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.Dd March 24, 2024 34.Dt MOUNT_NULLFS 8 35.Os 36.Sh NAME 37.Nm mount_nullfs 38.Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer" 39.Sh SYNOPSIS 40.Nm 41.Op Fl o Ar options 42.Ar target 43.Ar mount-point 44.Sh DESCRIPTION 45The 46.Nm 47utility creates a 48.Xr nullfs 4 49layer, duplicating a sub-tree of the file system 50name space under another part of the global file system namespace. 51This allows existing files and directories to be accessed 52using a different pathname. 53.Pp 54The primary differences between a virtual copy of the file system 55and a symbolic link are that the 56.Xr getcwd 3 57functions work correctly in the virtual copy, and that other file systems 58may be mounted on the virtual copy without affecting the original. 59A different device number for the virtual copy is returned by 60.Xr stat 2 , 61but in other respects it is indistinguishable from the original. 62.Pp 63The 64.Nm 65utility supports mounting both directories and single files. 66Both 67.Ar target 68and 69.Ar mount_point 70must be the same type. 71Mounting directories to files or files to 72directories is not supported. 73.Pp 74The 75.Nm 76file system differs from a traditional 77loopback file system in two respects: it is implemented using 78a stackable layers techniques, and its 79.Do null-node Dc Ns s 80stack above 81all lower-layer vnodes, not just over directory vnodes. 82.Pp 83The options are as follows: 84.Bl -tag -width indent 85.It Fl o 86Options are specified with a 87.Fl o 88flag followed by a comma separated string of options. 89See the 90.Xr mount 8 91man page for possible options and their meanings. 92Additionally the following option is supported: 93.Bl -tag -width nounixbypass 94.It Cm nocache 95Disable metadata caching in the null layer. 96Some lower-layer file systems may force this option. 97Depending on the access pattern, 98this may result in increased lock contention. 99.It Cm cache 100Force enable metadata caching. 101.It Cm nounixbypass 102Disable bypassing 103.Xr unix 4 104socket files used for 105.Xr bind 2 106and 107.Xr connect 2 , 108to the lower (mounted-from) filesystem layer. 109.Pp 110The effect is that lower and upper (bypassed) unix sockets 111are separate. 112.It Cm unixbypass 113Enable the bypass of unix socket file to lower filesystem layer. 114This is default. 115.Pp 116The effect is that 117.Xr bind 2 118and 119.Xr connect 2 120operations on a unix socket done from either the upper (nullfs) or lower 121layer path are performed on same unix socket. 122For instance, if a server 123.Xr bind 2 124is done on a socket in the lower layer, then 125.Xr connect 2 126on the socket file accessed via the nullfs mount, connects to the server. 127.El 128.El 129.Pp 130The 131.Dv vfs.nullfs.cache_vnodes 132sysctl specifies global default for mount-specific cache/nocache option. 133.Pp 134The null layer has two purposes. 135First, it serves as a demonstration of layering by providing a layer 136which does nothing. 137(It actually does everything the loopback file system does, 138which is slightly more than nothing.) 139Second, the null layer can serve as a prototype layer. 140Since it provides all necessary layer framework, 141new file system layers can be created very easily by starting 142with a null layer. 143.Pp 144The remainder of this man page examines the null layer as a basis 145for constructing new layers. 146.\" 147.\" 148.Sh INSTANTIATING NEW NULL LAYERS 149New null layers are created with 150.Nm . 151The 152.Nm 153utility takes two arguments, the pathname 154of the lower vfs (target-pn) and the pathname where the null 155layer will appear in the namespace (mount-point-pn). 156After 157the null layer is put into place, the contents 158of target-pn subtree will be aliased under mount-point-pn. 159.\" 160.\" 161.Sh OPERATION OF A NULL LAYER 162The null layer is the minimum file system layer, 163simply bypassing all possible operations to the lower layer 164for processing there. 