Date:   Wed, 20 Mar 2019 19:26:50 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Tim Murray <timmurray@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>,
        kernel-team <kernel-team@...roid.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: pidfd design

On Wed, Mar 20, 2019 at 07:33:51AM -0400, Joel Fernandes wrote:
> 
> 
> On March 20, 2019 3:02:32 AM EDT, Daniel Colascione <dancol@...gle.com> wrote:
> >On Tue, Mar 19, 2019 at 8:59 PM Christian Brauner
> ><christian@...uner.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@...lfernandes.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@...uner.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..

That's unrelated to the point made in the above paragraph.
Please note, I said that the pidfd api should work when proc is not
available not that they can't be dirfds.

> 
> Agreed. I can't imagine pidfd being anything but a proc pid directory handle. So I am confused what Christian meant. Pidfd *is* a procfs directory fid  always. That's what I gathered from his pidfd_send_signal patch but let me know if I'm way off in the woods.

(K9 Mail still hasn't learned to wrap lines at 80 it seems. :))

Again, I never said that pidfds should be a directory handle.
(Though I would like to point out that one of the original ideas I
discussed at LPC was to have something like this to get regular file
descriptors instead of dirfds:
https://gist.github.com/brauner/59eec91550c5624c9999eaebd95a70df)

> 
> For my next revision, I am thinking of adding the flag argument Christian mentioned to make translate_pid return an anon_inode FD which can be used for death status, given a <pid>. Since it is thought that translate_pid can be made to return a pid FD, I think it is ok to have it return a pid status FD for the purposes of the death status as well.

translate_pid() should just return you a pidfd. Having it return a pidfd
and a status fd feels like stuffing too much functionality in there. If
you're fine with it I'll finish prototyping what I had in mind. As I
said in previous mails I'm already working on this.

Would you be ok with prototyping the pidfd_wait() syscall you had in
mind? Especially the wait_fd part that you want to have I would like to
see how that is supposed to work, e.g. who is allowed to wait on the
process and how notifications will work for non-parent processes and so
on. I feel we won't get anywhere by talking in the abstrace and other
people are far more likely to review/comment once there's actual code.

Christian

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