Messages in this thread | | | From | Daniel Colascione <> | Date | Sun, 31 Mar 2019 08:21:39 -0700 | Subject | Re: [PATCH v2 0/5] pid: add pidfd_open() |
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On Sun, Mar 31, 2019 at 8:05 AM Christian Brauner <christian@brauner.io> wrote: > > On Sun, Mar 31, 2019 at 07:52:28AM -0700, Linus Torvalds wrote: > > On Sat, Mar 30, 2019 at 9:47 PM Jann Horn <jannh@google.com> wrote: > > > > > > Sure, given a pidfd_clone() syscall, as long as the parent of the > > > process is giving you a pidfd for it and you don't have to deal with > > > grandchildren created by fork() calls outside your control, that > > > works. > > > > Don't do pidfd_clone() and pidfd_wait(). > > > > Both of those existing system calls already get a "flags" argument. > > Just make a WPIDFD (for waitid) and CLONE_PIDFD (for clone) bit, and > > make the existing system calls just take/return a pidfd. > > Yes, that's one of the options I was considering but was afraid of > pitching it because of the very massive opposition I got > regarding"multiplexers". I'm perfectly happy with doing it this way.
This approach is fine by me, FWIW. I like it more than a general-purpose pidctl.
> Btw, the /proc/<pid> race issue that is mentioned constantly is simply > avoidable by placing the pid that the pidfd has stashed relative to the > callers' procfs mount's pid namespace in the pidfd's fdinfo. So there's > not even a need to really go through /proc/<pid> in the first place. A > caller wanting to get metadata access and avoid a race with pid > recycling can then simply do: > > int pidfd = pidfd_open(pid, 0); > int pid = parse_fdinfo("/proc/self/fdinfo/<pidfd>"); > int procpidfd = open("/proc/<pid>", ...);
IMHO, it's worth documenting this procedure in the pidfd man page.
> /* Test if process still exists by sending signal 0 through our pidfd. */
Are you planning on officially documenting this incantation in the pidfd man page?
> int ret = pidfd_send_signal(pid, 0, NULL, PIDFD_SIGNAL_THREAD); > if (ret < 0 && errno == ESRCH) { > /* pid has been recycled and procpidfd refers to another process */ > }
I was going to suggest that WNOHANG also works for this purpose, but that idea raises another question: normally, you can wait*(2) on a process only once. Are you imagining waitid on a pidfd succeeding more than one? ISTM that the pidfd would need to internally store not just a struct pid, but the exit status as well or some way to get to it.
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