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 August 24, 2026 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 directories, regular files and 66.Xr unix 4 67sockets. 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 nounixbypass 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.It Cm cache 102Force enable metadata caching. 103.It Cm nounixbypass 104Disable bypassing 105.Xr unix 4 106socket files used for 107.Xr bind 2 108and 109.Xr connect 2 , 110to the lower (mounted-from) filesystem layer. 111.Pp 112The effect is that lower and upper (bypassed) unix sockets 113are separate. 114.It Cm unixbypass 115Enable the bypass of unix socket file to lower filesystem layer. 116This is default. 117.Pp 118The effect is that 119.Xr bind 2 120and 121.Xr connect 2 122operations on a unix socket done from either the upper (nullfs) or lower 123layer path are performed on same unix socket. 124For instance, if a server 125.Xr bind 2 126is done on a socket in the lower layer, then 127.Xr connect 2 128on the socket file accessed via the nullfs mount, connects to the server. 129.El 130.El 131.Pp 132The 133.Dv vfs.nullfs.cache_vnodes 134sysctl specifies global default for mount-specific cache/nocache option. 135.Pp 136The null layer has two purposes. 137First, it serves as a demonstration of layering by providing a layer 138which does nothing. 139(It actually does everything the loopback file system does, 140which is slightly more than nothing.) 141Second, the null layer can serve as a prototype layer. 142Since it provides all necessary layer framework, 143new file system layers can be created very easily by starting 144with a null layer. 145.Pp 146The remainder of this man page examines the null layer as a basis 147for constructing new layers. 148.\" 149.\" 150.Sh INSTANTIATING NEW NULL LAYERS 151New null layers are created with 152.Nm . 153The 154.Nm 155utility takes two arguments, the pathname 156of the lower vfs (target-pn) and the pathname where the null 157layer will appear in the namespace (mount-point-pn). 158After 159the null layer is put into place, the contents 160of target-pn subtree will be aliased under mount-point-pn. 161.\" 162.\" 163.Sh OPERATION OF A NULL LAYER 164The null layer is the minimum file system layer, 165simply bypassing all possible operations to the lower layer 166for processing there. 167The majority of its activity centers 168on the bypass routine, through which nearly all vnode operations 169pass. 170.Pp 171The bypass routine accepts arbitrary vnode operations for 172handling by the lower layer. 173It begins by examining vnode 174operation arguments and replacing any null-nodes by their 175lower-layer equivalents. 176It then invokes the operation 177on the lower layer. 178Finally, it replaces the null-nodes 179in the arguments and, if a vnode is returned by the operation, 180stacks a null-node on top of the returned vnode. 181.Pp 182Although bypass handles most operations, 183.Em vop_getattr , 184.Em vop_inactive , 185.Em vop_reclaim , 186and 187.Em vop_print 188are not bypassed. 189.Em Vop_getattr 190must change the fsid being returned. 191.Em Vop_inactive 192and 193.Em vop_reclaim 194are not bypassed so that 195they can handle freeing null-layer specific data. 196.Em Vop_print 197is not bypassed to avoid excessive debugging 198information. 199.\" 200.\" 201.Sh INSTANTIATING VNODE STACKS 202Mounting associates the null layer with a lower layer, 203in effect stacking two VFSes. 204Vnode stacks are instead 205created on demand as files are accessed. 206.Pp 207The initial mount creates a single vnode stack for the 208root of the new null layer. 209All other vnode stacks 210are created as a result of vnode operations on 211this or other null vnode stacks. 212.Pp 213New vnode stacks come into existence as a result of 214an operation which returns a vnode. 215The bypass routine stacks a null-node above the new 216vnode before returning it to the caller. 217.Pp 218For example, imagine mounting a null layer with 219.Bd -literal -offset indent 220mount_nullfs /usr/include /dev/layer/null 221.Ed 222.Pp 223Changing directory to 224.Pa /dev/layer/null 225will assign 226the root null-node (which was created when the null layer was mounted). 227Now consider opening 228.Pa sys . 229A vop_lookup would be 230done on the root null-node. 231This operation would bypass through 232to the lower layer which would return a vnode representing 233the UFS 234.Pa sys . 235Null_bypass then builds a null-node 236aliasing the UFS 237.Pa sys 238and returns this to the caller. 239Later operations on the null-node 240.Pa sys 241will repeat this 242process when constructing other vnode stacks. 243.\" 244.\" 245.Sh CREATING OTHER FILE SYSTEM LAYERS 246One of the easiest ways to construct new file system layers is to make 247a copy of the null layer, rename all files and variables, and 248then begin modifying the copy. 249The 250.Xr sed 1 251utility can be used to easily rename 252all variables. 253.Pp 254The umap layer is an example of a layer descended from the 255null layer. 256.\" 257.\" 258.Sh INVOKING OPERATIONS ON LOWER LAYERS 259There are two techniques to invoke operations on a lower layer 260when the operation cannot be completely bypassed. 261Each method 262is appropriate in different situations. 263In both cases, 264it is the responsibility of the aliasing layer to make 265the operation arguments "correct" for the lower layer 266by mapping a vnode argument to the lower layer. 267.Pp 268The first approach is to call the aliasing layer's bypass routine. 269This method is most suitable when you wish to invoke the operation 270currently being handled on the lower layer. 271It has the advantage that 272the bypass routine already must do argument mapping. 273An example of this is 274.Em null_getattrs 275in the null layer. 276.Pp 277A second approach is to directly invoke vnode operations on 278the lower layer with the 279.Em VOP_OPERATIONNAME 280interface. 281The advantage of this method is that it is easy to invoke 282arbitrary operations on the lower layer. 283The disadvantage 284is that vnode arguments must be manually mapped. 285.\" 286.\" 287.Sh SEE ALSO 288.Xr nullfs 4 , 289.Xr mount 8 290.Pp 291UCLA Technical Report CSD-910056, 292.Em "Stackable Layers: an Architecture for File System Development" . 293.Sh HISTORY 294The 295.Nm mount_null 296utility first appeared in 297.Bx 4.4 . 298It was renamed to 299.Nm 300in 301.Fx 5.0 . 302