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

Aleksa Sarai cyphar at cyphar.com
Fri Dec 20 04:35:10 UTC 2019

On 2019-12-19, Sargun Dhillon <sargun at sargun.me> wrote:
> On Thu, Dec 19, 2019 at 2:35 AM Christian Brauner
> <christian.brauner at ubuntu.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
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