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

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Date:   Thu, 06 Dec 2018 14:20:21 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Jürg Billeter <j@...ron.ch>
Cc:     Christian Brauner <christian@...uner.io>,
        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

* Jürg Billeter:

> On Thu, 2018-12-06 at 14:12 +0100, Florian Weimer wrote:
>> * Jürg Billeter:
>> 
>> > On Thu, 2018-12-06 at 13:30 +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.
>> > 
>> > As I understand it, kill() behaves the same way. I think it's good that
>> > this new syscall keeps the behavior as close as possible to kill().
>> 
>> No, kill does not behave in this way because the PID can be reused.
>> The error condition is not stable there.
>
> The PID can't be reused as long as it's a zombie. It can only be reused
> when it has been wait()ed for. Or am I misunderstanding something?

Hmm, that's a fair point.  So the original interface is just broken.

Florian