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diff --git a/share/man/man4/fdescfs.4 b/share/man/man4/fdescfs.4 new file mode 100644 index 000000000000..4b51c2bad369 --- /dev/null +++ b/share/man/man4/fdescfs.4 @@ -0,0 +1,218 @@ +.\" Copyright (c) 2021 The FreeBSD Foundation, Inc. +.\" +.\" Copyright (c) 1996 +.\" Mike Pritchard <mpp@FreeBSD.org>. All rights reserved. +.\" +.\" Copyright (c) 1992, 1993, 1994 +.\" The Regents of the University of California. All rights reserved. +.\" All rights reserved. +.\" +.\" This code is derived from software donated to Berkeley by +.\" Jan-Simon Pendry. +.\" +.\" Parts of this documentation was written by +.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship +.\" from the FreeBSD Foundation. +.\" +.\" 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. +.\" +.Dd Jul 11, 2023 +.Dt FDESCFS 4 +.Os +.Sh NAME +.Nm fdescfs +.Nd file-descriptor file system +.Sh SYNOPSIS +.Bd -literal +fdescfs /dev/fd fdescfs rw 0 0 +.Ed +.Sh DESCRIPTION +The file-descriptor file system, or +.Nm , +provides access to the per-process file descriptor +namespace in the global file system namespace. +The conventional mount point is +.Pa /dev/fd . +.Pp +The file system's contents +appear as a list of numbered files +which correspond to the open files of the process reading the +directory. +The files +.Pa /dev/fd/0 +through +.Pa /dev/fd/# +refer to file descriptors which can be accessed through the file +system. +.Pp +The following mount options can be used when mounting +.Nm +filesystem: +.Bl -tag -width linrdlnk +.It Cm nodup +For file descriptors referencing vnodes, instead of the +.Xr dup 2 +semantic described above, implement re-opening of the referenced vnode. +See below for more details. +.It Cm linrdlnk +Report the type of the +.Nm +vnode as +.Dv VLNK +instead of +.Fx +traditional +.Dv VCHR . +For +.Xr linux 4 +ABI compatibility mount +.Nm +volume with the +.Cm linrdlnk +option. +.It Cm rdlnk +Treat +.Nm +vnodes as symbolic links consistently, in particular, follow +the resolved name for the name lookups. +This option is strictly stronger than the +.Cm linrdlnk +option, it changes not only the type returned by +.Xr stat 2 , +but also causes the +.Nm +files to behave as symlinks. +.El +.Pp +For +.Nm +mounted without the +.Cm nodup +mount option, +if the file descriptor is open and the mode the file is being opened +with is a subset of the mode of the existing descriptor, the call: +.Bd -literal -offset indent +fd = open("/dev/fd/0", mode); +.Ed +.Pp +and the call: +.Bd -literal -offset indent +fd = fcntl(0, F_DUPFD, 0); +.Ed +.Pp +are equivalent. +Flags to the +.Xr open 2 +call other than +.Dv O_RDONLY , +.Dv O_WRONLY +and +.Dv O_RDWR +are ignored. +.Pp +For +.Nm +mounted with the +.Cm nodup +option, and file descriptor referencing a vnode, the call: +.Bd -literal -offset indent +fd = open("/dev/fd/0", mode); +.Ed +.Pp +reopens the referenced vnode with the specified +.Fa mode . +In other words, the +.Fn open +call above is equivalent to +.Bd -literal -offset indent +fd = openat(0, "", O_EMPTY_PATH, mode); +.Ed +.Pp +In particular, if the file descriptor was opened with the +.Dv O_PATH +flag, then either +.Dv O_EMPTY_PATH +or +.Fn open +over +.Nm +mount with +.Cm nodup +option allows one to convert it to a regularly opened file, +assuming that the current permissions allow the requested +.Fa mode . +.Pp +.Em "Note:" +.Pa /dev/fd/0 , +.Pa /dev/fd/1 +and +.Pa /dev/fd/2 +files are created by default when devfs alone is mounted. +.Nm +creates entries for all file descriptors opened by the process. +.Sh FILES +.Bl -tag -width /dev/stderr -compact +.It Pa /dev/fd/# +.El +.Sh EXAMPLES +To mount a +.Nm +volume located on +.Pa /dev/fd : +.Pp +.Dl "mount -t fdescfs none /dev/fd" +.Pp +For +.Xr linux 4 +ABI compatibility: +.Pp +.Dl "mount -t fdescfs -o linrdlnk none /compat/linux/dev/fd" +.Pp +For substitute of +.Dv O_EMPTY_PATH +flag use: +.Pp +.Dl "mount -t fdescfs -o nodup none /dev/fdpath" +.Sh SEE ALSO +.Xr devfs 5 , +.Xr mount 8 +.Sh HISTORY +The +.Nm +file system first appeared in +.Bx 4.4 . +The +.Nm +manual page first appeared in +.Fx 2.2 . +.Sh AUTHORS +.An -nosplit +The +.Nm +manual page was written by +.An Mike Pritchard Aq Mt mpp@FreeBSD.org , +and was based on the +manual page written by +.An Jan-Simon Pendry . |