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>
---
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 ]
---
 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..68441c024dd8 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

On Tue, Jul 1, 2025 at 1:41 AM Alexander Mikhalitsyn
<aleksandr.mikhalitsyn@canonical.com> wrote:
>
> 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>
> ---
> 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 ]
> ---
>  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..68441c024dd8 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)

As you will need v3, I'd remove "__" because there's no-prefix-version.


> +{
> +       /* 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
>