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Date:   Thu, 6 Sep 2018 15:42:08 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     =?ISO-8859-1?Q?J=FCrg?= Billeter <j@bitron.ch>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Eric Biederman <ebiederm@xmission.com>,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/2] prctl: add PR_[GS]ET_KILLABLE
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On Fri,  3 Aug 2018 16:40:21 +0200 J?rg Billeter <j@bitron.ch> wrote:

> PR_SET_KILLABLE clears the SIGNAL_UNKILLABLE flag. This allows
> CLONE_NEWPID tasks to restore normal signal behavior, opting out of the
> special signal protection for init processes. This prctl does not allow
> setting the SIGNAL_UNKILLABLE flag, only clearing.
> 
> The SIGNAL_UNKILLABLE flag, which is implicitly set for tasks cloned
> with CLONE_NEWPID, has the effect of ignoring all signals (from
> userspace) if the corresponding handler is set to SIG_DFL. The only
> exceptions are SIGKILL and SIGSTOP and they are only accepted if raised
> from an ancestor namespace.
> 
> SIGINT, SIGQUIT and SIGTSTP are used in job control for ^C, ^\, ^Z.
> While a task with the SIGNAL_UNKILLABLE flag could install handlers for
> these signals, this is not sufficient to implement a shell that uses
> CLONE_NEWPID for child processes:
> 
>  * As SIGSTOP is ignored when raised from the SIGNAL_UNKILLABLE process
>    itself, it's not possible to implement the stop action in a custom
>    SIGTSTP handler.
>  * Many applications do not install handlers for these signals and
>    thus, job control won't work properly with unmodified applications.
> 
> There are other scenarios besides job control in a shell where
> applications rely on the default actions as described in signal(7) and
> PID isolation may be useful. This new prctl makes the signal protection
> for "init" processes optional, without breaking backward compatibility.

This one is above my pay grade.  Eric & Oleg: could you please provide
input?