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
