 153this or other null vnode stacks. 154.Pp 155New vnode stacks come into existence as a result of 156an operation which returns a vnode. 157The bypass routine stacks a null-node above the new 158vnode before returning it to the caller. 159.Pp 160For example, imagine mounting a null layer with 161.Bd -literal -offset indent 162mount_nullfs /usr/include /dev/layer/null 163.Ed 164Changing directory to 165.Pa /dev/layer/null 166will assign 167the root null-node (which was created when the null layer was mounted). 168Now consider opening 169.Pa sys . 170A vop_lookup would be 171done on the root null-node. This operation would bypass through 172to the lower layer which would return a vnode representing 173the UFS 174.Pa sys . 175Null_bypass then builds a null-node 176aliasing the UFS 177.Pa sys 178and returns this to the caller. 179Later operations on the null-node 180.Pa sys 181will repeat this 182process when constructing other vnode stacks. 183.\" 184.\" 185.Sh CREATING OTHER FILE SYSTEM LAYERS 186One of the easiest ways to construct new file system layers is to make 187a copy of the null layer, rename all files and variables, and 188then begin modifying the copy. 189The 190.Xr sed 1 191utility can be used to easily rename 192all 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. 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 BUGS 230THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK) 231AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR 232OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET. 233.Pp 234This code also needs an owner in order to be less dangerous - serious 235hackers can apply by sending mail to 236.Aq hackers@FreeBSD.org 237and announcing 238their intent to take it over. 239.Sh HISTORY 240The 241.Nm 242utility first appeared in 243.Bx 4.4 . 244