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SubjectRe: [PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor
On Mon, Mar 16, 2015 at 03:14:14PM -0700, Thiago Macieira wrote:
> On Monday 16 March 2015 14:44:20 Kees Cook wrote:
> > > 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...)
>
> Depends on CLONE_AUTOREAP. If it's on, then no one gets SIGCHLD, no one can
> wait() on it and the process autoreaps itself.

Minor nit: CLONE_AUTOREAP makes the process autoreap and nobody can wait
on it, but if you pass SIGCHLD or some other exit signal to clone then
you'll still get that signal.

> If it's no active, then the old rules apply: parent gets SIGCHILD and can
> wait(). If the parent exited first, then the child gets reparented to init,
> which can do the wait().

Right.

> A child without CLONE_AUTOREAP should be wait()able. If it gets wait()ed
> before the clonefd is read, the clonefd() will return a 0 read. If it gets
> read before wait, then wait() reaps another child or returns -ECHILD. That's
> no different than two threads doing simultaneous wait() on the same child.

Hrm? That isn't the semantics we implemented; you'll *always* get an
exit notification via the clonefd if you have it open, with or without
autoreap and whether or not a wait has occurred yet. And reading from
the clonefd does not serve as a wait; if you don't pass CLONE_AUTOREAP,
you'll still need to wait on the process.

- Josh Triplett


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