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. 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: src/sbin/mount_null/mount_null.8,v 1.11.2.6 2001/12/20 16:40:00 ru Exp $ 35.\" $DragonFly: src/sbin/mount_null/mount_null.8,v 1.8 2008/10/26 00:05:24 swildner Exp $ 36.\" 37.Dd September 28, 2008 38.Dt MOUNT_NULL 8 39.Os 40.Sh NAME 41.Nm mount_null 42.Nd "mount a loopback filesystem sub-tree; demonstrate the use of a null file system layer" 43.Sh SYNOPSIS 44.Nm 45.Op Fl o Ar options 46.Ar target 47.Ar mount-point 48.Nm 49.Fl u 50.Op Fl o Ar options 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.It Fl u 89Update the mount point. 90This is typically used to upgrade a mount to 91read-write or downgrade it to read-only. 92.El 93.Pp 94The null layer has three purposes. 95First, it serves as a demonstration of layering by providing a layer 96which does nothing. 97(It actually does everything the loopback file system does, 98which is slightly more than nothing.) 99Second, it is used for NFS exporting 100.Nm HAMMER 101PFSs. 102Third, the null layer can serve as a prototype layer. 103Since it provides all necessary layer framework, 104new file system layers can be created very easily by starting 105with a null layer. 106.Pp 107The remainder of this man page examines the null layer as a basis 108for constructing new layers. 109.\" 110.\" 111.Sh INSTANTIATING NEW NULL LAYERS 112New null layers are created with 113.Nm . 114.Nm Mount_null 115takes two arguments, the pathname 116of the lower vfs (target-pn) and the pathname where the null 117layer will appear in the namespace (mount-point-pn). After 118the null layer is put into place, the contents 119of target-pn subtree will be aliased under mount-point-pn. 120.\" 121.\" 122.Sh OPERATION OF A NULL LAYER 123The null layer is the minimum file system layer, 124simply bypassing all possible operations to the lower layer 125for processing there. The majority of its activity centers 126on the bypass routine, through which nearly all vnode operations 127pass. 128.Pp 129The bypass routine accepts arbitrary vnode operations for 130handling by the lower layer. It begins by examining vnode 131operation arguments and replacing any null-nodes by their 132lower-layer equivalents. It then invokes the operation 133on the lower layer. Finally, it replaces the null-nodes 134in the arguments and, if a vnode is returned by the operation, 135stacks a null-node on top of the returned vnode. 136.Pp 137Although bypass handles most operations, 138.Em vop_getattr , 139.Em vop_inactive , 140.Em vop_reclaim , 141and 142.Em vop_print 143are not bypassed. 144.Em Vop_getattr 145must change the fsid being returned. 146.Em Vop_inactive 147and 148.Em vop_reclaim 149are not bypassed so that 150they can handle freeing null-layer specific data. 151.Em Vop_print 152is not bypassed to avoid excessive debugging 153information. 154.\" 155.\" 156.Sh INSTANTIATING VNODE STACKS 157Mounting associates the null layer with a lower layer, 158in effect stacking two VFSes. Vnode stacks are instead 159created on demand as files are accessed. 160.Pp 161The initial mount creates a single vnode stack for the 162root of the new null layer. All other vnode stacks 163are created as a result of vnode operations on 164this or other null vnode stacks. 165.Pp 166New vnode stacks come into existence as a result of 167an operation which returns a vnode. 168The bypass routine stacks a null-node above the new 169vnode before returning it to the caller. 170.Pp 171For example, imagine mounting a null layer with 172.Bd -literal -offset indent 173mount_null /usr/include /dev/layer/null 174.Ed 175.Pp 176Changing directory to 177.Pa /dev/layer/null 178will assign 179the root null-node (which was created when the null layer was mounted). 180Now consider opening 181.Pa sys . 182A 183.Em vop_lookup 184would be 185done on the root null-node. This operation would bypass through 186to the lower layer which would return a vnode representing 187the 188.Xr UFS 5 189.Pa sys 190(assuming that the lower layer is an 191.Xr UFS 5 192file system). 193Null_bypass then builds a null-node 194aliasing the 195.Xr UFS 5 196.Pa sys 197and returns this to the caller. 198Later operations on the null-node 199.Pa sys 200will repeat this 201process when constructing other vnode stacks. 202.\" 203.\" 204.Sh CREATING OTHER FILE SYSTEM LAYERS 205One of the easiest ways to construct new file system layers is to make 206a copy of the null layer, rename all files and variables, and 207then begin modifying the copy. 208.Xr Sed 1 209can be used to easily rename 210all variables. 211.\" 212.\" 213.Sh INVOKING OPERATIONS ON LOWER LAYERS 214There are two techniques to invoke operations on a lower layer 215when the operation cannot be completely bypassed. Each method 216is appropriate in different situations. In both cases, 217it is the responsibility of the aliasing layer to make 218the operation arguments "correct" for the lower layer 219by mapping a vnode argument to the lower layer. 220.Pp 221The first approach is to call the aliasing layer's bypass routine. 222This method is most suitable when you wish to invoke the operation 223currently being handled on the lower layer. 224It has the advantage that 225the bypass routine already must do argument mapping. 226An example of this is 227.Em null_getattrs 228in the null layer. 229.Pp 230A second approach is to directly invoke vnode operations on 231the lower layer with the 232.Em VOP_OPERATIONNAME 233interface. 234The advantage of this method is that it is easy to invoke 235arbitrary operations on the lower layer. The disadvantage 236is that vnode arguments must be manually mapped. 237.\" 238.\" 239.Sh SEE ALSO 240.Xr HAMMER 5 , 241.Xr mount 8 242.Pp 243UCLA Technical Report CSD-910056, 244.Em "Stackable Layers: an Architecture for File System Development" . 245.Sh HISTORY 246The 247.Nm 248utility first appeared in 249.Bx 4.4 . 250.An Matthew Dillon 251made 252.Nm 253work in 254.Dx 1.7 , 255after it had been broken for some time. 256