linux-kernel - Re: [PATCH v4 1/2] compiler, atomics: Provide READ_ONCE

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Message-ID: <20151016110837.GU3816@twins.programming.kicks-ass.net>
Date:	Fri, 16 Oct 2015 13:08:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	kasan-dev <kasan-dev@...glegroups.com>,
	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Wolfram Gloger <wmglo@...t.med.uni-muenchen.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 1/2] compiler, atomics: Provide READ_ONCE_NOKSAN()

On Fri, Oct 16, 2015 at 01:54:44PM +0300, Andrey Ryabinin wrote:
> On 10/16/2015 01:00 PM, Peter Zijlstra wrote:
> > On Fri, Oct 16, 2015 at 12:44:53PM +0300, Andrey Ryabinin wrote:
> >> Some code may perform racy by design memory reads. This could be
> >> harmless, yet such code may produce KASAN warnings.
> >>
> >> To hide such accesses from KASAN this patch introduces
> >> READ_ONCE_NOKSAN() macro. KASAN will not check the memory
> >> accessed by READ_ONCE_NOKSAN(). The KernelThreadSanitizer (KTSAN)
> >> is going to ignore it as well.
> >>
> >> This patch creates __read_once_size_noksan() a clone of
> >> __read_once_size(). The only difference between them is
> >> 'no_sanitized_address' attribute appended to '*_nokasan' function.
> >> This attribute tells the compiler that instrumentation of memory
> >> accesses should not be applied to that function. We declare it as
> >> static '__maybe_unsed' because GCC is not capable to inline such
> >> function: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
> >>
> >> With KASAN=n READ_ONCE_NOKSAN() is just a clone of READ_ONCE().
> > 
> > Would we need a similar annotation for things like
> > mutex_spin_on_owner()'s dereference of owner, or is that considered safe
> > by KASAN?
> > 
> > (its not actually safe; as I remember we have a problem with using
> > rcu_read_lock for tasks like that)
> > 
> 
> How exactly it's not safe? 

I was worried perhaps KASAN would trip over the speculative nature of
the owner pointer, but we do verify it, so I suppose its allright.

> If we could dereference freed owner, I'd say we need to fix this,
> but not hide.

We should, just not 'trivial' and we seem to get distracted while
thinking of possible fixes :/
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