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From: Florian Weimer <fweimer@redhat.com>
To: Christian Brauner <christian@brauner.io>
Cc: linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	luto@kernel.org, arnd@arndb.de, ebiederm@xmission.com,
	serge@hallyn.com, jannh@google.com, akpm@linux-foundation.org,
	oleg@redhat.com, cyphar@cyphar.com, viro@zeniv.linux.org.uk,
	linux-fsdevel@vger.kernel.org, dancol@google.com,
	timmurray@google.com, linux-man@vger.kernel.org,
	keescook@chromium.org, tglx@linutronix.de, x86@kernel.org
Subject: Re: [PATCH v4] signal: add taskfd_send_signal() syscall
Date: Thu, 06 Dec 2018 14:17:09 +0100
Message-ID: <87lg526bhm.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20181206125354.ef3zlg3o75w32ymx@brauner.io> (Christian Brauner's message of "Thu, 6 Dec 2018 13:53:57 +0100")

* Christian Brauner:

> On Thu, Dec 06, 2018 at 01:30:19PM +0100, Florian Weimer wrote:
>> * Christian Brauner:
>> 
>> > /* zombies */
>> > Zombies can be signaled just as any other process. No special error will be
>> > reported since a zombie state is an unreliable state (cf. [3]).
>> 
>> I still disagree with this analysis.  If I know that the target process
>> is still alive, and it is not, this is a persistent error condition
>> which can be reliably reported.  Given that someone might send SIGKILL
>> to the process behind my back, detecting this error condition could be
>> useful.
>
> Apart from my objection that this is not actually a reliable state
> because of timing issues between e.g. calling wait and a process
> exiting

The point is that if you are in an error state, the error state does not
go away, *especially* if you do not expect the process to terminate and
have not arranged for something calling waitpid on the PID.

> I have two more concerns and one helpful suggestion.
> First, this is hooking pretty deep into kernel internals. So far
> EXIT_ZOMBIE is only exposed in kernel/exit.c and I don't see enough
> value to drag all of this into kernel/signal.c
> Second, all other signal syscalls don't do report errors when signaling
> to zombies as well.

They cannot do this reliably because the error state is not persistent:
the PID can be reused.  So for the legacy interface, a difference in
error signaling would just have encouraged a bad programming model.

> It would be odd if this one suddenly did.

I don't think so.  My point is that the FD-based mechanism finally
allows to cope with this in a reasonable way.

> Third, if this really becomes such a big issue for userspace in the
> future that we want to do that work then we can add a flag like
> TASKFD_DETECT_ZOMBIE (or some such name) that will allow userspace to
> get an error back when signaling a zombie.

I can live with that.

Thanks,
Florian

  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-06 12:18 Christian Brauner
2018-12-06 12:30 ` Florian Weimer
2018-12-06 12:45   ` Jürg Billeter
2018-12-06 13:12     ` Florian Weimer
2018-12-06 13:18       ` Jürg Billeter
2018-12-06 13:20         ` Florian Weimer
2018-12-06 13:40       ` Eric W. Biederman
2018-12-06 13:44         ` Florian Weimer
2018-12-06 14:27           ` Aleksa Sarai
2018-12-06 14:46           ` Eric W. Biederman
2018-12-06 12:53   ` Christian Brauner
2018-12-06 13:17     ` Florian Weimer [this message]
2018-12-06 15:01 ` Eric W. Biederman
2018-12-06 16:17   ` Daniel Colascione
2018-12-06 17:24     ` Eric W. Biederman
2018-12-06 17:41       ` Christian Brauner
2018-12-06 18:30         ` Kees Cook
2018-12-06 22:27           ` Serge E. Hallyn
2018-12-06 17:14   ` Christian Brauner
2018-12-06 19:17     ` Eric W. Biederman
2018-12-06 19:30       ` Christian Brauner
2018-12-06 20:29         ` Eric W. Biederman
2018-12-06 20:37           ` Daniel Colascione
2018-12-06 22:22             ` Eric W. Biederman
2018-12-06 22:43               ` Daniel Colascione
2018-12-06 21:31           ` Christian Brauner
2018-12-06 21:46             ` Eric W. Biederman
2018-12-06 22:01               ` Daniel Colascione
2018-12-06 22:39               ` Christian Brauner
2018-12-06 23:17                 ` Christian Brauner
2018-12-07  0:31                   ` Serge E. Hallyn
2018-12-07  0:34                     ` Daniel Colascione
2018-12-07  0:59                       ` Serge E. Hallyn
2018-12-07  1:39                         ` Daniel Colascione
2018-12-07  1:54                           ` Christian Brauner
2018-12-07 16:49                             ` Serge E. Hallyn
2018-12-07 16:47 ` Serge E. Hallyn
2018-12-08 21:46 ` kbuild test robot

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