[PATCH v2 3/8] man/man7/unix.7: Autobind: Clarify when it happens, and mention SO_PASSPIDFD

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

Autobind happens if either one of SO_PASSCRED/SO_PASSPIDFD is set and
either connect() or sendmsg() happens.  Mention SO_PASSPIDFD and
clarify that autobind does not happen immediately when setting these
socket options.

In linux.git net/unix/af_unix.c, unix_autobind() is called by
unix_dgram_connect(), unix_stream_connect() and unix_dgram_sendmsg(),
if the socket has no address and unix_may_passcred() is true.

unix_may_passcred() is also true when SO_PASSPIDFD is set, which was
added in linux.git 5e2ff6704a27 (2023-06-12; "scm: add SO_PASSPIDFD and
SCM_PIDFD"), Linux 6.5.

Also, the old text said "not yet connected", but the condition is
about the socket not being bound.

Fixes: 9d1fd1157306 (1999-06-15, 2022-12-19; "man-pages 1.24")
Fixes: 0cf2caa41ea6 (2010-10-10; "unix.7: Document the autobind feature")
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---

Notes:
    Rephrased the LLM-generated text so that the canonical description is
    in the "Autobind feature" section and the other place just refers to
    it.
    
    SO_PASSPIDFD is unfortunately not documented yet at this point in
    time, but it's still correct to mention it here.

 man/man7/unix.7 | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/man/man7/unix.7 b/man/man7/unix.7
index 1695f1c120c7..61f06a74359b 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -296,8 +296,14 @@ .SS Socket options
 .B SCM_CREDENTIALS
 ancillary data.
 .IP
-When this option is set and the socket is not yet connected,
-a unique name in the abstract namespace will be generated automatically.
+When this option is set on a socket that is not bound to an address,
+the socket is autobound to a unique name in the abstract namespace
+when
+.BR connect (2)
+or
+.BR sendmsg (2)
+is called on it;
+see "Autobind feature", below.
 .IP
 The value given as an argument to
 .BR setsockopt (2)
@@ -365,10 +371,18 @@ .SS Autobind feature
 as
 .IR sizeof(sa_family_t) ,
 .\" i.e., sizeof(short)
-or the
+or if
+.BR connect (2)
+or
+.BR sendmsg (2)
+is called on a socket that was
+not explicitly bound to an address
+and that has the
 .B SO_PASSCRED
-socket option was specified for a socket that was
-not explicitly bound to an address,
+or
+.B SO_PASSPIDFD
+(since Linux 6.5)
+socket option enabled,
 then the socket is autobound to an abstract address.
 The address consists of a null byte
 followed by 5 bytes in the character set
-- 
2.56.0