f9b1e711f0
The 'nodup' option forces fdescfs to return real vnode behind file
descriptor instead of the fdescfs fd vnode, on lookup. The end result
is that e.g. stat("/dev/fd/3") returns the stat data for the underlying
vnode, if any. Similarly, fchdir(2) works in the expected way.
For open(2), if applied over file descriptor opened with O_PATH, it
effectively re-open that vnode into normal file descriptor which has the
specified access mode, assuming the current vnode permissions allow it.
If the file descriptor does not reference vnode, the behavior is unchanged.
This is done by a mount option, because permission check on open(2) breaks
established fdescfs open semantic of dup(2)-ing the descriptor. So it
is not suitable for /dev/fd mount.
Tested by: Andrew Walker <awalker@ixsystems.com>
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30140
209 lines
5.0 KiB
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209 lines
5.0 KiB
Groff
.\" Copyright (c) 2021 The FreeBSD Foundation, Inc.
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.\"
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.\" Copyright (c) 1996
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.\" Mike Pritchard <mpp@FreeBSD.org>. All rights reserved.
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.\"
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.\" Copyright (c) 1992, 1993, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\" 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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.\" Parts of this documentation was written by
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.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
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.\" from the FreeBSD Foundation.
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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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.\" $FreeBSD$
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.\"
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.Dd May 17, 2021
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.Dt FDESCFS 5
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.Os
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.Sh NAME
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.Nm fdescfs
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.Nd file-descriptor file system
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.Sh SYNOPSIS
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.Bd -literal
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fdescfs /dev/fd fdescfs rw 0 0
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.Ed
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.Sh DESCRIPTION
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The file-descriptor file system, or
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.Nm ,
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provides access to the per-process file descriptor
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namespace in the global file system namespace.
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The conventional mount point is
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.Pa /dev/fd .
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.Pp
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The file system's contents
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appear as a list of numbered files
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which correspond to the open files of the process reading the
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directory.
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The files
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.Pa /dev/fd/0
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through
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.Pa /dev/fd/#
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refer to file descriptors which can be accessed through the file
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system.
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.Pp
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The following mount options can be used when mounting
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.Nm
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filesystem:
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.Bl -tag -width linrdlnk
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.It Cm nodup
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For file descriptors referencing vnodes, instead of the
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.Xr dup 2
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semantic described above, implement re-opening of the referenced vnode.
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See below for more details.
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.It Cm linrdlnk
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Report the type of the
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.Nm
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vnode as
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.Dv VLNK
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instead of
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.Fx
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traditional
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.Dv VCHR .
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For
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.Xr linux 4
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ABI compatibility mount
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.Nm
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volume with the
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.Cm linrdlnk
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option.
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.El
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.Pp
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For
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.Nm
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mounted without the
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.Cm nodup
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mount option,
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if the file descriptor is open and the mode the file is being opened
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with is a subset of the mode of the existing descriptor, the call:
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.Bd -literal -offset indent
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fd = open("/dev/fd/0", mode);
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.Ed
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.Pp
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and the call:
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.Bd -literal -offset indent
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fd = fcntl(0, F_DUPFD, 0);
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.Ed
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.Pp
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are equivalent.
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Flags to the
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.Xr open 2
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call other than
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.Dv O_RDONLY ,
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.Dv O_WRONLY
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and
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.Dv O_RDWR
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are ignored.
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.Pp
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For
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.Nm
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mounted with the
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.Cm nodup
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option, and file descriptor referencing a vnode, the call:
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.Bd -literal -offset indent
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fd = open("/dev/fd/0", mode);
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.Ed
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.Pp
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reopens the referenced vnode with the specified
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.Fa mode .
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In other words, the
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.Fn open
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call above is equivalent to
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.Bd -literal -offset indent
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fd = openat(0, "", O_EMPTY_PATH, mode);
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.Ed
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.Pp
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In particular, if the file descriptor was opened with the
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.Dv O_PATH
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flag, then either
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.Dv O_EMPTY_PATH
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or
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.Fn open
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over
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.Nm
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mount with
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.Cm nodup
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option allows one to convert it to a regularly opened file,
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assuming that the current permissions allow the requested
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.Fa mode .
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.Pp
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.Em "Note:"
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.Pa /dev/fd/0 ,
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.Pa /dev/fd/1
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and
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.Pa /dev/fd/2
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files are created by default when devfs alone is mounted.
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.Nm
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creates entries for all file descriptors opened by the process.
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.Sh FILES
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.Bl -tag -width /dev/stderr -compact
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.It Pa /dev/fd/#
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.El
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.Sh EXAMPLES
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To mount a
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.Nm
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volume located on
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.Pa /dev/fd :
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.Pp
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.Dl "mount -t fdescfs none /dev/fd"
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.Pp
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For
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.Xr linux 4
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ABI compatibility:
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.Pp
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.Dl "mount -t fdescfs -o linrdlnk none /compat/linux/dev/fd"
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.Pp
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For substitute of
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.Dv O_EMPTY_PATH
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flag use:
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.Pp
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.Dl "mount -t fdescfs -o nodup none /dev/fdpath"
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.Sh SEE ALSO
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.Xr devfs 5 ,
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.Xr mount 8
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.Sh HISTORY
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The
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.Nm
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file system first appeared in
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.Bx 4.4 .
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The
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.Nm
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manual page first appeared in
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.Fx 2.2 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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manual page was written by
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.An Mike Pritchard Aq Mt mpp@FreeBSD.org ,
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and was based on the
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manual page written by
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.An Jan-Simon Pendry .
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