Re: [PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor

[josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org]

On Mon, Mar 16, 2015 at 02:44:20PM -0700, Kees Cook wrote:
> On Sun, Mar 15, 2015 at 12:59 AM, Josh Triplett <josh-iaAMLnmF4Un9QnwtaJ6UQQ@public.gmane.orgg> wrote:
> > - Make poll on a CLONE_FD for an exited task also return POLLHUP, for
> >   compatibility with FreeBSD's pdfork.  Thanks to David Drysdale for calling
> >   attention to pdfork.
> 
> I think POLLHUP should be mentioned in the manpage (now it only
> mentions POLLIN).

Added for v3.

> >        CLONE_FD
> >               Return  a file descriptor associated with the new process, stor‐
> >               ing it in location clonefd in the parent's address space.   When
> >               the new process exits, the file descriptor will become available
> >               for reading.
> >
> >               Unlike using  signalfd(2)  for  the  SIGCHLD  signal,  the  file
> >               descriptor  returned  by  clone4()  with the CLONE_FD flag works
> >               even with SIGCHLD unblocked in one or more threads of the parent
> >               process,  allowing  the  process  to have different handlers for
> >               different child processes, such as those created by  a  library,
> >               without  introducing  race conditions around process-wide signal
> >               handling.
> >
> >               clonefd_flags may contain the following additional flags for use
> >               with CLONE_FD:
> >
> >
> >               O_CLOEXEC
> >                      Set  the  close-on-exec  flag on the new file descriptor.
> >                      See the description of the O_CLOEXEC flag in open(2)  for
> >                      reasons why this may be useful.
> 
> This begs the question: what happens when all CLONE_FD fds for a
> process are closed? Will the parent get SIGCHLD instead, will it
> auto-reap, or will it be un-wait-able (I assume not this...)

Whether the parent gets SIGCHLD is determined only by what signal you
request in clone; if you clone with CLONE_FD | SIGCHLD (or
CLONE_AUTOREAP | CLONE_FD | SIGCHLD), you'll get notification via both
clonefd (if you have one) and signal (if you have a handler).  If you
pass a 0 signal (just CLONE_FD or CLONE_AUTOREAP | CLONE_FD), you'll
receive no signal, only the notification via clonefd.  Independently, if
you have CLONE_AUTOREAP set, the process will autoreap.

Those are all orthogonal now.

If you close the clonefd, nothing special happens other than a
put_task_struct.  While this is conceptually somewhat like a pipe, the
data is actually generated at read time, so the task exit doesn't care
whether there's a live clonefd or not.  (Or, in the future, if there are
multiple live clonefds for the same process.)

> Looks promising!

Thanks!

And thanks for catching the manpage issue.  I'd definitely welcome any
comments you have on the implementation as well.

- Josh Triplett