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SubjectRe: [PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor
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@joshtriplett.org> 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


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