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_NULL 8 42.Os BSD 4.4 43.Sh NAME 44.Nm mount_null 45.Nd mount a loopback filesystem sub-tree; 46demonstrate 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 63.Xr getcwd 3 64functions 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 76null-node 77.Dc s 78stack above 79all lower-layer vnodes, not just over directory vnodes. 80.Pp 81The options are as follows: 82.Bl -tag -width indent 83.It Fl o 84Options are specified with a 85.Fl o 86flag followed by a comma separated string of options. 87See the 88.Xr mount 8 89man page for possible options and their meanings. 90.El 91.Pp 92The null layer has two purposes. 93First, it serves as a demonstration of layering by providing a layer 94which does nothing. 95(It actually does everything the loopback file system does, 96which is slightly more than nothing.) 97Second, the null layer can serve as a prototype layer. 98Since it provides all necessary layer framework, 99new file system layers can be created very easily by starting 100with a null layer. 101.Pp 102The remainder of this man page examines the null layer as a basis 103for constructing new layers. 104.\" 105.\" 106.Sh INSTANTIATING NEW NULL LAYERS 107New null layers are created with 108.Xr mount_null 8 . 109.Xr Mount_null 8 110takes two arguments, the pathname 111of the lower vfs (target-pn) and the pathname where the null 112layer will appear in the namespace (mount-point-pn). After 113the null layer is put into place, the contents 114of target-pn subtree will be aliased under mount-point-pn. 115.\" 116.\" 117.Sh OPERATION OF A NULL LAYER 118The null layer is the minimum file system layer, 119simply bypassing all possible operations to the lower layer 120for processing there. The majority of its activity centers 121on the bypass routine, through which nearly all vnode operations 122pass. 123.Pp 124The bypass routine accepts arbitrary vnode operations for 125handling by the lower layer. It begins by examining vnode 126operation arguments and replacing any null-nodes by their 127lower-layer equivalents. It then invokes the operation 128on the lower layer. Finally, it replaces the null-nodes 129in the arguments and, if a vnode is returned by the operation, 130stacks a null-node on top of the returned vnode. 131.Pp 132Although bypass handles most operations, 133.Em vop_getattr , 134.Em vop_inactive , 135.Em vop_reclaim , 136and 137.Em vop_print 138are not bypassed. 139.Em Vop_getattr 140must change the fsid being returned. 141.Em Vop_inactive 142and 143.Em vop_reclaim 144are not bypassed so that 145they can handle freeing null-layer specific data. 146.Em Vop_print 147is not bypassed to avoid excessive debugging 148information. 149.\" 150.\" 151.Sh INSTANTIATING VNODE STACKS 152Mounting associates the null layer with a lower layer, 153in effect stacking two VFSes. Vnode stacks are instead 154created on demand as files are accessed. 155.Pp 156The initial mount creates a single vnode stack for the 157root of the new null layer. All other vnode stacks 158are created as a result of vnode operations on 159this or other null vnode stacks. 160.Pp 161New vnode stacks come into existence as a result of 162an operation which returns a vnode. 163The bypass routine stacks a null-node above the new 164vnode before returning it to the caller. 165.Pp 166For example, imagine mounting a null layer with 167.Bd -literal -offset indent 168mount_null /usr/include /dev/layer/null 169.Ed 170Changing directory to 171.Pa /dev/layer/null 172will assign 173the root null-node (which was created when the null layer was mounted). 174Now consider opening 175.Pa sys . 176A vop_lookup would be 177done on the root null-node. This operation would bypass through 178to the lower layer which would return a vnode representing 179the UFS 180.Pa sys . 181Null_bypass then builds a null-node 182aliasing the UFS 183.Pa sys 184and returns this to the caller. 185Later operations on the null-node 186.Pa sys 187will repeat this 188process when constructing other vnode stacks. 189.\" 190.\" 191.Sh CREATING OTHER FILE SYSTEM LAYERS 192One of the easiest ways to construct new file system layers is to make 193a copy of the null layer, rename all files and variables, and 194then begin modifying the copy. 195.Xr Sed 1 196can be used to easily rename 197all variables. 198.Pp 199The umap layer is an example of a layer descended from the 200null layer. 201.\" 202.\" 203.Sh INVOKING OPERATIONS ON LOWER LAYERS 204There are two techniques to invoke operations on a lower layer 205when the operation cannot be completely bypassed. Each method 206is appropriate in different situations. In both cases, 207it is the responsibility of the aliasing layer to make 208the operation arguments "correct" for the lower layer 209by mapping a vnode argument to the lower layer. 210.Pp 211The first approach is to call the aliasing layer's bypass routine. 212This method is most suitable when you wish to invoke the operation 213currently being handled on the lower layer. 214It has the advantage that 215the bypass routine already must do argument mapping. 216An example of this is 217.Em null_getattrs 218in the null layer. 219.Pp 220A second approach is to directly invoke vnode operations on 221the lower layer with the 222.Em VOP_OPERATIONNAME 223interface. 224The advantage of this method is that it is easy to invoke 225arbitrary operations on the lower layer. The disadvantage 226is that vnode arguments must be manually mapped. 227.\" 228.\" 229.Sh SEE ALSO 230.Xr mount 8 231.Pp 232UCLA Technical Report CSD-910056, 233.Em "Stackable Layers: an Architecture for File System Development" . 234.Sh BUGS 235 236THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK) 237AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR 238OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET. 239 240This code also needs an owner in order to be less dangerous - serious 241hackers can apply by sending mail to hackers@freebsd.org and announcing 242their intent to take it over. 243 244.Sh HISTORY 245The 246.Nm 247utility first appeared in 248.Bx 4.4 . 249