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.\" $FreeBSD$ 35.\" 36.Dd December 19, 2022 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 67utility supports mounting both directories and single files. 68Both 69.Ar target 70and 71.Ar mount_point 72must be the same type. 73Mounting directories to files or files to 74directories is not supported. 75.Pp 76The 77.Nm 78file system differs from a traditional 79loopback file system in two respects: it is implemented using 80a stackable layers techniques, and its 81.Do null-node Dc Ns s 82stack above 83all lower-layer vnodes, not just over directory vnodes. 84.Pp 85The options are as follows: 86.Bl -tag -width indent 87.It Fl o 88Options are specified with a 89.Fl o 90flag followed by a comma separated string of options. 91See the 92.Xr mount 8 93man page for possible options and their meanings. 94Additionally the following option is supported: 95.Bl -tag -width indent 96.It Cm nocache 97Disable metadata caching in the null layer. 98Some lower-layer file systems may force this option. 99Depending on the access pattern, 100this may result in increased lock contention. 101.El 102.El 103.Pp 104The null layer has two purposes. 105First, it serves as a demonstration of layering by providing a layer 106which does nothing. 107(It actually does everything the loopback file system does, 108which is slightly more than nothing.) 109Second, the null layer can serve as a prototype layer. 110Since it provides all necessary layer framework, 111new file system layers can be created very easily by starting 112with a null layer. 113.Pp 114The remainder of this man page examines the null layer as a basis 115for constructing new layers. 116.\" 117.\" 118.Sh INSTANTIATING NEW NULL LAYERS 119New null layers are created with 120.Nm . 121The 122.Nm 123utility takes two arguments, the pathname 124of the lower vfs (target-pn) and the pathname where the null 125layer will appear in the namespace (mount-point-pn). 126After 127the null layer is put into place, the contents 128of target-pn subtree will be aliased under mount-point-pn. 129.\" 130.\" 131.Sh OPERATION OF A NULL LAYER 132The null layer is the minimum file system layer, 133simply bypassing all possible operations to the lower layer 134for processing there. 135The majority of its activity centers 136on the bypass routine, through which nearly all vnode operations 137pass. 138.Pp 139The bypass routine accepts arbitrary vnode operations for 140handling by the lower layer. 141It begins by examining vnode 142operation arguments and replacing any null-nodes by their 143lower-layer equivalents. 144It then invokes the operation 145on the lower layer. 146Finally, it replaces the null-nodes 147in the arguments and, if a vnode is returned by the operation, 148stacks a null-node on top of the returned vnode. 149.Pp 150Although bypass handles most operations, 151.Em vop_getattr , 152.Em vop_inactive , 153.Em vop_reclaim , 154and 155.Em vop_print 156are not bypassed. 157.Em Vop_getattr 158must change the fsid being returned. 159.Em Vop_inactive 160and 161.Em vop_reclaim 162are not bypassed so that 163they can handle freeing null-layer specific data. 164.Em Vop_print 165is not bypassed to avoid excessive debugging 166information. 167.\" 168.\" 169.Sh INSTANTIATING VNODE STACKS 170Mounting associates the null layer with a lower layer, 171in effect stacking two VFSes. 172Vnode stacks are instead 173created on demand as files are accessed. 174.Pp 175The initial mount creates a single vnode stack for the 176root of the new null layer. 177All other vnode stacks 178are created as a result of vnode operations on 179this or other null vnode stacks. 180.Pp 181New vnode stacks come into existence as a result of 182an operation which returns a vnode. 183The bypass routine stacks a null-node above the new 184vnode before returning it to the caller. 185.Pp 186For example, imagine mounting a null layer with 187.Bd -literal -offset indent 188mount_nullfs /usr/include /dev/layer/null 189.Ed 190.Pp 191Changing directory to 192.Pa /dev/layer/null 193will assign 194the root null-node (which was created when the null layer was mounted). 195Now consider opening 196.Pa sys . 197A vop_lookup would be 198done on the root null-node. 199This operation would bypass through 200to the lower layer which would return a vnode representing 201the UFS 202.Pa sys . 203Null_bypass then builds a null-node 204aliasing the UFS 205.Pa sys 206and returns this to the caller. 207Later operations on the null-node 208.Pa sys 209will repeat this 210process when constructing other vnode stacks. 211.\" 212.\" 213.Sh CREATING OTHER FILE SYSTEM LAYERS 214One of the easiest ways to construct new file system layers is to make 215a copy of the null layer, rename all files and variables, and 216then begin modifying the copy. 217The 218.Xr sed 1 219utility can be used to easily rename 220all variables. 221.Pp 222The umap layer is an example of a layer descended from the 223null layer. 224.\" 225.\" 226.Sh INVOKING OPERATIONS ON LOWER LAYERS 227There are two techniques to invoke operations on a lower layer 228when the operation cannot be completely bypassed. 229Each method 230is appropriate in different situations. 231In both cases, 232it is the responsibility of the aliasing layer to make 233the operation arguments "correct" for the lower layer 234by mapping a vnode argument to the lower layer. 235.Pp 236The first approach is to call the aliasing layer's bypass routine. 237This method is most suitable when you wish to invoke the operation 238currently being handled on the lower layer. 239It has the advantage that 240the bypass routine already must do argument mapping. 241An example of this is 242.Em null_getattrs 243in the null layer. 244.Pp 245A second approach is to directly invoke vnode operations on 246the lower layer with the 247.Em VOP_OPERATIONNAME 248interface. 249The advantage of this method is that it is easy to invoke 250arbitrary operations on the lower layer. 251The disadvantage 252is that vnode arguments must be manually mapped. 253.\" 254.\" 255.Sh SEE ALSO 256.Xr mount 8 257.Pp 258UCLA Technical Report CSD-910056, 259.Em "Stackable Layers: an Architecture for File System Development" . 260.Sh HISTORY 261The 262.Nm mount_null 263utility first appeared in 264.Bx 4.4 . 265It was renamed to 266.Nm 267in 268.Fx 5.0 . 269