linux-kernel - [PATCH net-next v3 3/7] af_unix: introduce and use scm_replace

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Message-ID: <20250703222314.309967-4-aleksandr.mikhalitsyn@canonical.com>
Date: Fri,  4 Jul 2025 00:23:07 +0200
From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: kuniyu@...gle.com
Cc: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Willem de Bruijn <willemb@...gle.com>,
	Leon Romanovsky <leon@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Christian Brauner <brauner@...nel.org>,
	Lennart Poettering <mzxreary@...inter.de>,
	Luca Boccassi <bluca@...ian.org>,
	David Rheinsberg <david@...dahead.eu>
Subject: [PATCH net-next v3 3/7] 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@...r.kernel.org
Cc: netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>
Cc: Willem de Bruijn <willemb@...gle.com>
Cc: Leon Romanovsky <leon@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: Lennart Poettering <mzxreary@...inter.de>
Cc: Luca Boccassi <bluca@...ian.org>
Cc: David Rheinsberg <david@...dahead.eu>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
---
v2:
	- don't do get_pid() in __scm_replace_pid() [ as Kuniyuki suggested ]
	- move __scm_replace_pid() from scm.h to scm.c [ as Kuniyuki suggested ]
v3:
	- rename __scm_replace_pid() to scm_replace_pid() [ as Kuniyuki suggested ]
---
 net/core/scm.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/net/core/scm.c b/net/core/scm.c
index 0225bd94170f..045ab5bdac7d 100644
--- a/net/core/scm.c
+++ b/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;
-- 
2.43.0