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

On Mon, Jun 30, 2025 at 9:46 PM Kuniyuki Iwashima [off-list ref] wrote:

[dropped my previous email address]

On Sun, Jun 29, 2025 at 2:45 PM Alexander Mikhalitsyn
[off-list ref] wrote:

quoted

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 <redacted>
Cc: Luca Boccassi <redacted>
Cc: David Rheinsberg <david@readahead.eu>
Signed-off-by: Alexander Mikhalitsyn <redacted>
---
 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)

It seems this function is only called from __scm_send() so this should
be moved to .c (and inlined ?).

sure!

quoted

+{
+       /* drop all previous references */
+       scm_destroy_cred(scm);
+
+       scm->pid = get_pid(pid);

This looks redundant.  Maybe move the put_pid() under if (error)
in __scm_send().

yep, fixed in v2.

quoted

+       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