Re: [PATCH] proc: Disable /proc/$pid/wchan

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 Weiß" <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>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Qi Zheng" <zhengqi.arch@bytedance.com>,
	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
Date: Wed, 29 Sep 2021 14:10:23 -0700	[thread overview]
Message-ID: <20210929211023.runlsoqfto7hrl36@treble> (raw)
In-Reply-To: <20210929194026.GA4323@worktop.programming.kicks-ass.net>

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