[PATCH v2 6/8] man/man7/unix.7: Document SO_PASSPIDFD, SO_PEERPIDFD, and SCM_PIDFD

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

These were added in linux.git 5e2ff6704a27 (2023-06-12; "scm: add
SO_PASSPIDFD and SCM_PIDFD") and 7b26952a91cf (2023-06-12; "net: core:
add getsockopt SO_PEERPIDFD"), both in Linux 6.5.  They are the PID
file descriptor counterparts of SO_PASSCRED/SCM_CREDENTIALS and
SO_PEERCRED.

SO_PASSPIDFD
	sk_setsockopt() and sk_getsockopt() in net/core/sock.c handle it
	as a boolean flag for AF_UNIX sockets (EOPNOTSUPP otherwise).
	Like SO_PASSCRED, it triggers autobind, which was already
	mentioned in "Autobind feature".

SCM_PIDFD
	scm_pidfd_recv() in net/core/scm.c creates the PID file
	descriptor from scm->pid, which is the PID given in the sender's
	SCM_CREDENTIALS, or the sender's own PID by default.  The fd is
	created with O_CLOEXEC by pidfd_prepare().  If pidfd_prepare()
	fails, its negative error number is put into the cmsg data.
	Sending SCM_PIDFD is rejected by __scm_send() (EINVAL).

	Since Linux 6.17, with c679d17d3f2d (2025-07-04; "af_unix:
	enable handing out pidfds for reaped tasks in SCM_PIDFD"),
	PIDFD_STALE is passed, so this also works for reaped processes.
	Before that, pidfd_prepare() returned -EINVAL for them.

SO_PEERPIDFD
	sk_getsockopt() returns a PID file descriptor for sk_peer_pid,
	or fails with ENODATA if there is none.

	Since Linux 6.16, with 20b70e58961b (2025-04-28; "net, pidfs:
	enable handing out pidfds for reaped sk->sk_peer_pid"), this also
	works for reaped processes on AF_UNIX sockets.  Before that, it
	failed with EINVAL.

Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Eric Dumazet <edumazet@kernel.org>
Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 man/man7/unix.7 | 100 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 100 insertions(+)

diff --git a/man/man7/unix.7 b/man/man7/unix.7
index 93a0a335e791..140e77ea12ee 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -311,6 +311,32 @@ .SS Socket options
 .BR getsockopt (2)
 is an integer boolean flag.
 .TP
+.BR SO_PASSPIDFD " (since Linux 6.5)"
+Enabling this socket option causes receipt of a PID file descriptor
+(see
+.BR pidfd_open (2))
+referring to the sending process in an
+.B SCM_PIDFD
+ancillary message in each subsequently received message
+(see below).
+.IP
+As with
+.BR SO_PASSCRED ,
+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)
+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
@@ -362,6 +388,43 @@ .SS Socket options
 .B AF_UNIX
 socket pairs created using
 .BR socketpair (2).
+.TP
+.BR SO_PEERPIDFD " (since Linux 6.5)"
+.\" commit 7b26952a91cf65ff1cc867a2382a8964d8c0ee7d
+This read-only socket option returns a new PID file descriptor
+(see
+.BR pidfd_open (2))
+referring to the peer process connected to this socket.
+This is the process whose credentials are returned by
+.BR SO_PEERCRED .
+Unlike the PID returned by
+.BR SO_PEERCRED ,
+the PID file descriptor cannot come to refer to a different process
+if the peer process terminates and its PID is reused.
+.IP
+The argument to
+.BR getsockopt (2)
+is a pointer to an
+.IR int ,
+in which the PID file descriptor is returned.
+The close-on-exec flag
+.RB ( FD_CLOEXEC )
+is set on the new file descriptor.
+.IP
+If the socket has no peer process,
+.BR getsockopt (2)
+fails with the error
+.BR ENODATA .
+.IP
+Since Linux 6.16,
+.\" commit 20b70e58961b855e61f2c6a7a19a38a64ad1bed6
+a PID file descriptor is returned
+even if the peer process has already terminated and been reaped.
+Before Linux 6.16,
+.BR getsockopt (2)
+failed with the error
+.B EINVAL
+in this case.
 .\"
 .SS Autobind feature
 If a
@@ -537,6 +600,42 @@ .SS Ancillary messages
 .B SO_PASSCRED
 option must be enabled on the socket.
 .TP
+.BR SCM_PIDFD " (since Linux 6.5)"
+.\" commit 5e2ff6704a275be009be8979af17c52361b79b89
+Receive a PID file descriptor
+(see
+.BR pidfd_open (2))
+referring to the sending process.
+The data portion contains a single
+.IR int .
+The PID file descriptor refers to the process whose PID would be
+received in an
+.B SCM_CREDENTIALS
+message;
+if the sender specified
+.B SCM_CREDENTIALS
+ancillary data,
+it refers to the process with the PID given there.
+The close-on-exec flag
+.RB ( FD_CLOEXEC )
+is set on the new file descriptor.
+.IP
+If the PID file descriptor cannot be created,
+the data portion contains a negative error number instead.
+Since Linux 6.17,
+.\" commit c679d17d3f2d895b34e660673141ad250889831f
+a PID file descriptor is created
+even if the sending process has already terminated and been reaped.
+Before Linux 6.17,
+.B \-EINVAL
+was received in this case.
+.IP
+This message cannot be sent;
+it is generated by the kernel.
+To receive it, the
+.B SO_PASSPIDFD
+option must be enabled on the socket.
+.TP
 .B SCM_SECURITY
 Receive the security context (the security label)
 of the peer socket,
@@ -1252,6 +1351,7 @@ .SS Program source
 and
 .BR seccomp_unotify (2).
 .SH SEE ALSO
+.BR pidfd_open (2),
 .BR recvmsg (2),
 .BR sendmsg (2),
 .BR socket (2),
-- 
2.56.0