linux-kernel - Re: [PATCH] signal: don't remove SIGNAL

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Date:   Fri, 5 May 2017 18:26:07 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jamie Iles <jamie.iles@...cle.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] signal: don't remove SIGNAL_UNKILLABLE for traced
        tasks.

Hi Jamie,

I am sorry for being slow... yes, probably we should start with this
simple change, and perhaps we do not really need anything else. But
let me think about this till Monday, perhaps we can remove this "clear
SIGNAL_UNKILLABLE" logic in force_sig_info() altogether.

On 05/04, Jamie Iles wrote:
>
> When forcing a signal, SIGNAL_UNKILLABLE is removed to prevent recursive
> faults, but this is undesirable when tracing.  For example, debugging an
> init process (whether global or namespace), hitting a breakpoint and
> SIGTRAP will force SIGTRAP and then remove SIGNAL_UNKILLABLE.
> Everything continues fine, but then once debugging has finished, the
> init process is left killable which is unlikely what the user expects,
> resulting in either an accidentally killed init or an init that stops
> reaping zombies.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Jamie Iles <jamie.iles@...cle.com>
> ---
>  kernel/signal.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 7e59ebc2c25e..5516a0cda668 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1185,7 +1185,11 @@ force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
>  			recalc_sigpending_and_wake(t);
>  		}
>  	}
> -	if (action->sa.sa_handler == SIG_DFL)
> +	/*
> +	 * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
> +	 * debugging to leave init killable.
> +	 */
> +	if (action->sa.sa_handler == SIG_DFL && !t->ptrace)
>  		t->signal->flags &= ~SIGNAL_UNKILLABLE;
>  	ret = specific_send_sig_info(sig, info, t);
>  	spin_unlock_irqrestore(&t->sighand->siglock, flags);
> -- 
> 2.12.0.rc0
>