HardenedBSD/sbin/mount_unionfs/mount_unionfs.8

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.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.
.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.