[PATCH 7/7] man/man7/unix.7: Document SO_PASSRIGHTS

["Günther Noack" <gnoack3000@gmail.com>]

SO_PASSRIGHTS was added in linux.git 77cbe1a6d873 (2025-05-23;
"af_unix: Introduce SO_PASSRIGHTS."), in Linux 6.16.  It lets a
receiver refuse file descriptors passed with SCM_RIGHTS, so that an
untrusted peer cannot leave it with file descriptors whose final
close() blocks for a long time (for example, a file on a hung NFS
mount or a FUSE-controlled file).

The option is enabled by default (unix_create1()).  When the receiving
socket has it disabled, unix_dgram_sendmsg(), unix_stream_sendmsg(),
and queue_oob() fail with EPERM if the message carries file
descriptors.  The check happens at send time, so messages that are
already queued are unaffected.

Also add the new EPERM case to ERRORS.

Link: [1] <https://uapi-group.org/kernel-features/#disabling-reception-of-scm_rights-for-af_unix-sockets>
Link: [2] <https://lore.kernel.org/all/20250519205820.66184-1-kuniyu@amazon.com/>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 man/man7/unix.7 | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/man/man7/unix.7 b/man/man7/unix.7
index a4457508fd2a..1aa223b2c79d 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -343,6 +343,35 @@ and returned as the result of
 .BR getsockopt (2)
 is an integer boolean flag.
 .TP
+.BR SO_PASSRIGHTS " (since Linux 6.16)"
+.\" commit 77cbe1a6d8730a07f99f9263c2d5f2304cf5e830
+This socket option controls whether the socket accepts
+file descriptors from its peers in
+.B SCM_RIGHTS
+ancillary messages (see below).
+It is enabled by default.
+When it is disabled,
+.BR sendmsg (2)
+calls that pass file descriptors to this socket
+fail with the error
+.BR EPERM .
+The option is checked when the message is sent;
+disabling it does not affect messages
+that are already queued on the socket.
+.IP
+Processes that do not expect to receive file descriptors
+can disable this option.
+Once the file descriptors have been passed,
+closing them is the responsibility of the receiving process,
+and this may block for a long time,
+for example for a file on an unresponsive network file system.
+.IP
+The value given as an argument to
+.BR setsockopt (2)
+and returned as the result of
+.BR getsockopt (2)
+is an integer boolean flag.
+.TP
 .B SO_PASSSEC
 Enables receiving of the security context of the peer socket
 in an ancillary message of type
@@ -893,6 +922,14 @@ or
 specified a datagram socket as the remote address,
 but that socket is connected to a different socket.
 .TP
+.B EPERM
+The sender passed file descriptors in an
+.B SCM_RIGHTS
+ancillary message,
+but the receiving socket has the
+.B SO_PASSRIGHTS
+option disabled.
+.TP
 .B EPIPE
 Remote socket was closed on a stream socket.
 If enabled, a
-- 
2.56.0