Re: pidfd design

[Daniel Colascione]

On Wed, Mar 20, 2019 at 11:52 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> On Wed, Mar 20, 2019 at 11:38:35AM -0700, Daniel Colascione wrote:
> > On Wed, Mar 20, 2019 at 11:26 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > 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@xxxxxxxxxx> wrote:
> > > > >On Tue, Mar 19, 2019 at 8:59 PM Christian Brauner
> > > > ><christian@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> 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.
> >
> > What do you mean by "not available"? CONFIG_PROCFS=n? If pidfds
>
> I'm talking about the ability to clone processes and get fd handles on
> them via pidfd_clone() or CLONE_NEWFD.

I wouldn't call that situation "proc [not being] available". We need
pidfd_clone to return a pidfd for atomicity reasons, not /proc
availability reasons. Again, it doesn't make any sense to support this
stuff when CONFIG_PROCFS=n, and CONFIG_PROCFS=n shouldn't even be a
supported configuration.

> > > 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.
> >
> > translate_pid also needs to *accept* pidfds, at least optionally.
> > Unless you have a function from pidfd to pidfd, you race.
>
> You're misunderstanding. Again, I said in my previous mails it should
> accept pidfds optionally as arguments, yes. But I don't want it to
> return the status fds that you previously wanted pidfd_wait() to return.

Agreed. There should be a different way to get these wait handle FDs.

> I really want to see Joel's pidfd_wait() patchset and have more people
> review the actual code.

Sure. But it's also unpleasant to have people write code and then have
to throw it away due to guessing incorrectly about unclear
requirements.