linux-kernel - Re: [PATCH] prctl: add PR_[GS]ET

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Date:   Mon, 30 Jul 2018 21:39:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jürg Billeter <j@...ron.ch>
cc:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] prctl: add PR_[GS]ET_KILLABLE

On Mon, 30 Jul 2018, Jürg Billeter wrote:

> On Mon, 2018-07-30 at 12:17 +0200, Oleg Nesterov wrote:
> > On 07/30, Jürg Billeter wrote:
> > > 
> > > This is required for job control in a shell that uses CLONE_NEWPID for
> > > child processes.
> > 
> > Could you explain in more details?
> 
> 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, I don't think it's 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.
> 
> Job control in a shell is just an example. There are other scenarios,
> of course, where applications rely on the default actions as described
> in signal(7), and PID isolation may be useful. In my opinion, the
> kernel support for preventing accidental killing of the "init" process
> should really be optional and this new prctl provides this without
> breaking backward compatibility.

This makes sense and exactly that information needs to be in the changelog.

Thanks,

	tglx