linux-kernel - Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes

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Message-ID: <CAMp4zn_z-CCQYMpT=GjZeGVLobjHBCSbmfha1rtWdmptOQ8JtA@mail.gmail.com>
Date:   Thu, 19 Dec 2019 08:15:29 -0800
From:   Sargun Dhillon <sargun@...gun.me>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Tycho Andersen <tycho@...ho.ws>, Jann Horn <jannh@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...capital.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Gian-Carlo Pascutto <gpascutto@...illa.com>,
        Emilio Cobos Álvarez <ealvarez@...illa.com>,
        Jed Davis <jld@...illa.com>
Subject: Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file
 descriptors from processes

On Thu, Dec 19, 2019 at 2:35 AM Christian Brauner
<christian.brauner@...ntu.com> wrote:
> I guess this is the remaining question we should settle, i.e. what do we
> prefer.
> I still think that adding a new syscall for this seems a bit rich. On
> the other hand it seems that a lot more people agree that using a
> dedicated syscall instead of an ioctl is the correct way; especially
> when it touches core kernel functionality. I mean that was one of the
> takeaways from the pidfd API ioctl-vs-syscall discussion.
>
> A syscall is nicer especially for core-kernel code like this.
> So I guess the only way to find out is to try the syscall approach and
> either get yelled and switch to an ioctl() or have it accepted.
>
> What does everyone else think? Arnd, still in favor of a syscall I take
> it. Oleg, you had suggested a syscall too, right? Florian, any
> thoughts/worries on/about this from the glibc side?
>
> Christian

My feelings towards this are that syscalls might pose a problem if we
ever want to extend this API. Of course we can have a reserved
"flags" field, and populate it later, but what if we turn out to need
a proper struct? I already know we're going to want to add one
around cgroup metadata (net_cls), and likely we'll want to add
a "steal" flag as well. As Arnd mentioned earlier, this is trivial to
fix in a traditional ioctl environment, as ioctls are "cheap". How
do we feel about potentially adding a pidfd_getfd2? Or are we
confident that reserved flags will save us?