[PATCH v2] socket.7,unix.7: add initial description for SO_PEERSEC

[Stephen Smalley <stephen.smalley.work@gmail.com>]

SO_PEERSEC was introduced for AF_UNIX stream sockets connected via
connect(2) in Linux 2.6.2 [1] and later augmented to support AF_UNIX stream
and datagram sockets created via socketpair(2) in Linux 4.18 [2].  Document
SO_PEERSEC in the socket.7 and unix.7 man pages following the example
of the existing SO_PEERCRED descriptions.  SO_PEERSEC is also supported
on AF_INET sockets when using labeled IPSEC or NetLabel but defer
adding a description of that support to a separate patch.

The module-independent description of the security context returned
by SO_PEERSEC is from Simon McVittie <smcv@collabora.com>.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=da6e57a2e6bd7939f610d957afacaf6a131e75ed

[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0b811db2cb2aabc910e53d34ebb95a15997c33e7

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
---
v2 adds kernel commit info to the description and man page and uses
the suggested text from Simon McVittie for the description of
the security context string in a module-neutral way.

 man7/socket.7 |  5 +++++
 man7/unix.7   | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/man7/socket.7 b/man7/socket.7
index 21e891791..c3635f95b 100644
--- a/man7/socket.7
+++ b/man7/socket.7
@@ -690,6 +690,11 @@ Return the credentials of the peer process connected to this socket.
 For further details, see
 .BR unix (7).
 .TP
+.BR SO_PEERSEC " (since Linux 2.6.2)"
+Return the security context of the peer socket connected to this socket.
+For further details, see
+.BR unix (7).
+.TP
 .B SO_PRIORITY
 Set the protocol-defined priority for all packets to be sent on
 this socket.
diff --git a/man7/unix.7 b/man7/unix.7
index 50828a5bc..298521d4a 100644
--- a/man7/unix.7
+++ b/man7/unix.7
@@ -349,6 +349,52 @@ stream sockets and for
 .B AF_UNIX
 stream and datagram socket pairs created using
 .BR socketpair (2).
+.TP
+.B SO_PEERSEC
+This read-only socket option returns the
+security context of the peer socket connected to this socket.
+By default, this will be the same as the security context of
+the process that created the peer socket unless overridden
+by the policy or by a process with the required permissions.
+.IP
+The argument to
+.BR getsockopt (2)
+is a pointer to a
+buffer of the specified length in bytes
+into which the security context string will be copied.
+If the buffer length is less than the length of the security
+context string, then
+.BR getsockopt (2)
+will return the required length
+via
+.I optlen
+and return \-1 and sets
+.I errno
+to
+.BR ERANGE .
+The caller should allocate at least
+.BR NAME_MAX
+bytes for the buffer initially although this is not guaranteed
+to be sufficient.  Resizing the buffer to the returned length
+and retrying may be necessary.
+.IP
+The security context string may include a terminating null character
+in the returned length, but is not guaranteed to do so: a security
+context "foo" might be represented as either {'f','o','o'} of length 3
+or {'f','o','o','\\0'} of length 4, which are considered to be
+interchangeable. It is printable, does not contain non-terminating
+null characters, and is in an unspecified encoding (in particular it
+is not guaranteed to be ASCII or UTF-8).
+.IP
+The use of this option for sockets in the
+.B AF_UNIX
+address family
+is supported since Linux 2.6.2 for connected stream sockets and
+since Linux 4.18,
+.\" commit 0b811db2cb2aabc910e53d34ebb95a15997c33e7
+also for stream and datagram socket pairs created
+using
+.BR socketpair (2).
 .\"
 .SS Autobind feature
 If a
-- 
2.25.1