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

[Christian Brauner]

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.

> 
> And also for the translation the other way, add a syscall or modify
> translate_fd or something, to covert a anon_inode pidfd into a /proc/pid
> directory fd. Then the user is welcomed to do openat(2) on _that_ directory fd.
> Then we modify pidfd_send_signal to only send signals to pure pidfd fds, not
> to /proc/pid directory fds.
> 
> Should we work on patches for these? Please let us know if this idea makes
> sense and thanks a lot for adding us to the review as well.
> 
> Best,
> 
>  - Joel