linux-kernel - Re: [PATCH 17/17] ptrace: implement PTRACE

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Date:	Mon, 13 Jun 2011 22:33:41 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	vda.linux@...glemail.com, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu, bdonlan@...il.com,
	pedro@...esourcery.com
Subject: Re: [PATCH 17/17] ptrace: implement PTRACE_LISTEN

Hello Tejun,

On 06/13, Tejun Heo wrote:
>
> On Thu, Jun 02, 2011 at 07:33:30PM +0200, Oleg Nesterov wrote:
> > > +		/*
> > > +		 * If NOTIFY is set, it means event happened between start
> > > +		 * of this trap and now.  Trigger re-trap immediately.
> > > +		 */
> > > +		if (child->jobctl & JOBCTL_TRAP_NOTIFY)
> > > +			signal_wake_up(child, true);
> >
> > Again, I won't insist if you prefer signal_wake_up(), but afaics
> > wake_up_state(__TASK_TRACED) should be enough.
>
> Re-trapping from attach/detach paths are already using
> signal_wake_up()

because attach sets TRAP_STOP which contributes to recalc_sigpending().

If JOBCTL_TRAP_NOTIFY is set, TIF_SIGPENDING should be already set as
well by the same reason. And in any case ptrace_stop() does
recalc_sigpending_tsk() before return, TIF_SIGPENDING is never really
needed when we are going to wake up the TASK_TRACED task.

However,

> and I think it would be better to keep it consistent.

OK, I don't really mind, up to you.

> > OK. The only thing I can't understand is why prepare_signal(SIGCONT)
> > calls ptrace_trap_notify() unconditionally. How about
> >
> > 		if (likely(!(t->ptrace & PT_SEIZED)))
> > 			wake_up_state(t, __TASK_STOPPED);
> > 	-	else
> > 	+	else if (why)
> > 			ptrace_trap_notify(t);
> >
> > ?
>
> I'm having a Deja Vu.

Me too...

> Did I reply to this already?  Anyways, here are
> my rationales.
>
> * Tracer should be able to handle seemingly spurious notifications.
> ...
> SIGCONT always generating notification is correct

Yes, I didn't say this is wrong.

>   and I don't see
>   good reasons to optimize it.  Moreover, I think it doesn't hurt to
>   have a way to reliably trigger spurious notification.

Well. I don't really understand why, but OK. Let's keep it this way.

> * If we're gonna optimize out SIGCONT processing if the target process
>   doesn't need it, the proper way would be testing stopped state and
>   exit before walking through the group list.

We can't, at least we need rm_from_queue(SIG_KERNEL_STOP_MASK) and
task_clear_jobctl_pending().


> However, I think it's
>   done the current way for a reason - always trying to wake up on
>   SIGCONT is more robust in case something went out of sync

Hmm. I am wondering if we can ever see why == 0 && __TASK_STOPPED with
the recent fixes...

> So, I'd like to keep this one as it currently is.

OK.

Oleg.

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