netdev - Re: [PATCH net-next 1/3] scm: add SO_PASSPIDFD and SCM

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Date:   Thu, 16 Mar 2023 16:32:03 +0100
From:   Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        David Ahern <dsahern@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <brauner@...nel.org>,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

On Thu, Mar 16, 2023 at 3:34 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Mar 16, 2023 at 6:16 AM Alexander Mikhalitsyn
> <aleksandr.mikhalitsyn@...onical.com> wrote:
> >
> > Implement SCM_PIDFD, a new type of CMSG type analogical to SCM_CREDENTIALS,
> > but it contains pidfd instead of plain pid, which allows programmers not
> > to care about PID reuse problem.
>
> Hi Alexander

Hi Eric

Thanks for the fast reply! ;-)

>
> This would add yet another conditional in af_unix fast path.
>
> It seems that we already can use pidfd_open() (since linux-5.3), and
> pass the resulting fd in af_unix SCM_RIGHTS message ?

Yes, it's possible, but it means that from the receiver side we need
to trust the sent pidfd (in SCM_RIGHTS),
or always use combination of SCM_RIGHTS+SCM_CREDENTIALS, then we can
extract pidfd from SCM_RIGHTS,
then acquire plain pid from pidfd and after compare it with the pid
from SCM_CREDENTIALS.

>
> If you think this is not suitable, it should at least be mentioned in
> the changelog.

Kind regards,
Alex

>
> Thanks.