[PATCH net-next 3/6] af_unix: introduce and use __scm_replace_pid() helper

[Alexander Mikhalitsyn]

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@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Simon Horman <horms@xxxxxxxxxx>
Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
Cc: Leon Romanovsky <leon@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Cc: Lennart Poettering <mzxreary@xxxxxxxxxxx>
Cc: Luca Boccassi <bluca@xxxxxxxxxx>
Cc: David Rheinsberg <david@xxxxxxxxxxxx>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
---
 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