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

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Date:   Thu, 8 Jun 2017 11:07:07 +0100
From:   Jamie Iles <jamie.iles@...cle.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jamie Iles <jamie.iles@...cle.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] signal: don't remove SIGNAL_UNKILLABLE for traced tasks.

Hi Oleg,

On Fri, May 05, 2017 at 06:26:07PM +0200, Oleg Nesterov wrote:
> 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.

I was wondering if you had given this any more thought?

Thanks,

Jamie

> 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
> > 
>