Re: [PATCH RFC 1/2] Add polling support to pidfd

From: Daniel Colascione <dancol@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christian Brauner <christian@brauner.io>,
	Joel Fernandes <joel@joelfernandes.org>,
	Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	kernel list <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrei Vagin <avagin@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Kees Cook <keescook@chromium.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>, Michal Hocko <mhocko@suse.com>,
	Nadav Amit <namit@vmware.com>, Serge Hallyn <serge@hallyn.com>,
	Shuah Khan <shuah@kernel.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Taehee Yoo <ap420073@gmail.com>, Tejun Heo <tj@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	kernel-team <kernel-team@android.com>,
	Tycho Andersen <tycho@tycho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
Date: Fri, 19 Apr 2019 16:36:24 -0700	[thread overview]
Message-ID: <CAKOZuesaWK1Y7c85jNv71LNx7sKoGgn_eLzQVPnoi6e4eUx4aQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wh8DmfsOFY4OrqRjLYCYi+2Ej7uLbidmiPg+BhBOGZVcA@mail.gmail.com>

On Fri, Apr 19, 2019 at 4:33 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Apr 19, 2019 at 4:20 PM Christian Brauner <christian@brauner.io> wrote:
> >
> > On Sat, Apr 20, 2019 at 1:11 AM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > It's also worth noting that POLLERR/POLLHUP/POLLNVAL cannot be masked
> > > for "poll()". Even if you only ask for POLLIN/POLLOUT, you will always
> > > get POLLERR/POLLHUP notification. That is again historical behavior,
> > > and it's kind of a "you can't poll a hung up fd". But it once again
> > > means that you should consider POLLHUP to be something *exceptional*
> > > and final, where no further or other state changes can happen or are
> > > relevant.
> >
> > Which kind of makes sense for process exit. So the historical behavior
> > here is in our favor and having POLLIN | POLLHUP rather fitting.
> > It just seems right that POLLHUP indicates "there can be
> > no more state transitions".
>
> Note that that is *not* true of process exit.
>
> The final state transition isn't "exit", it is actually "process has
> been reaped".  That's the point where data no longer exists.

FWIW, I think the exit status should be available via pidfd even after
process reaping. A non-parent holder of a pidfd has no ability to
control when the parent reaps the child, or even if reaping is
necessary at all --- the parent could make SIGCHLD SIG_IGN. Someone
trying to read exit status via a pidfd shouldn't fail to get that exit
status just because he lost the race with a concurrent waitpid().