af_unix: introduce and use scm_replace_pid() helper

Existing logic in __scm_send() related to filling an struct scm_cookie
with a proper struct pid reference is already pretty tricky. Let's
simplify it a bit by introducing a new helper. This helper will be
extended in one of the next patches.

Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Luca Boccassi <bluca@debian.org>
Cc: David Rheinsberg <david@readahead.eu>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Link: https://lore.kernel.org/20250703222314.309967-4-aleksandr.mikhalitsyn@canonical.com
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mihalicyn

net/core/scm.c

@@ -145,6 +145,16 @@ void __scm_destroy(struct scm_cookie *scm)
 }
 EXPORT_SYMBOL(__scm_destroy);
 
+static inline int scm_replace_pid(struct scm_cookie *scm, struct pid *pid)
+{
+	/* drop all previous references */
+	scm_destroy_cred(scm);
+
+	scm->pid = pid;
+	scm->creds.pid = pid_vnr(pid);
+	return 0;
+}
+
 int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
 {
 	const struct proto_ops *ops = READ_ONCE(sock->ops);
@@ -189,15 +199,21 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
 			if (err)
 				goto error;
 
-			p->creds.pid = creds.pid;
 			if (!p->pid || pid_vnr(p->pid) != creds.pid) {
 				struct pid *pid;
 				err = -ESRCH;
 				pid = find_get_pid(creds.pid);
 				if (!pid)
 					goto error;
-				put_pid(p->pid);
-				p->pid = pid;
+
+				/* pass a struct pid reference from
+				 * find_get_pid() to scm_replace_pid().
+				 */
+				err = scm_replace_pid(p, pid);
+				if (err) {
+					put_pid(pid);
+					goto error;
+				}
 			}
 
 			err = -EINVAL;