linux-kernel - Re: [PATCH v4] signal: add taskfd_send

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Message-ID: <20181206125354.ef3zlg3o75w32ymx@brauner.io>
Date:   Thu, 6 Dec 2018 13:53:57 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        luto@...nel.org, arnd@...db.de, ebiederm@...ssion.com,
        serge@...lyn.com, jannh@...gle.com, akpm@...ux-foundation.org,
        oleg@...hat.com, cyphar@...har.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, dancol@...gle.com,
        timmurray@...gle.com, linux-man@...r.kernel.org,
        keescook@...omium.org, tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH v4] signal: add taskfd_send_signal() syscall

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
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. It would be odd if this one suddenly did.
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.
As far as I'm concerned, this is out of scope for an initial
implementation. We are going to use fds for tasks that's enough
excitement for one patchset!

> 
> Rest looks good to me (with the usual caveats).

I take it that's your way of saying Acked-by? :)

Christian