1.\" Copyright (c) 1994 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software donated to Berkeley by 5.\" Jan-Simon Pendry. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94 36.\" $FreeBSD$ 37.\" 38.Dd March 27, 1994 39.Dt MOUNT_UNION 8 40.Os BSD 4.4 41.Sh NAME 42.Nm mount_union 43.Nd mount union filesystems 44.Sh SYNOPSIS 45.Nm mount_union 46.Op Fl br 47.Op Fl o Ar options 48.Ar directory 49.Ar uniondir 50.Sh DESCRIPTION 51The 52.Nm 53command 54attaches 55.Ar directory 56above 57.Ar uniondir 58in such a way that the contents of both directory trees remain visible. 59By default, 60.Ar directory 61becomes the 62.Em upper 63layer and 64.Ar uniondir 65becomes the 66.Em lower 67layer. 68.Pp 69The options are as follows: 70.Bl -tag -width indent 71.It Fl b 72Invert the default position, so that 73.Ar directory 74becomes the lower layer and 75.Ar uniondir 76becomes the upper layer. 77However, 78.Ar uniondir 79remains the mount point. 80.It Fl o 81Options are specified with a 82.Fl o 83flag followed by a comma separated string of options. 84See the 85.Xr mount 8 86man page for possible options and their meanings. 87.It Fl r 88Hide the lower layer completely in the same way as mounting with 89.Xr mount_null 8 . 90.El 91.Pp 92To enforce filesystem security, the user mounting the filesystem 93must be superuser or else have write permission on the mounted-on 94directory. 95.Pp 96Filenames are looked up in the upper layer and then in the 97lower layer. 98If a directory is found in the lower layer, and there is no entry 99in the upper layer, then a 100.Em shadow 101directory will be created in the upper layer. 102It will be owned by the user who originally did the union mount, 103with mode 104.Dq rwxrwxrwx 105(0777) modified by the umask in effect at that time. 106.Pp 107If a file exists in the upper layer then there is no way to access 108a file with the same name in the lower layer. 109If necessary, a combination of loopback and union mounts can be made 110which will still allow the lower files to be accessed by a different 111pathname. 112.Pp 113Except in the case of a directory, 114access to an object is granted via the normal filesystem access checks. 115For directories, the current user must have access to both the upper 116and lower directories (should they both exist). 117.Pp 118Requests to create or modify objects in 119.Ar uniondir 120are passed to the upper layer with the exception of a few special cases. 121An attempt to open for writing a file which exists in the lower layer 122causes a copy of the 123.Em entire 124file to be made to the upper layer, and then for the upper layer copy 125to be opened. 126Similarly, an attempt to truncate a lower layer file to zero length 127causes an empty file to be created in the upper layer. 128Any other operation which would ultimately require modification to 129the lower layer fails with 130.Dv EROFS . 131.Pp 132The union filesystem manipulates the namespace, rather than 133individual filesystems. 134The union operation applies recursively down the directory tree 135now rooted at 136.Ar uniondir . 137Thus any filesystems which are mounted under 138.Ar uniondir 139will take part in the union operation. 140This differs from the 141.Em union 142option to 143.Xr mount 8 144which only applies the union operation to the mount point itself, 145and then only for lookups. 146.Sh EXAMPLES 147The commands 148.Bd -literal -offset indent 149mount -t cd9660 -o ro /dev/cd0a /usr/src 150mount -t union /var/obj /usr/src 151.Ed 152.Pp 153mount the CD-ROM drive 154.Pa /dev/cd0a 155on 156.Pa /usr/src 157and then attaches 158.Pa /var/obj 159on top. 160For most purposes the effect of this is to make the 161source tree appear writable 162even though it is stored on a CD-ROM. 163.Pp 164The command 165.Bd -literal -offset indent 166mount -t union -o -b /sys $HOME/sys 167.Ed 168.Pp 169attaches the system source tree below the 170.Pa sys 171directory in the user's home directory. 172This allows individual users to make private changes 173to the source, and build new kernels, without those 174changes becoming visible to other users. 175Note that the files in the lower layer remain 176accessible via 177.Pa /sys . 178.Sh SEE ALSO 179.Xr intro 2 , 180.Xr mount 2 , 181.Xr unmount 2 , 182.Xr fstab 5 , 183.Xr mount 8 , 184.Xr mount_null 8 185.Sh BUGS 186 187THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK) 188AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR 189OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET. 190 191This code also needs an owner in order to be less dangerous - serious 192hackers can apply by sending mail to hackers@freebsd.org and announcing 193their intent to take it over. 194 195Without whiteout support from the filesystem backing the upper layer, 196there is no way that delete and rename operations on lower layer 197objects can be done. 198.Dv EROFS 199is returned for this kind of operations along with any others 200which would make modifications to the lower layer, such as 201.Xr chmod 1 . 202.Pp 203Running 204.Xr find 1 205over a union tree has the side-effect of creating 206a tree of shadow directories in the upper layer. 207.Sh HISTORY 208The 209.Nm 210command first appeared in 211.Bx 4.4 . 212It first worked in FreeBSD-(fill this in). 213