.\" Copyright (c) 1994
.\" The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software donated to Berkeley by
.\" Jan-Simon Pendry.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\"	@(#)mount_union.8	8.6 (Berkeley) 3/27/94
.\" $FreeBSD: src/sbin/mount_union/mount_union.8,v 1.6.2.2 2001/12/20 16:46:05 ru Exp $
.\" $DragonFly: src/sbin/mount_union/mount_union.8,v 1.5 2006/02/17 19:33:33 swildner Exp $
.\"
.Dd March 27, 1994
.Dt MOUNT_UNION 8
.Os
.Sh NAME
.Nm mount_union
.Nd mount union filesystems
.Sh SYNOPSIS
.Nm
.Op Fl br
.Op Fl o Ar options
.Ar directory
.Ar uniondir
.Sh DESCRIPTION
The
.Nm
command
attaches
.Ar directory
above
.Ar uniondir
in such a way that the contents of both directory trees remain visible.
By default,
.Ar directory
becomes the
.Em upper
layer and
.Ar uniondir
becomes the
.Em lower
layer.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl b
Invert the default position, so that
.Ar directory
becomes the lower layer and
.Ar uniondir
becomes the upper layer.
However,
.Ar uniondir
remains the mount point.
.It Fl o
Options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.It Fl r
Hide the lower layer completely in the same way as mounting with
.Xr mount_null 8 .
.El
.Pp
To enforce filesystem security, the user mounting the filesystem
must be superuser or else have write permission on the mounted-on
directory.
In addition, the
.Em vfs.usermount
.Xr sysctl 3
variable must be set to 1 to permit file system mounting by ordinary users.
.Pp
Filenames are looked up in the upper layer and then in the
lower layer.
If a directory is found in the lower layer, and there is no entry
in the upper layer, then a
.Em shadow
directory will be created in the upper layer.
It will be owned by the user who originally did the union mount,
with mode
.Dq rwxrwxrwx
(0777) modified by the umask in effect at that time.
.Pp
If a file exists in the upper layer then there is no way to access
a file with the same name in the lower layer.
If necessary, a combination of loopback and union mounts can be made
which will still allow the lower files to be accessed by a different
pathname.
.Pp
Except in the case of a directory,
access to an object is granted via the normal filesystem access checks.
For directories, the current user must have access to both the upper
and lower directories (should they both exist).
.Pp
Requests to create or modify objects in
.Ar uniondir
are passed to the upper layer with the exception of a few special cases.
An attempt to open for writing a file which exists in the lower layer
causes a copy of the
.Em entire
file to be made to the upper layer, and then for the upper layer copy
to be opened.
Similarly, an attempt to truncate a lower layer file to zero length
causes an empty file to be created in the upper layer.
Any other operation which would ultimately require modification to
the lower layer fails with
.Er EROFS .
.Pp
The union filesystem manipulates the namespace, rather than
individual filesystems.
The union operation applies recursively down the directory tree
now rooted at
.Ar uniondir .
Thus any filesystems which are mounted under
.Ar uniondir
will take part in the union operation.
This differs from the
.Em union
option to
.Xr mount 8
which only applies the union operation to the mount point itself,
and then only for lookups.
.Sh EXAMPLES
The commands
.Bd -literal -offset indent
mount -t cd9660 -o ro /dev/cd0a /usr/src
mount -t union /var/obj /usr/src
.Ed
.Pp
mount the CD-ROM drive
.Pa /dev/cd0a
on
.Pa /usr/src
and then attaches
.Pa /var/obj
on top.
For most purposes the effect of this is to make the
source tree appear writable
even though it is stored on a CD-ROM.
.Pp
The command
.Bd -literal -offset indent
mount -t union -o -b /sys $HOME/sys
.Ed
.Pp
attaches the system source tree below the
.Pa sys
directory in the user's home directory.
This allows individual users to make private changes
to the source, and build new kernels, without those
changes becoming visible to other users.
Note that the files in the lower layer remain
accessible via
.Pa /sys .
.Sh SEE ALSO
.Xr intro 2 ,
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr fstab 5 ,
.Xr mount 8 ,
.Xr mount_null 8
.Sh HISTORY
The
.Nm
command first appeared in
.Bx 4.4 .
It first worked in
.Fx Ns -(fill this in) .
.Sh BUGS
THIS FILESYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.  USE AT YOUR
OWN RISK.  BEWARE OF DOG.  SLIPPERY WHEN WET.
.Pp
This code also needs an owner in order to be less dangerous - serious
hackers can apply by sending mail to
.Aq kernel@crater.dragonflybsd.org
and announcing
their intent to take it over.
.Pp
Without whiteout support from the filesystem backing the upper layer,
there is no way that delete and rename operations on lower layer
objects can be done.
.Er EROFS
is returned for this kind of operations along with any others
which would make modifications to the lower layer, such as
.Xr chmod 1 .
.Pp
Running
.Xr find 1
over a union tree has the side-effect of creating
a tree of shadow directories in the upper layer.