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Date:   Wed, 29 Sep 2021 14:10:23 -0700
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Vito Caputo <vcaputo@pengaru.com>,
        Jann Horn <jannh@google.com>,
        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 =?utf-8?B?V2Vpw58=?= <michael.weiss@aisec.fraunhofer.de>,
        Anand K Mistry <amistry@google.com>,
        Alexey Gladkov <legion@kernel.org>,
        Michal Hocko <mhocko@suse.com>, Helge Deller <deller@gmx.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andrea Righi <andrea.righi@canonical.com>,
        Ohhoon Kwon <ohoono.kwon@samsung.com>,
        Kalesh Singh <kaleshsingh@google.com>,
        YiFei Zhu <yifeifz2@illinois.edu>,
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        linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-fsdevel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
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On Wed, Sep 29, 2021 at 09:40:26PM +0200, Peter Zijlstra wrote:
> 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;

Ah, cool :-)

I'd also add that I don't see any reason to use the "reliable" unwinding
variant.  AFAIK, just basic stack_trace_save_tsk() should be sufficient.

-- 
Josh