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e301fd984a
Document AF_UNIX control messages in unix(4) only, not split between unix(4) and recv(2). Also, warn about LOCAL_CREDS effective uid/gid fields, since the write could be from a setuid or setgid program (with the explicit SCM_CREDS and LOCAL_PEERCRED, the credentials are read at such a time that it can be assumed that the process intends for them to be used in this context). Reviewed by: wblock MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D9298
364 lines
8.9 KiB
Groff
364 lines
8.9 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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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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.\" @(#)unix.4 8.1 (Berkeley) 6/9/93
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.\" $FreeBSD$
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.\"
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.Dd February 3, 2017
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.Dt UNIX 4
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.Os
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.Sh NAME
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.Nm unix
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.Nd UNIX-domain protocol family
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/un.h
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.Sh DESCRIPTION
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The
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.Ux Ns -domain
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protocol family is a collection of protocols
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that provides local (on-machine) interprocess
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communication through the normal
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.Xr socket 2
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mechanisms.
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The
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.Ux Ns -domain
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family supports the
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.Dv SOCK_STREAM ,
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.Dv SOCK_SEQPACKET ,
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and
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.Dv SOCK_DGRAM
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socket types and uses
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file system pathnames for addressing.
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.Sh ADDRESSING
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.Ux Ns -domain
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addresses are variable-length file system pathnames of
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at most 104 characters.
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The include file
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.In sys/un.h
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defines this address:
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.Bd -literal -offset indent
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struct sockaddr_un {
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u_char sun_len;
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u_char sun_family;
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char sun_path[104];
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};
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.Ed
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.Pp
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Binding a name to a
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.Ux Ns -domain
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socket with
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.Xr bind 2
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causes a socket file to be created in the file system.
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This file is
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.Em not
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removed when the socket is closed \(em
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.Xr unlink 2
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must be used to remove the file.
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.Pp
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The length of
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.Ux Ns -domain
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address, required by
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.Xr bind 2
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and
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.Xr connect 2 ,
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can be calculated by the macro
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.Fn SUN_LEN
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defined in
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.In sys/un.h .
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The
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.Va sun_path
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field must be terminated by a
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.Dv NUL
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character to be used with
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.Fn SUN_LEN ,
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but the terminating
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.Dv NUL
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is
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.Em not
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part of the address.
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.Pp
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The
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.Ux Ns -domain
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protocol family does not support broadcast addressing or any form
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of
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.Dq wildcard
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matching on incoming messages.
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All addresses are absolute- or relative-pathnames
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of other
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.Ux Ns -domain
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sockets.
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Normal file system access-control mechanisms are also
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applied when referencing pathnames; e.g., the destination
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of a
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.Xr connect 2
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or
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.Xr sendto 2
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must be writable.
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.Sh CONTROL MESSAGES
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The
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.Ux Ns -domain
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sockets support the communication of
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.Ux
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file descriptors and process credentials through the use of the
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.Va msg_control
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field in the
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.Fa msg
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argument to
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.Xr sendmsg 2
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and
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.Xr recvmsg 2 .
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The items to be passed are described using a
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.Vt "struct cmsghdr"
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that is defined in the include file
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.In sys/socket.h .
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.Pp
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To send file descriptors, the type of the message is
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.Dv SCM_RIGHTS ,
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and the data portion of the messages is an array of integers
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representing the file descriptors to be passed.
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The number of descriptors being passed is defined
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by the length field of the message;
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the length field is the sum of the size of the header
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plus the size of the array of file descriptors.
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.Pp
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The received descriptor is a
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.Em duplicate
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of the sender's descriptor, as if it were created via
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.Li dup(fd)
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or
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.Li fcntl(fd, F_DUPFD_CLOEXEC, 0)
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depending on whether
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.Dv MSG_CMSG_CLOEXEC
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is passed in the
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.Xr recvmsg 2
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call.
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Descriptors that are awaiting delivery, or that are
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purposely not received, are automatically closed by the system
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when the destination socket is closed.
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.Pp
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Credentials of the sending process can be transmitted explicitly using a
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control message of type
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.Dv SCM_CREDS
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with a data portion of type
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.Vt "struct cmsgcred" ,
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defined in
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.In sys/socket.h
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as follows:
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.Bd -literal
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struct cmsgcred {
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pid_t cmcred_pid; /* PID of sending process */
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uid_t cmcred_uid; /* real UID of sending process */
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uid_t cmcred_euid; /* effective UID of sending process */
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gid_t cmcred_gid; /* real GID of sending process */
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short cmcred_ngroups; /* number of groups */
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gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
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};
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.Ed
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.Pp
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The sender should pass a zeroed buffer which will be filled in by the system.
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.Pp
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The group list is truncated to at most
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.Dv CMGROUP_MAX
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GIDs.
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.Pp
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The process ID
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.Fa cmcred_pid
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should not be looked up (such as via the
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.Dv KERN_PROC_PID
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sysctl) for making security decisions.
