1.\" $NetBSD: mount_union.8,v 1.10 2001/06/05 11:22:51 wiz Exp $ 2.\" 3.\" Copyright (c) 1994 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software donated to Berkeley by 7.\" Jan-Simon Pendry. 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_union.8 8.7 (Berkeley) 5/1/95 38.\" 39.Dd May 1, 1995 40.Dt MOUNT_UNION 8 41.Os 42.Sh NAME 43.Nm mount_union 44.Nd mount union filesystems 45.Sh SYNOPSIS 46.Nm "" 47.Op Fl br 48.Op Fl o Ar options 49.Ar directory 50.Ar uniondir 51.Sh DESCRIPTION 52The 53.Nm 54command 55attaches 56.Ar directory 57above 58.Ar uniondir 59in such a way that the contents of both directory trees remain visible. 60By default, 61.Ar directory 62becomes the 63.Em upper 64layer and 65.Ar uniondir 66becomes the 67.Em lower 68layer. 69.Pp 70The options are as follows: 71.Bl -tag -width indent 72.It Fl b 73Invert the default position, so that 74.Ar directory 75becomes the lower layer and 76.Ar uniondir 77becomes the upper layer. 78However, 79.Ar uniondir 80remains the mount point. 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 r 89Hide the lower layer completely in the same way as mounting with 90.Xr mount_null 8 . 91.El 92.Pp 93To enforce filesystem security, the user mounting the filesystem 94must be superuser or else have write permission on the mounted-on 95directory. 96.Pp 97Filenames are looked up in the upper layer and then in the 98lower layer. 99If a directory is found in the lower layer, and there is no entry 100in the upper layer, then a 101.Em shadow 102directory will be created in the upper layer. 103It will be owned by the user who originally did the union mount, 104with mode 105.Dq rwxrwxrwx 106(0777) modified by the umask in effect at that time. 107.Pp 108If a file exists in the upper layer then there is no way to access 109a file with the same name in the lower layer. 110If necessary, a combination of loopback and union mounts can be made 111which will still allow the lower files to be accessed by a different 112pathname. 113.Pp 114Except in the case of a directory, 115access to an object is granted via the normal filesystem access checks. 116For directories, the current user must have access to both the upper 117and lower directories (should they both exist). 118.Pp 119Requests to create or modify objects in 120.Ar uniondir 121are passed to the upper layer with the exception of a few special cases. 122An attempt to open for writing a file which exists in the lower layer 123causes a copy of the 124.Em entire 125file to be made to the upper layer, and then for the upper layer copy 126to be opened. 127Similarly, an attempt to truncate a lower layer file to zero length 128causes an empty file to be created in the upper layer. 129Any other operation which would ultimately require modification to 130the lower layer fails with 131.Dv EROFS . 132.Pp 133The union filesystem manipulates the namespace, rather than 134individual filesystems. 135The union operation applies recursively down the directory tree 136now rooted at 137.Ar uniondir . 138Thus any filesystems which are mounted under 139.Ar uniondir 140will take part in the union operation. 141This differs from the 142.Em union 143option to 144.Xr mount 8 145which only applies the union operation to the mount point itself, 146and then only for lookups. 147.Sh EXAMPLES 148The commands 149.Bd -literal -offset indent 150mount -t cd9660 -o ro /dev/cd0a /usr/src 151mount -t union /var/obj /usr/src 152.Ed 153.Pp 154mount the CD-ROM drive 155.Pa /dev/cd0a 156on 157.Pa /usr/src 158and then attaches 159.Pa /var/obj 160on top. 161For most purposes the effect of this is to make the 162source tree appear writable 163even though it is stored on a CD-ROM. 164.Pp 165The command 166.Bd -literal -offset indent 167mount -t union -o -b /sys $HOME/sys 168.Ed 169.Pp 170attaches the system source tree below the 171.Pa sys 172directory in the user's home directory. 173This allows individual users to make private changes 174to the source, and build new kernels, without those 175changes becoming visible to other users. 176Note that the files in the lower layer remain 177accessible via 178.Pa /sys . 179.Sh SEE ALSO 180.Xr intro 2 , 181.Xr mount 2 , 182.Xr unmount 2 , 183.Xr fstab 5 , 184.Xr mount 8 , 185.Xr mount_null 8 , 186.Xr fsck_ffs 8 187.Sh BUGS 188Without whiteout support from the filesystem backing the upper layer, 189there is no way that delete and rename operations on lower layer 190objects can be done. 191An attempt to mount a union directory under one which does not 192have whiteout support will return 193.Dv EOPNOTSUPP 194(``Operation not supported''). 195Whiteout support can be added to an existing FFS filesystem 196by using the 197.Fl c 198option of 199.Xr fsck_ffs 8 . 200.Pp 201Running 202.Xr find 1 203over a union tree has the side-effect of creating 204a tree of shadow directories in the upper layer. 205.Sh HISTORY 206The 207.Nm 208command first appeared in 209.Bx 4.4 . 210