linux-kernel - Re: [PATCH v2] prctl: add PR_[GS]ET

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Date:   Tue, 31 Jul 2018 16:39:50 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jürg Billeter <j@...ron.ch>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] prctl: add PR_[GS]ET_KILLABLE

On 07/31, Jürg Billeter 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:

Ah. My question wasn't clear, sorry.

Could you explain your use-case? Why a shell wants to use CLONE_NEWPID?
And what do we actually want in, say, ^Z case? Just stop the child reaper
or may be it would be better to stop the whole pid namespace?

>  * 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.

Yes. So may be we actually want to change __isig() paths to use
SEND_SIG_FORCED (this is not that simple), or perhaps we can change
__send_signal() to not drop SIGSTOP sent to itself, or may be we can even
introduce SIG_DFL_EVEN_IF_INIT, I dunno.

>  * Many applications do not install handlers for these signals and
>    thus, job control won't work properly with unmodified applications.

I can't understand this. An application should be changed anyway to do
PR_SET_KILLABLE?

Let me clarify. I am not arguing with this patch, probably it makes sense in
any case. I am just trying to understand your real motivation for this change.

> +	case PR_SET_KILLABLE:
> +		if (arg2 != 1 || arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		spin_lock_irq(&me->sighand->siglock);
> +		me->signal->flags &= ~SIGNAL_UNKILLABLE;
> +		spin_unlock_irq(&me->sighand->siglock);

OK, but then you need to change the CLONE_PARENT/SIGNAL_UNKILLABLE check
in copy_process().

Oleg.