Re: pidfd design

[Daniel Colascione <dancol@google.com>]

On Tue, Mar 19, 2019 at 8:59 PM Christian Brauner <christian@brauner.io> wrote:
>
> On Tue, Mar 19, 2019 at 07:42:52PM -0700, Daniel Colascione wrote:
> > On Tue, Mar 19, 2019 at 6:52 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > >
> > > On Wed, Mar 20, 2019 at 12:10:23AM +0100, Christian Brauner wrote:
> > > > On Tue, Mar 19, 2019 at 03:48:32PM -0700, Daniel Colascione wrote:
> > > > > On Tue, Mar 19, 2019 at 3:14 PM Christian Brauner <christian@brauner.io> wrote:
> > > > > > So I dislike the idea of allocating new inodes from the procfs super
> > > > > > block. I would like to avoid pinning the whole pidfd concept exclusively
> > > > > > to proc. The idea is that the pidfd API will be useable through procfs
> > > > > > via open("/proc/<pid>") because that is what users expect and really
> > > > > > wanted to have for a long time. So it makes sense to have this working.
> > > > > > But it should really be useable without it. That's why translate_pid()
> > > > > > and pidfd_clone() are on the table.  What I'm saying is, once the pidfd
> > > > > > api is "complete" you should be able to set CONFIG_PROCFS=N - even
> > > > > > though that's crazy - and still be able to use pidfds. This is also a
> > > > > > point akpm asked about when I did the pidfd_send_signal work.
> > > > >
> > > > > I agree that you shouldn't need CONFIG_PROCFS=Y to use pidfds. One
> > > > > crazy idea that I was discussing with Joel the other day is to just
> > > > > make CONFIG_PROCFS=Y mandatory and provide a new get_procfs_root()
> > > > > system call that returned, out of thin air and independent of the
> > > > > mount table, a procfs root directory file descriptor for the caller's
> > > > > PID namspace and suitable for use with openat(2).
> > > >
> > > > Even if this works I'm pretty sure that Al and a lot of others will not
> > > > be happy about this. A syscall to get an fd to /proc?
> >
> > Why not? procfs provides access to a lot of core kernel functionality.
> > Why should you need a mountpoint to get to it?
> >
> > > That's not going
> > > > to happen and I don't see the need for a separate syscall just for that.
> >
> > We need a system call for the same reason we need a getrandom(2): you
> > have to bootstrap somehow when you're in a minimal environment.
> >
> > > > (I do see the point of making CONFIG_PROCFS=y the default btw.)
> >
> > I'm not proposing that we make CONFIG_PROCFS=y the default. I'm
> > proposing that we *hardwire* it as the default and just declare that
> > it's not possible to build a Linux kernel that doesn't include procfs.
> > Why do we even have that button?
> >
> > > I think his point here was that he wanted a handle to procfs no matter where
> > > it was mounted and then can later use openat on that. Agreed that it may be
> > > unnecessary unless there is a usecase for it, and especially if the /proc
> > > directory being the defacto mountpoint for procfs is a universal convention.
> >
> > If it's a universal convention and, in practice, everyone needs proc
> > mounted anyway, so what's the harm in hardwiring CONFIG_PROCFS=y? If
> > we advertise /proc as not merely some kind of optional debug interface
> > but *the* way certain kernel features are exposed --- and there's
> > nothing wrong with that --- then we should give programs access to
> > these core kernel features in a way that doesn't depend on userspace
> > kernel configuration, and you do that by either providing a
> > procfs-root-getting system call or just hardwiring the "/proc/" prefix
> > into VFS.
> >
> > > > Inode allocation from the procfs mount for the file descriptors Joel
> > > > wants is not correct. Their not really procfs file descriptors so this
> > > > is a nack. We can't just hook into proc that way.
> > >
> > > I was not particular about using procfs mount for the FDs but that's the only
> > > way I knew how to do it until you pointed out anon_inode (my grep skills
> > > missed that), so thank you!
> > >
> > > > > C'mon: /proc is used by everyone today and almost every program breaks
> > > > > if it's not around. The string "/proc" is already de facto kernel ABI.
> > > > > Let's just drop the pretense of /proc being optional and bake it into
> > > > > the kernel proper, then give programs a way to get to /proc that isn't
> > > > > tied to any particular mount configuration. This way, we don't need a
> > > > > translate_pid(), since callers can just use procfs to do the same
> > > > > thing. (That is, if I understand correctly what translate_pid does.)
> > > >
> > > > I'm not sure what you think translate_pid() is doing since you're not
> > > > saying what you think it does.
> > > > Examples from the old patchset:
> > > > translate_pid(pid, ns, -1)      - get pid in our pid namespace
> >
> > Ah, it's a bit different from what I had in mind. It's fair to want to
> > translate PIDs between namespaces, but the only way to make the
> > translate_pid under discussion robust is to have it accept and produce
> > pidfds. (At that point, you might as well call it translate_pidfd.) We
> > should not be adding new APIs to the kernel that accept numeric PIDs:
>
> The traditional pid-based api is not going away. There are users that
> have the requirement to translate pids between namespaces and also doing
> introspection on these namespaces independent of pidfds. We will not
> restrict the usefulness of this syscall by making it only work with
> pidfds.
>
> > it's not possible to use these APIs correctly except under very
> > limited circumstances --- mostly, talking about init or a parent
>
> The pid-based api is one of the most widely used apis of the kernel and
> people have been using it quite successfully for a long time. Yes, it's
> rac, but it's here to stay.
>
> > talking about its child.
> >
> > Really, we need a few related operations, and we shouldn't necessarily
> > mingle them.
>
> Yes, we've established that previously.
>
> >
> > 1) Given a numeric PID, give me a pidfd: that works today: you just
> > open /proc/<pid>
>
> Agreed.
>
> >
> > 2) Given a pidfd, give me a numeric PID: that works today: you just
> > openat(pidfd, "stat", O_RDONLY) and read the first token (which is
> > always the numeric PID).
>
> Agreed.
>
> >
> > 3) Given a pidfd, send a signal: that's what pidfd_send_signal does,
> > and it's a good start on the rest of these operations.
>
> Agreed.
>
> > 5) Given a pidfd in NS1, get a pidfd in NS2. That's what translate_pid
> > is for. My preferred signature for this routine is translate_pid(int
> > pidfd, int nsfd) -> pidfd. We don't need two namespace arguments. Why
> > not? Because the pidfd *already* names a single process, uniquely!
>
> Given that people are interested in pids we can't just always return a
> pidfd. That would mean a user would need to do get the pidfd read from
> <pidfd>/stat and then close the pidfd. If you do that for a 100 pids or
> more you end up allocating and closing file descriptors constantly for
> no reason. We can't just debate pids away. So it will also need to be
> able to yield pids e.g. through a flag argument.

Sure, but that's still not a reason that we should care about pidfds
working separately from procfs.