165The majority of its activity centers 166on the bypass routine, through which nearly all vnode operations 167pass. 168.Pp 169The bypass routine accepts arbitrary vnode operations for 170handling by the lower layer. 171It begins by examining vnode 172operation arguments and replacing any null-nodes by their 173lower-layer equivalents. 174It then invokes the operation 175on the lower layer. 176Finally, it replaces the null-nodes 177in the arguments and, if a vnode is returned by the operation, 178stacks a null-node on top of the returned vnode. 179.Pp 180Although bypass handles most operations, 181.Em vop_getattr , 182.Em vop_inactive , 183.Em vop_reclaim , 184and 185.Em vop_print 186are not bypassed. 187.Em Vop_getattr 188must change the fsid being returned. 189.Em Vop_inactive 190and 191.Em vop_reclaim 192are not bypassed so that 193they can handle freeing null-layer specific data. 194.Em Vop_print 195is not bypassed to avoid excessive debugging 196information. 197.\" 198.\" 199.Sh INSTANTIATING VNODE STACKS 200Mounting associates the null layer with a lower layer, 201in effect stacking two VFSes. 202Vnode stacks are instead 203created on demand as files are accessed. 204.Pp 205The initial mount creates a single vnode stack for the 206root of the new null layer. 207All other vnode stacks 208are created as a result of vnode operations on 209this or other null vnode stacks. 210.Pp 211New vnode stacks come into existence as a result of 212an operation which returns a vnode. 213The bypass routine stacks a null-node above the new 214vnode before returning it to the caller. 215.Pp 216For example, imagine mounting a null layer with 217.Bd -literal -offset indent 218mount_nullfs /usr/include /dev/layer/null 219.Ed 220.Pp 221Changing directory to 222.Pa /dev/layer/null 223will assign 224the root null-node (which was created when the null layer was mounted). 225Now consider opening 226.Pa sys . 227A vop_lookup would be 228done on the root null-node. 229This operation would bypass through 230to the lower layer which would return a vnode representing 231the UFS 232.Pa sys . 233Null_bypass then builds a null-node 234aliasing the UFS 235.Pa sys 236and returns this to the caller. 237Later operations on the null-node 238.Pa sys 239will repeat this 240process when constructing other vnode stacks. 241.\" 242.\" 243.Sh CREATING OTHER FILE SYSTEM LAYERS 244One of the easiest ways to construct new file system layers is to make 245a copy of the null layer, rename all files and variables, and 246then begin modifying the copy. 247The 248.Xr sed 1 249utility can be used to easily rename 250all variables. 251.Pp 252The umap layer is an example of a layer descended from the 253null layer. 254.\" 255.\" 256.Sh INVOKING OPERATIONS ON LOWER LAYERS 257There are two techniques to invoke operations on a lower layer 258when the operation cannot be completely bypassed. 259Each method 260is appropriate in different situations. 261In both cases, 262it is the responsibility of the aliasing layer to make 263the operation arguments "correct" for the lower layer 264by mapping a vnode argument to the lower layer. 265.Pp 266The first approach is to call the aliasing layer's bypass routine. 267This method is most suitable when you wish to invoke the operation 268currently being handled on the lower layer. 269It has the advantage that 270the bypass routine already must do argument mapping. 271An example of this is 272.Em null_getattrs 273in the null layer. 274.Pp 275A second approach is to directly invoke vnode operations on 276the lower layer with the 277.Em VOP_OPERATIONNAME 278interface. 279The advantage of this method is that it is easy to invoke 280arbitrary operations on the lower layer. 281The disadvantage 282is that vnode arguments must be manually mapped. 283.\" 284.\" 285.Sh SEE ALSO 286.Xr nullfs 4 , 287.Xr mount 8 288.Pp 289UCLA Technical Report CSD-910056, 290.Em "Stackable Layers: an Architecture for File System Development" . 291.Sh HISTORY 292The 293.Nm mount_null 294utility first appeared in 295.Bx 4.4 . 296It was renamed to 297.Nm 298in 299.Fx 5.0 . 300