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Editing: mount_unionfs.8
.\" 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. 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 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94 .\" $FreeBSD$ .\" .Dd October 3, 2016 .Dt MOUNT_UNIONFS 8 .Os .Sh NAME .Nm mount_unionfs .Nd mount union file systems .Sh SYNOPSIS .Nm .Op Fl b .Op Fl o Ar options .Ar directory .Ar uniondir .Sh DESCRIPTION The .Nm utility 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 Deprecated. Use .Fl o Cm below instead. .It Fl o Options are specified with the .Fl o flag followed by an option. The following options are available: .Bl -tag -width indent .It Cm below Inverts 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 Cm copymode No = Cm traditional | transparent | masquerade Specifies the way to create a file or a directory in the upper layer automatically when needed. The .Cm traditional mode uses the same way as the old unionfs for backward compatibility, and .Cm transparent duplicates the file and directory mode bits and the ownership in the lower layer to the created file in the upper layer. For behavior of the .Cm masquerade mode, see .Sx MASQUERADE MODE below. .It Cm whiteout No = Cm always | whenneeded Specifies whether whiteouts should always be made in the upper layer when removing a file or directory or only when it already exists in the lower layer. .It Cm udir Ns = Ns Ar mode Specifies directory mode bits in octal for .Cm masquerade mode. .It Cm ufile Ns = Ns Ar mode Specifies file mode bits in octal for .Cm masquerade mode. .It Cm gid Ns = Ns Ar gid Specifies group for .Cm masquerade mode. .It Cm uid Ns = Ns Ar uid Specifies user for .Cm masquerade mode. .El .El .Pp To enforce file system security, the user mounting a file system must be superuser or else have write permission on the mounted-on directory. In addition, the .Va vfs.usermount .Xr sysctl 8 variable must be set to 1 to permit file system mounting by ordinary users. However, note that .Cm transparent and .Cm masquerade modes require .Va vfs.usermount to be set to 0 because this functionality can only be used by superusers. .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. The ownership and the mode bits are set depending on the .Cm copymode option. In .Cm traditional mode, it will be owned by the user who originally did the union mount, with mode 0777 .Pq Dq Li rwxrwxrwx 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 file system 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 file system manipulates the namespace, rather than individual file systems. The union operation applies recursively down the directory tree now rooted at .Ar uniondir . Thus any file systems which are mounted under .Ar uniondir will take part in the union operation. This differs from the .Cm union option to .Xr mount 8 which only applies the union operation to the mount point itself, and then only for lookups. .Sh MASQUERADE MODE When a file (or a directory) is created in the upper layer, the .Cm masquerade mode sets it the fixed access mode bits given in .Cm ufile (for files) or .Cm udir (for directories) option and the owner given in .Cm udir and .Cm gid options, instead of ones in the lower layer. Note that in the .Cm masquerade mode and when owner of the file or directory matches one specified in .Cm uid option, only mode bits for the owner will be modified. More specifically, the file mode bits in the upper layer will be (mode in the lower layer) OR (mode given in .Cm ufile AND 0700), and the ownership will be the same as one in the lower layer. .Pp The default values for .Cm ufile , udir , uid , and .Cm gid are as follow: .Pp .Bl -bullet -compact .It If none of .Cm ufile and .Cm udir were specified, access mode bits in the mount point will be used. .It If none of .Cm uid and .Cm gid were specified, ownership in the mount point will be used. .It If one of .Cm udir or .Cm ufile is not specified, the value of the other option will be used. .It If one of .Cm uid or .Cm gid is not specified, the value of the other option will be used. .El .Sh EXAMPLES The commands .Bd -literal -offset indent mount -t cd9660 -o ro /dev/cd0 /usr/src mount -t unionfs -o noatime /var/obj /usr/src .Ed .Pp mount the CD-ROM drive .Pa /dev/cd0 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. The .Fl o Cm noatime option is useful to avoid unnecessary copying from the lower to the upper layer. .Pp The commands .Bd -literal -offset indent mount -t cd9660 -o ro /dev/cd0 /usr/src chown 2020 /usr/src mount -t unionfs -o noatime -o copymode=masquerade -o uid=builder \\ -o udir=755 -o ufile=644 /var/obj /usr/src .Ed .Pp also mount the CD-ROM drive .Pa /dev/cd0 on .Pa /usr/src and then attaches .Pa /var/obj on top. Furthermore, the owner of all files and directories in .Pa /usr/src is a regular user with UID 2020 when seen from the upper layer. Note that for the access mode bits, ones in the lower layer (on the CD-ROM, in this example) are still used without change. Thus, write privilege to the upper layer can be controlled independently from access mode bits and ownership in the lower layer. If a user does not have read privilege from the lower layer, one cannot still read even when the upper layer is mounted by using .Cm masquerade mode. .Pp The command .Bd -literal -offset indent mount -t unionfs -o noatime -o below /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_nullfs 8 .Sh HISTORY The .Nm mount_null utility first appeared in .Bx 4.4 . It was renamed to .Nm in .Fx 5.0 . .Pp The .Fl r option for hiding the lower layer completely was removed in .Fx 7.0 because this is identical to using .Xr mount_nullfs 8 . .Sh AUTHORS .An -nosplit In .Fx 7.0 , .An Masanori OZAWA Aq Mt ozawa@ongs.co.jp reimplemented handling of locking, whiteout, and file mode bits, and .An Hiroki Sato Aq Mt hrs@FreeBSD.org wrote about the changes in this manual page. .Sh BUGS THIS FILE SYSTEM 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. BATTERIES NOT INCLUDED. .Pp This code also needs an owner in order to be less dangerous - serious hackers can apply by sending mail to .Aq Mt freebsd-fs@FreeBSD.org and announcing their intent to take it over. .Pp Without whiteout support from the file system backing the upper layer, there is no way that delete and rename operations on lower layer objects can be done. .Er EOPNOTSUPP is returned for this kind of operations as generated by VOP_WHITEOUT() 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. .Pp The current implementation does not support copying extended attributes for .Xr acl 9 , .Xr mac 9 , or so on to the upper layer. Note that this may be a security issue. .Pp A shadow directory, which is one automatically created in the upper layer when it exists in the lower layer and does not exist in the upper layer, is always created with the superuser privilege. However, a file copied from the lower layer in the same way is created by the user who accessed it. Because of this, if the user is not the superuser, even in .Cm transparent mode the access mode bits in the copied file in the upper layer will not always be the same as ones in the lower layer. This behavior should be fixed.
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