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John Ericson
6563dcb6b1
Accept an empty `sun_path` when `fd` is not `AT_FDCWD`: the descriptor then names the peer unix socket directly, instead of being the starting directory for a pathname lookup. The held file reference keeps the peer PCB stable, playing the role `unp_vp_mtxpool` plays in the pathname path. The descriptor must carry `CAP_CONNECTAT` and refer to an `AF_UNIX` socket (`EPROTOTYPE` otherwise, `ENOTSOCK` for non-sockets). As with a pathname, a stream/seqpacket peer must be listening. No filesystem permission or MAC vnode check applies on this path: possession of the descriptor is the authorization, as with descriptor passing. Note this makes it possible to connect a datagram socket to an unbound peer, which no pathname could previously name. `connect(2)` and the implicit-connect send path pass `AT_FDCWD` and still reject an empty path with `EINVAL`. The `unp_sun_path()` call is hoisted out of `unp_connectat()` because the early exit conditions for the two system calls (`connect(2)` and `connectat(2)`) are slightly different. Additionally, support `/dev/fd/<N>`. In a world with `connectat(2)`, this is largely overkill, but this also allows me to add support for direct peer connections with plain `connect(2)`. I think that is a wise choice because this will allow me to propose this functionality for Linux too without a new system call (saving that conversation for later). Ultimately, I want to see multiple operating systems support this to foster broader userland adoption, which should benefit everyone including FreeBSD --- it's nicer if more 3rd party in addition to 1st party software uses the new kernel functionality. Therefore, I hope this additional feature is also acceptable. Signed-off-by: John Ericson <[email protected]> Assisted-by: Claude Code (Claude Opus 4.8 and Fable 5) Reviewed by: markj MFC after: 2 months Differential Revision: https://reviews.freebsd.org/D58405
157 lines
4.0 KiB
Plaintext
157 lines
4.0 KiB
Plaintext
.\" Copyright (c) 2013 The FreeBSD Foundation
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.\"
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.\" This documentation was written by Pawel Jakub Dawidek under sponsorship from
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.\" 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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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS 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 AUTHORS 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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.Dd July 25, 2026
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.Dt CONNECTAT 2
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.Os
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.Sh NAME
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.Nm connectat
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.Nd initiate a connection on a socket
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/socket.h
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.Pp
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.In fcntl.h
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.Ft int
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.Fn connectat "int fd" "int s" "const struct sockaddr *name" "socklen_t namelen"
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.Sh DESCRIPTION
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The
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.Fn connectat
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system call initiates a connection on the socket
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.Fa s .
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When passed the special value
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.Dv AT_FDCWD
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in the
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.Fa fd
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parameter, the behavior is identical to a call to
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.Xr connect 2 .
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Otherwise,
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.Fn connectat
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works like the
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.Xr connect 2
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system call with two exceptions:
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.Pp
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.Bl -enum -offset indent -compact
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.It
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It is limited to sockets in the PF_LOCAL domain.
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.Pp
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.It
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If the file path stored in the
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.Fa sun_path
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field of the sockaddr_un structure is a relative path, it is located relative
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to the directory associated with the file descriptor
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.Fa fd .
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.Pp
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.It
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If the
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.Fa sun_path
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field is empty, that is,
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.Fa namelen
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equals
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.Li offsetof(struct sockaddr_un, sun_path) ,
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then
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.Fa fd
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does not resolve a path but instead names the peer socket directly.
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.Pp
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The descriptor may be the peer socket itself, or a descriptor for a file, such
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as an
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.Dv O_PATH
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handle opened with
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.Xr open 2 .
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In the latter case, the
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.Dv O_PATH
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file description may point either to a traditional socket file bound to the
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peer, or to a
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.Pa /dev/fd/N
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node.
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The
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.Pa /dev/fd/N
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node must name the socket descriptor itself; the named descriptor is resolved a
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single level and not chased further.
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With a standard
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.Xr fdescfs 5
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mount this means a node naming a traditional socket file, or another
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.Pa /dev/fd/N
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node, does not resolve to a peer.
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.Pp
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In all cases,
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.Fa fd
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must carry the
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.Dv CAP_CONNECTAT
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capability right, and the target of a
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.Pa /dev/fd/N
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node must also carry that right; see
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.Xr unix 4
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for the details of this form.
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.El
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.Sh RETURN VALUES
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.Rv -std connectat
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.Sh ERRORS
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The
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.Fn connectat
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system call may fail with the same errors as the
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.Xr connect 2
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system call or with the following errors:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa sun_path
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field does not specify an absolute path and the
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.Fa fd
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argument is neither
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.Dv AT_FDCWD
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nor a valid file descriptor.
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.It Bq Er ENOTDIR
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The
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.Fa sun_path
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field is not an absolute path and
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.Fa fd
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is neither
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.Dv AT_FDCWD
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nor a file descriptor associated with a directory.
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.It Bq Er EINVAL
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The
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.Fa sun_path
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field is empty but
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.Fa fd
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is
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.Dv AT_FDCWD ,
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so no descriptor names the peer.
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.El
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.Sh SEE ALSO
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.Xr bindat 2 ,
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.Xr connect 2 ,
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.Xr socket 2 ,
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.Xr unix 4
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.Sh AUTHORS
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The
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.Nm
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was developed by
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.An Pawel Jakub Dawidek Aq Mt [email protected]
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under sponsorship from the FreeBSD Foundation.
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