linux-kernel - Re: [PATCH net-next v6 1/3] scm: add SO_PASSPIDFD and SCM

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Message-ID: <20230523-flechten-ortsschild-e5724ecc4ed0@brauner>
Date:   Tue, 23 May 2023 11:49:10 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        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>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Luca Boccassi <bluca@...ian.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH net-next v6 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

On Mon, May 22, 2023 at 01:34:09PM -0700, Jakub Kicinski wrote:
> On Mon, 22 May 2023 15:24:37 +0200 Alexander Mikhalitsyn wrote:
> > v6:
> > 	- disable feature when CONFIG_UNIX=n/m (pidfd_prepare API is not exported to modules)
> 
> IMHO hiding the code under #if IS_BUILTIN(CONFIG_UNRELATED) is
> surprising to the user and.. ugly?
> 
> Can we move scm_pidfd_recv() into a C source and export that?
> That should be less controversial than exporting pidfd_prepare()
> directly?

I really would like to avoid that because it will just mean that someone
else will abuse that function and then make an argument why we should
export the other function.

I think it would be ok if we required that unix support is built in
because it's not unprecedented either and we're not breaking anything.
Bpf has the same requirement:

  #if IS_BUILTIN(CONFIG_UNIX) && defined(CONFIG_BPF_SYSCALL)
  struct bpf_unix_iter_state {
          struct seq_net_private p;
          unsigned int cur_sk;
          unsigned int end_sk;
          unsigned int max_sk;
          struct sock **batch;
          bool st_bucket_done;
  };

and

  #if IS_BUILTIN(CONFIG_UNIX) && defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_PROC_FS)
  DEFINE_BPF_ITER_FUNC(unix, struct bpf_iter_meta *meta,
                       struct unix_sock *unix_sk, uid_t uid)