Re: [PATCH 0/4] pid: add pidctl()

[Joel Fernandes]

On Mon, Mar 25, 2019 at 10:19:26PM +0100, Jann Horn wrote:
> On Mon, Mar 25, 2019 at 10:11 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > On Mon, Mar 25, 2019 at 09:15:45PM +0100, Christian Brauner wrote:
> > > On Mon, Mar 25, 2019 at 01:36:14PM -0400, Joel Fernandes wrote:
> > > > On Mon, Mar 25, 2019 at 09:48:43AM -0700, Daniel Colascione wrote:
> > > > > On Mon, Mar 25, 2019 at 9:21 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > > > > The pidctl() syscalls builds on, extends, and improves translate_pid() [4].
> > > > > > I quote Konstantins original patchset first that has already been acked and
> > > > > > picked up by Eric before and whose functionality is preserved in this
> > > > > > syscall. Multiple people have asked when this patchset will be sent in
> > > > > > for merging (cf. [1], [2]). It has recently been revived by Nagarathnam
> > > > > > Muthusamy from Oracle [3].
> > > > > >
> > > > > > The intention of the original translate_pid() syscall was twofold:
> > > > > > 1. Provide translation of pids between pid namespaces
> > > > > > 2. Provide implicit pid namespace introspection
> > > > > >
> > > > > > Both functionalities are preserved. The latter task has been improved
> > > > > > upon though. In the original version of the pachset passing pid as 1
> > > > > > would allow to deterimine the relationship between the pid namespaces.
> > > > > > This is inherhently racy. If pid 1 inside a pid namespace has died it
> > > > > > would report false negatives. For example, if pid 1 inside of the target
> > > > > > pid namespace already died, it would report that the target pid
> > > > > > namespace cannot be reached from the source pid namespace because it
> > > > > > couldn't find the pid inside of the target pid namespace and thus
> > > > > > falsely report to the user that the two pid namespaces are not related.
> > > > > > This problem is simple to avoid. In the new version we simply walk the
> > > > > > list of ancestors and check whether the namespace are related to each
> > > > > > other. By doing it this way we can reliably report what the relationship
> > > > > > between two pid namespace file descriptors looks like.
> > > > > >
> > > > > > Additionally, this syscall has been extended to allow the retrieval of
> > > > > > pidfds independent of procfs. These pidfds can e.g. be used with the new
> > > > > > pidfd_send_signal() syscall we recently merged. The ability to retrieve
> > > > > > pidfds independent of procfs had already been requested in the
> > > > > > pidfd_send_signal patchset by e.g. Andrew [4] and later again by Alexey
> > > > > > [5]. A use-case where a kernel is compiled without procfs but where
> > > > > > pidfds are still useful has been outlined by Andy in [6]. Regular
> > > > > > anon-inode based file descriptors are used that stash a reference to
> > > > > > struct pid in file->private_data and drop that reference on close.
> > > > > >
> > > > > > With this translate_pid() has three closely related but still distinct
> > > > > > functionalities. To clarify the semantics and to make it easier for
> > > > > > userspace to use the syscall it has:
> > > > > > - gained a command argument and three commands clearly reflecting the
> > > > > >   distinct functionalities (PIDCMD_QUERY_PID, PIDCMD_QUERY_PIDNS,
> > > > > >   PIDCMD_GET_PIDFD).
> > > > > > - been renamed to pidctl()
> > > > >
> > > > [snip]
> > > > > Also, I'm still confused about how metadata access is supposed to work
> > > > > for these procfs-less pidfs. If I use PIDCMD_GET_PIDFD on a process,
> > > > > You snipped out a portion of a previous email in which I asked about
> > > > > your thoughts on this question. With the PIDCMD_GET_PIDFD command in
> > > > > place, we have two different kinds of file descriptors for processes,
> > > > > one derived from procfs and one that's independent. The former works
> > > > > with openat(2). The latter does not. To be very specific; if I'm
> > > > > writing a function that accepts a pidfd and I get a pidfd that comes
> > > > > from PIDCMD_GET_PIDFD, how am I supposed to get the equivalent of
> > > > > smaps or oom_score_adj or statm for the named process in a race-free
> > > > > manner?
> > > >
> > > > This is true, that such usecase will not be supportable.  But the advantage
> > > > on the other hand, is that suchs "pidfd" can be made pollable or readable in
> > > > the future. Potentially allowing us to return exit status without a new
> > > > syscall (?). And we can add IOCTLs to the pidfd descriptor which we cannot do
> > > > with proc.
> > > >
> > > > But.. one thing we could do for Daniel usecase is if a /proc/pid directory fd
> > > > can be translated into a "pidfd" using another syscall or even a node, like
> > > > /proc/pid/handle or something. I think this is what Christian suggested in
> > > > the previous threads.
> > >
> > > Andy - and Jann who I just talked to - have proposed solutions for this.
> > > Jann's idea is similar to what you suggested, Joel. You could e.g. do an
> > > ioctl() handler for /proc that would give you a dirfd back for a given
> > > pidfd. The advantage is that pidfd_clone() can then give back pidfds
> > > without having to care in what procfs the process is supposed to live.
> > > That makes things a lot easier. But pidfds for the general case should
> > > be anon inodes. It's clean, it's simple and it is way more secure.
> >
> > That makes sense to me, it is clean and I agree let us do that.
> >
> > Also for the "blocking on pid exit status" usecase, instead of adding a new
> > syscall like pidfd_wait, lets just make that a new IOCTL to the
> > file_operations of the anon_inode pidfd file. This will lets us specify
> > exactly what to wait on (wait on death or wait on zombie) and lets us avoid
> > having a new syscall and create new fd just for waiting. Let me know if you
> > disagree, but otherwise I am thinking of modifying my patches that way and
> > avoid adding a new syscall.
> 
> But often you don't just want to wait for a single thing to happen;
> you want to wait for many things at once, and react as soon as any one
> of them happens. This is why the kernel has epoll and all the other
> "wait for event on FD" APIs. If waiting for a process isn't possible
> with fd-based APIs like epoll, users of this API have to spin up
> useless helper threads.

This is true. I almost forgot about the polling requirement, sorry. So then a
new syscall it is.. About what to wait for, that can be a separate parameter
to pidfd_wait then.

Thanks.

 - Joel