395 lines
11 KiB
Groff
395 lines
11 KiB
Groff
.\" Copyright (c) 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software donated to Berkeley by
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.\" Jan-Simon Pendry.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94
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.\"
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.Dd October 3, 2016
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.Dt MOUNT_UNIONFS 8
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.Os
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.Sh NAME
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.Nm mount_unionfs
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.Nd mount union file systems
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.Sh SYNOPSIS
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.Nm
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.Op Fl b
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.Op Fl o Ar options
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.Ar directory
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.Ar uniondir
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.Sh DESCRIPTION
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The
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.Nm
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utility attaches
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.Ar directory
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above
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.Ar uniondir
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in such a way that the contents of both directory trees remain visible.
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By default,
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.Ar directory
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becomes the
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.Em upper
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layer and
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.Ar uniondir
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becomes the
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.Em lower
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layer.
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.Pp
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The options are as follows:
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.Bl -tag -width indent
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.It Fl b
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Deprecated.
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Use
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.Fl o Cm below
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instead.
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.It Fl o
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Options are specified with the
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.Fl o
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flag followed by an option.
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The following options are available:
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.Bl -tag -width indent
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.It Cm below
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Inverts the default position, so that
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.Ar directory
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becomes the lower layer and
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.Ar uniondir
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becomes the upper layer.
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However,
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.Ar uniondir
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remains the mount point.
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.It Cm copymode No = Cm traditional | transparent | masquerade
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Specifies the way to create a file or a directory in the upper layer
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automatically when needed.
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The
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.Cm traditional
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mode
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uses the same way as the old unionfs for backward compatibility, and
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.Cm transparent
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duplicates the file and directory mode bits and the ownership in the
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lower layer to the created file in the upper layer.
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For behavior of the
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.Cm masquerade
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mode, see
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.Sx MASQUERADE MODE
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below.
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.It Cm whiteout No = Cm always | whenneeded
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Specifies whether whiteouts should always be made in the upper layer
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when removing a file or directory or only when it already exists in the
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lower layer.
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.It Cm udir Ns = Ns Ar mode
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Specifies directory mode bits in octal for
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.Cm masquerade
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mode.
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.It Cm ufile Ns = Ns Ar mode
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Specifies file mode bits in octal for
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.Cm masquerade
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mode.
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.It Cm gid Ns = Ns Ar gid
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Specifies group for
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.Cm masquerade
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mode.
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.It Cm uid Ns = Ns Ar uid
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Specifies user for
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.Cm masquerade
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mode.
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.El
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.El
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.Pp
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To enforce file system security, the user mounting a file system
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must be superuser or else have write permission on the mounted-on
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directory.
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In addition, the
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.Va vfs.usermount
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.Xr sysctl 8
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variable must be set to 1 to permit file system mounting by ordinary users.
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However, note that
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.Cm transparent
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and
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.Cm masquerade
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modes require
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.Va vfs.usermount
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to be set to 0 because this functionality can only be used by superusers.
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.Pp
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Filenames are looked up in the upper layer and then in the
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lower layer.
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If a directory is found in the lower layer, and there is no entry
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in the upper layer, then a
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.Em shadow
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directory will be created in the upper layer.
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The ownership and the mode bits are set depending on the
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.Cm copymode
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option.
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In
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.Cm traditional
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mode, it will be owned by the user who originally did the
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union mount, with mode 0777
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.Pq Dq Li rwxrwxrwx
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modified by the umask in effect at that time.
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.Pp
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If a file exists in the upper layer then there is no way to access
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a file with the same name in the lower layer.
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If necessary, a combination of loopback and union mounts can be made
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which will still allow the lower files to be accessed by a different
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pathname.
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.Pp
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Except in the case of a directory,
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access to an object is granted via the normal file system access checks.
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For directories, the current user must have access to both the upper
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and lower directories (should they both exist).
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.Pp
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Requests to create or modify objects in
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.Ar uniondir
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are passed to the upper layer with the exception of a few special cases.
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An attempt to open for writing a file which exists in the lower layer
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causes a copy of the
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.Em entire
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file to be made to the upper layer, and then for the upper layer copy
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to be opened.
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Similarly, an attempt to truncate a lower layer file to zero length
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causes an empty file to be created in the upper layer.
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Any other operation which would ultimately require modification to
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the lower layer fails with
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.Er EROFS .
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.Pp
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The union file system manipulates the namespace, rather than
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individual file systems.
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The union operation applies recursively down the directory tree
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now rooted at
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.Ar uniondir .
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Thus any file systems which are mounted under
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.Ar uniondir
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will take part in the union operation.
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This differs from the
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.Cm union
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option to
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.Xr mount 8
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which only applies the union operation to the mount point itself,
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and then only for lookups.
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.Sh MASQUERADE MODE
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When a file
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(or a directory)
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is created in the upper layer, the
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.Cm masquerade
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mode sets it the fixed access mode bits given in
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.Cm ufile
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(for files)
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or
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.Cm udir
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(for directories)
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option and the owner given in
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.Cm udir
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and
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.Cm gid
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options, instead of ones in the lower layer.
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Note that in the
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.Cm masquerade
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mode and when owner of the file or directory matches
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one specified in
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.Cm uid
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option, only mode bits for the owner will be modified.
