linux-kernel - Re: [PATCH v10 06/17] mm: slub: introduce metadata_access_enable()/metadata_access

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Date:	Fri, 30 Jan 2015 15:16:43 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	Andrey Ryabinin <a.ryabinin@...sung.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Konstantin Serebryany <kcc@...gle.com>,
	Dmitry Chernenkov <dmitryc@...gle.com>,
	Andrey Konovalov <adech.fo@...il.com>,
	Yuri Gribov <tetra2005@...il.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v10 06/17] mm: slub: introduce
 metadata_access_enable()/metadata_access_disable()

On Sat, 31 Jan 2015 03:11:55 +0400 Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:

> >> > kasan_disable_local/kasan_enable_local are also undocumented doesn't
> >> > help.
> >> >
> >>
> >> Ok, How about this?
> >>
> >> /*
> >>  * This hooks separate payload access from metadata access.
> >>  * Useful for memory checkers that have to know when slub
> >>  * accesses metadata.
> >>  */
> >
> > "These hooks".
> >
> > I still don't understand :( Maybe I'm having a more-stupid-than-usual
> > day.
> 
> I think it's me being stupid today ;) I'll try to explain better.
> 
> > How can a function "separate access"?  What does this mean?  More
> > details, please.  I think I've only once seen a comment which had too
> > much info!
> >
> 
> slub could access memory marked by kasan as inaccessible (object's metadata).
> Kasan shouldn't print report in that case because this access is valid.
> Disabling instrumentation of slub.c code is not enough to achieve this
> because slub passes pointer to object's metadata into memchr_inv().
> 
> We can't disable instrumentation for memchr_inv() because this is quite
> generic function.
> 
> So metadata_access_enable/metadata_access_disable wrap some
> places in slub.c where access to object's metadata starts/end.
> And kasan_disable_local/kasan_enable_local just disable/enable
> error reporting in this places.

ooh, I see.  Something like this?

/*
 * slub is about to manipulate internal object metadata.  This memory lies
 * outside the range of the allocated object, so accessing it would normally
 * be reported by kasan as a bounds error.  metadata_access_enable() is used
 * to tell kasan that these accesses are OK.
 */
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