linux-kernel - [RESEND PATCH net-next 3/6] af_unix: introduce and use __scm_replace

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Message-ID: <20250629214449.14462-4-aleksandr.mikhalitsyn@canonical.com>
Date: Sun, 29 Jun 2025 23:44:40 +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>,
	Kuniyuki Iwashima <kuniyu@...zon.com>
Subject: [RESEND PATCH net-next 3/6] 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>
---
 include/net/scm.h | 10 ++++++++++
 net/core/scm.c    | 11 ++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/net/scm.h b/include/net/scm.h
index 84c4707e78a5..856eb3a380f6 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -88,6 +88,16 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
 		__scm_destroy(scm);
 }
 
+static __inline__ int __scm_replace_pid(struct scm_cookie *scm, struct pid *pid)
+{
+	/* drop all previous references */
+	scm_destroy_cred(scm);
+
+	scm->pid = get_pid(pid);
+	scm->creds.pid = pid_vnr(pid);
+	return 0;
+}
+
 static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
 			       struct scm_cookie *scm, bool forcecreds)
 {
diff --git a/net/core/scm.c b/net/core/scm.c
index 0225bd94170f..0e71d5a249a1 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -189,15 +189,20 @@ 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;
+
+				err = __scm_replace_pid(p, pid);
+				/* Release what we get from find_get_pid() as
+				 * __scm_replace_pid() takes all necessary refcounts.
+				 */
+				put_pid(pid);
+				if (err)
+					goto error;
 			}
 
 			err = -EINVAL;
-- 
2.43.0