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The sending process could have exited and its process ID already been
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reused for a new process.
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.Sh SOCKET OPTIONS
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.Tn UNIX
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domain sockets support a number of socket options which can be set with
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.Xr setsockopt 2
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and tested with
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.Xr getsockopt 2 :
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.Bl -tag -width ".Dv LOCAL_CONNWAIT"
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.It Dv LOCAL_CREDS
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This option may be enabled on
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.Dv SOCK_DGRAM ,
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.Dv SOCK_SEQPACKET ,
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or a
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.Dv SOCK_STREAM
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socket.
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This option provides a mechanism for the receiver to
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receive the credentials of the process calling
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.Xr write 2 ,
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.Xr send 2 ,
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.Xr sendto 2
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or
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.Xr sendmsg 2
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as a
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.Xr recvmsg 2
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control message.
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The
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.Va msg_control
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field in the
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.Vt msghdr
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structure points to a buffer that contains a
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.Vt cmsghdr
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structure followed by a variable length
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.Vt sockcred
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structure, defined in
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.In sys/socket.h
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as follows:
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.Bd -literal
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struct sockcred {
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uid_t sc_uid; /* real user id */
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uid_t sc_euid; /* effective user id */
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gid_t sc_gid; /* real group id */
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gid_t sc_egid; /* effective group id */
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int sc_ngroups; /* number of supplemental groups */
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gid_t sc_groups[1]; /* variable length */
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};
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.Ed
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.Pp
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The current implementation truncates the group list to at most
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.Dv CMGROUP_MAX
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groups.
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.Pp
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The
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.Fn SOCKCREDSIZE
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macro computes the size of the
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.Vt sockcred
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structure for a specified number
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of groups.
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The
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.Vt cmsghdr
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fields have the following values:
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.Bd -literal
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cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups))
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cmsg_level = SOL_SOCKET
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cmsg_type = SCM_CREDS
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.Ed
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.Pp
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On
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.Dv SOCK_STREAM
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and
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.Dv SOCK_SEQPACKET
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sockets credentials are passed only on the first read from a socket,
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then the system clears the option on the socket.
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.Pp
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This option and the above explicit
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.Vt "struct cmsgcred"
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both use the same value
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.Dv SCM_CREDS
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but incompatible control messages.
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If this option is enabled and the sender attached a
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.Dv SCM_CREDS
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control message with a
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.Vt "struct cmsgcred" ,
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it will be discarded and a
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.Vt "struct sockcred"
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will be included.
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.Pp
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Many setuid programs will
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.Xr write 2
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data at least partially controlled by the invoker,
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such as error messages.
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Therefore, a message accompanied by a particular
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.Fa sc_euid
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value should not be trusted as being from that user.
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.It Dv LOCAL_CONNWAIT
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Used with
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.Dv SOCK_STREAM
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sockets, this option causes the
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.Xr connect 2
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function to block until
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.Xr accept 2
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has been called on the listening socket.
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.It Dv LOCAL_PEERCRED
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Requested via
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.Xr getsockopt 2
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on a
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.Dv SOCK_STREAM
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socket returns credentials of the remote side.
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These will arrive in the form of a filled in
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.Vt xucred
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structure, defined in
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.In sys/ucred.h
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as follows:
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.Bd -literal
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struct xucred {
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u_int cr_version; /* structure layout version */
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uid_t cr_uid; /* effective user id */
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short cr_ngroups; /* number of groups */
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gid_t cr_groups[XU_NGROUPS]; /* groups */
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};
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.Ed
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The
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.Vt cr_version
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fields should be checked against
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.Dv XUCRED_VERSION
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define.
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.Pp
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The credentials presented to the server (the
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.Xr listen 2
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caller) are those of the client when it called
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.Xr connect 2 ;
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the credentials presented to the client (the
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.Xr connect 2
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caller) are those of the server when it called
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.Xr listen 2 .
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This mechanism is reliable; there is no way for either party to influence
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the credentials presented to its peer except by calling the appropriate
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system call (e.g.,
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.Xr connect 2
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or
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.Xr listen 2 )
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under different effective credentials.
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.Pp
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To reliably obtain peer credentials on a
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.Dv SOCK_DGRAM
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socket refer to the
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.Dv LOCAL_CREDS
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socket option.
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.El
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.Sh SEE ALSO
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.Xr connect 2 ,
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.Xr dup 2 ,
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.Xr fcntl 2 ,
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.Xr getsockopt 2 ,
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.Xr listen 2 ,
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.Xr recvmsg 2 ,
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.Xr sendto 2 ,
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.Xr setsockopt 2 ,
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.Xr socket 2 ,
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.Xr intro 4
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.Rs
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.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
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.%B PS1
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.%N 7
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.Re
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.Rs
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.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
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.%B PS1
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.%N 8
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.Re
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