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More specifically, the file mode bits in the upper layer will
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be
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(mode in the lower layer)
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OR
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(mode given in
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.Cm ufile
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AND 0700), and the ownership will be the same as one in the lower layer.
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.Pp
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The default values for
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.Cm ufile , udir , uid ,
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and
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.Cm gid
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are as follow:
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.Pp
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.Bl -bullet -compact
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.It
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If none of
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.Cm ufile
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and
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.Cm udir
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were specified, access mode bits in the mount point will be used.
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.It
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If none of
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.Cm uid
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and
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.Cm gid
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were specified, ownership in the mount point will be used.
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.It
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If one of
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.Cm udir
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or
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.Cm ufile
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is not specified, the value of the other option will be used.
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.It
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If one of
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.Cm uid
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or
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.Cm gid
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is not specified, the value of the other option will be used.
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.El
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.Sh EXAMPLES
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The commands
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.Bd -literal -offset indent
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mount -t cd9660 -o ro /dev/cd0 /usr/src
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mount -t unionfs -o noatime /var/obj /usr/src
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.Ed
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.Pp
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mount the CD-ROM drive
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.Pa /dev/cd0
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on
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.Pa /usr/src
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and then attaches
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.Pa /var/obj
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on top.
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For most purposes the effect of this is to make the
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source tree appear writable
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even though it is stored on a CD-ROM.
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The
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.Fl o Cm noatime
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option is useful to avoid unnecessary copying from the lower to the
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upper layer.
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.Pp
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The commands
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.Bd -literal -offset indent
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mount -t cd9660 -o ro /dev/cd0 /usr/src
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chown 2020 /usr/src
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mount -t unionfs -o noatime -o copymode=masquerade -o uid=builder \\
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-o udir=755 -o ufile=644 /var/obj /usr/src
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.Ed
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.Pp
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also mount the CD-ROM drive
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.Pa /dev/cd0
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on
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.Pa /usr/src
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and then attaches
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.Pa /var/obj
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on top.
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Furthermore, the owner of all files and directories in
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.Pa /usr/src
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is a regular user with UID 2020
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when seen from the upper layer.
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Note that for the access mode bits,
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ones in the lower layer
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(on the CD-ROM, in this example)
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are still used without change.
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Thus, write privilege to the upper layer can be controlled
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independently from access mode bits and ownership in the lower layer.
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If a user does not have read privilege from the lower layer,
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one cannot still read even when the upper layer is mounted by using
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.Cm masquerade
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mode.
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.Pp
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The command
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.Bd -literal -offset indent
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mount -t unionfs -o noatime -o below /sys $HOME/sys
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.Ed
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.Pp
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attaches the system source tree below the
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.Pa sys
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directory in the user's home directory.
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This allows individual users to make private changes
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to the source, and build new kernels, without those
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changes becoming visible to other users.
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Note that the files in the lower layer remain
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accessible via
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.Pa /sys .
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.Sh SEE ALSO
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.Xr intro 2 ,
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.Xr mount 2 ,
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.Xr unmount 2 ,
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.Xr fstab 5 ,
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.Xr mount 8 ,
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.Xr mount_nullfs 8
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.Sh HISTORY
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The
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.Nm mount_null
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utility first appeared in
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.Bx 4.4 .
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It was renamed to
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.Nm
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in
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.Fx 5.0 .
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.Pp
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The
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.Fl r
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option for hiding the lower layer completely was removed in
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.Fx 7.0
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because this is identical to using
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.Xr mount_nullfs 8 .
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.Sh AUTHORS
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.An -nosplit
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In
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.Fx 7.0 ,
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.An Masanori OZAWA Aq Mt ozawa@ongs.co.jp
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reimplemented handling of locking, whiteout, and file mode bits, and
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.An Hiroki Sato Aq Mt hrs@FreeBSD.org
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wrote about the changes in this manual page.
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.Sh BUGS
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THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
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AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.
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USE AT YOUR OWN RISK.
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.Pp
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This code also needs an owner in order to be less dangerous - serious
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hackers can apply by sending mail to
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.Aq Mt freebsd-fs@FreeBSD.org
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and announcing
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their intent to take it over.
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.Pp
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Without whiteout support from the file system backing the upper layer,
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there is no way that delete and rename operations on lower layer
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objects can be done.
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.Er EOPNOTSUPP
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is returned for this kind of operations as generated by VOP_WHITEOUT()
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along with any others which would make modifications to the lower
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layer, such as
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.Xr chmod 1 .
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.Pp
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Running
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.Xr find 1
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over a union tree has the side-effect of creating
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a tree of shadow directories in the upper layer.
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.Pp
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The current implementation does not support copying extended attributes
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for
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.Xr acl 9 ,
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.Xr mac 9 ,
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or so on to the upper layer.
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Note that this may be a security issue.
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.Pp
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A shadow directory, which is one automatically created in the upper
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layer when it exists in the lower layer and does not exist in the
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upper layer, is always created with the superuser privilege.
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However, a file copied from the lower layer in the same way
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is created by the user who accessed it.
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Because of this,
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if the user is not the superuser, even in
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.Cm transparent
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mode the access mode bits in the copied file in the upper layer
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will not always be the same as ones in the lower layer.
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This behavior should be fixed.
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