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

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Message-ID: <CAAVpQUAG+=PP67sCFSYJ6zubsEODUt+1Tf-2KtiPUcJTYAF_EQ@mail.gmail.com>
Date: Mon, 30 Jun 2025 12:45:57 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Cc: 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: Re: [RESEND PATCH net-next 3/6] af_unix: introduce and use
 __scm_replace_pid() helper

[dropped my previous email address]

On Sun, Jun 29, 2025 at 2:45 PM Alexander Mikhalitsyn
<aleksandr.mikhalitsyn@...onical.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@...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)

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

> +{
> +       /* 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().

> +       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
>