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Date:   Wed, 29 Sep 2021 21:40:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Vito Caputo <vcaputo@pengaru.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
        Stefan Metzmacher <metze@samba.org>,
        Andy Lutomirski <luto@kernel.org>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kenta.Tada@sony.com" <Kenta.Tada@sony.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Michael =?iso-8859-1?Q?Wei=DF?= 
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Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
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On Wed, Sep 29, 2021 at 11:54:55AM -0700, Kees Cook wrote:

> > > > > > It's supposed to show where a blocked task is blocked; the "wait
> > > > > > channel".

> Since I think we're considering get_wchan() to be slow-path, can we just
> lock the runqueue and use arch_stack_walk_reliable()?

Funny thing, when a task is blocked it isn't on the runqueue :-)

So if all we want to do is capture a blocked task and fail otherwise we
don't need the rq->lock at all.

Something like:

	unsigned long ip = 0;

	raw_spin_lock_irq(&p->pi_lock);
	state = READ_ONCE(p->__state);
	smp_rmb(); /* see try_to_wake_up() */
	if (state == TASK_RUNNING || state == TASK_WAKING || p->on_rq)
		goto unlock;

	ip = /* do actual stack walk on a blocked task */
unlock:
	raw_spin_unlock_irq(&p->pi_lock);

	return ip;


Should get us there.