77d3337c9f
Linux specific things to the native fdescfs file system.
Unlike FreeBSD, the Linux fdescfs is a directory containing a symbolic
links to the actual files, which the process has open.
A readlink(2) call on this file returns a full path in case of regular file
or a string in a special format (type:[inode], anon_inode:<file-type>, etc..).
As well as in a FreeBSD, opening the file in the Linux fdescfs directory is
equivalent to duplicating the corresponding file descriptor.
Here we have mutually exclusive requirements:
- in case of readlink(2) call fdescfs lookup() method should return VLNK
vnode otherwise our kern_readlink() fail with EINVAL error;
- in the other calls fdescfs lookup() method should return non VLNK vnode.
For what new vnode v_flag VV_READLINK was added, which is set if fdescfs has beed
mounted with linrdlnk option an modified kern_readlinkat() to properly handle it.
For now For Linux ABI compatibility mount fdescfs volume with linrdlnk option:
mount -t fdescfs -o linrdlnk null /compat/linux/dev/fd
Reviewed by: kib@
MFC after: 1 week
Relnotes: yes
141 lines
3.6 KiB
Groff
141 lines
3.6 KiB
Groff
.\" 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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.\" 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 August 1, 2017
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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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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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.Pp
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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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.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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.Pp
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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
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.Cm linrdlnk
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option.
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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 null /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 null /compat/linux/dev/fd"
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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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