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

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Message-ID: <20191220043510.r5h6wvsp2p5glyjv@yavin.dot.cyphar.com>
Date:   Fri, 20 Dec 2019 15:35:10 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        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>,
        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 2019-12-19, Sargun Dhillon <sargun@...gun.me> wrote:
> 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?

If we end up making this a syscall, then we can re-use the
copy_struct_from_user() API to make it both extensible and compatible in
both directions. I wasn't aware that this was frowned upon for ioctls
(sorry for the extra work) but there are several syscalls which use this
model for extendability (clone3, openat2, sched_setattr,
perf_events_open) so there shouldn't be any such complaints for a
syscall which is extensible.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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