2 .\" The Regents of the University of California. All rights reserved.
4 .\" This code is derived from software donated to Berkeley by
7 .\" Redistribution and use in source and binary forms, with or without
8 .\" modification, are permitted provided that the following conditions
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 .\" 4. Neither the name of the University nor the names of its contributors
16 .\" may be used to endorse or promote products derived from this software
17 .\" without specific prior written permission.
19 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
20 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
21 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
22 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
23 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
24 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
25 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
26 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
27 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
28 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
31 .\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94
39 .Nd mount union file systems
53 in such a way that the contents of both directory trees remain visible.
64 The options are as follows:
65 .Bl -tag -width indent
72 Options are specified with the
74 flag followed by an option.
75 The following options are available:
76 .Bl -tag -width indent
78 Inverts the default position, so that
80 becomes the lower layer and
82 becomes the upper layer.
85 remains the mount point.
86 .It Sm Cm copymode No = Cm traditional | transparent | masquerade Sm
87 Specifies the way to create a file or a directory in the upper layer
88 automatically when needed.
92 uses the same way as the old unionfs for backward compatibility, and
94 duplicates the file and directory mode bits and the ownership in the
95 lower layer to the created file in the upper layer.
101 .It Cm udir Ns = Ns Ar mode
102 Specifies directory mode bits in octal for
105 .It Cm ufile Ns = Ns Ar mode
106 Specifies file mode bits in octal for
109 .It Cm gid Ns = Ns Ar gid
113 .It Cm uid Ns = Ns Ar uid
120 To enforce file system security, the user mounting a file system
121 must be superuser or else have write permission on the mounted-on
126 variable must be set to 1 to permit file system mounting by ordinary users.
133 to be set to 0 because this functionality can only be used by superusers.
135 Filenames are looked up in the upper layer and then in the
137 If a directory is found in the lower layer, and there is no entry
138 in the upper layer, then a
140 directory will be created in the upper layer.
141 The ownership and the mode bits are set depending on the
146 mode, it will be owned by the user who originally did the
147 union mount, with mode 0777
149 modified by the umask in effect at that time.
151 If a file exists in the upper layer then there is no way to access
152 a file with the same name in the lower layer.
153 If necessary, a combination of loopback and union mounts can be made
154 which will still allow the lower files to be accessed by a different
157 Except in the case of a directory,
158 access to an object is granted via the normal file system access checks.
159 For directories, the current user must have access to both the upper
160 and lower directories (should they both exist).
162 Requests to create or modify objects in
164 are passed to the upper layer with the exception of a few special cases.
165 An attempt to open for writing a file which exists in the lower layer
168 file to be made to the upper layer, and then for the upper layer copy
170 Similarly, an attempt to truncate a lower layer file to zero length
171 causes an empty file to be created in the upper layer.
172 Any other operation which would ultimately require modification to
173 the lower layer fails with
176 The union file system manipulates the namespace, rather than
177 individual file systems.
178 The union operation applies recursively down the directory tree
181 Thus any file systems which are mounted under
183 will take part in the union operation.
184 This differs from the
188 which only applies the union operation to the mount point itself,
189 and then only for lookups.
193 is created in the upper layer, the
195 mode sets it the fixed access mode bits given in
201 option and the owner given in
205 options, instead of ones in the lower layer.
208 mode and when owner of the file or directory matches
211 option, only mode bits for the owner will be modified.
212 More specifically, the file mode bits in the upper layer will
214 (mode in the lower layer)
218 AND 0700), and the ownership will be the same as one in the lower layer.
220 The default values for
221 .Cm ufile , udir , uid ,
232 were specified, access mode bits in the mount point will be used.
238 were specified, ownership in the mount point will be used.
244 is not specified, the value of the other option will be used.
250 is not specified, the value of the other option will be used.
254 .Bd -literal -offset indent
255 mount -t cd9660 -o ro /dev/cd0 /usr/src
256 mount -t unionfs -o noatime /var/obj /usr/src
259 mount the CD-ROM drive
266 For most purposes the effect of this is to make the
267 source tree appear writable
268 even though it is stored on a CD-ROM.
271 option is useful to avoid unnecessary copying from the lower to the
275 .Bd -literal -offset indent
276 mount -t cd9660 -o ro /dev/cd0 /usr/src
278 mount -t unionfs -o noatime -o copymode=masquerade -o uid=builder \\
279 -o udir=755 -o ufile=644 /var/obj /usr/src
282 also mount the CD-ROM drive
289 Furthermore, the owner of all files and directories in
291 is a regular user with UID 2020
292 when seen from the upper layer.
293 Note that for the access mode bits,
294 ones in the lower layer
295 (on the CD-ROM, in this example)
296 are still used without change.
297 Thus, write privilege to the upper layer can be controlled
298 independently from access mode bits and ownership in the lower layer.
299 If a user does not have read privilege from the lower layer,
300 one cannot still read even when the upper layer is mounted by using
305 .Bd -literal -offset indent
306 mount -t unionfs -o noatime -o below /sys $HOME/sys
309 attaches the system source tree below the
311 directory in the user's home directory.
312 This allows individual users to make private changes
313 to the source, and build new kernels, without those
314 changes becoming visible to other users.
315 Note that the files in the lower layer remain
328 utility first appeared in
333 option for hiding the lower layer completely was removed in
335 because this is identical to using
341 .An Masanori OZAWA Aq ozawa@ongs.co.jp
342 reimplemented handling of locking, whiteout, and file mode bits, and
343 .An Hiroki Sato Aq hrs@FreeBSD.org
344 wrote about the changes in this manual page.
346 THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
347 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.
353 This code also needs an owner in order to be less dangerous - serious
354 hackers can apply by sending mail to
355 .Aq freebsd-fs@FreeBSD.org
357 their intent to take it over.
359 Without whiteout support from the file system backing the upper layer,
360 there is no way that delete and rename operations on lower layer
363 is returned for this kind of operations along with any others
364 which would make modifications to the lower layer, such as
369 over a union tree has the side-effect of creating
370 a tree of shadow directories in the upper layer.
372 The current implementation does not support copying extended attributes
376 or so on to the upper layer.
377 Note that this may be a security issue.
379 A shadow directory, which is one automatically created in the upper
380 layer when it exists in the lower layer and does not exist in the
381 upper layer, is always created with the superuser privilege.
382 However, a file copied from the lower layer in the same way
383 is created by the user who accessed it.
385 if the user is not the superuser, even in
387 mode the access mode bits in the copied file in the upper layer
388 will not always be the same as ones in the lower layer.
389 This behavior should be fixed.