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Date:	Tue, 6 Oct 2015 09:26:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.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>,
	Sasha Levin <sasha.levin@...cle.com>,
	Wolfram Gloger <wmglo@...t.med.uni-muenchen.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86/process: Silence KASAN warnings in get_wchan()


* Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:

> On 10/05/2015 07:39 PM, Andi Kleen wrote:
> >> But, I think I have the solution.
> >> We could have some blacklist - list of function names which we should be ignored.
> >> In kasan_report() we could resolve return address to function name and compare it with name in list.
> >> If name in list -> ignore report.
> > 
> > I think annotating statements is cleaner than functions, even if it
> > is more code. Much better documentation
> > 
> 
> I agree with that, that's why I suggested to add READ_ONCE_NOCHECK():
> 	READ_ONCE_NOCHECK()
> 	{
> 		kasan_disable_current();
> 		READ_ONCE();
> 		kasan_enable_current();
> 	}
> 
> Anywone objects?

Sounds good to me! As long as it's hidden from plain .c files I'm a happy camper.

This should probably also be faster for KASAN than triggering a warning and having 
to parse a blacklist, right?

> > If disabling with an attribute doesn't work, you could put it into a special 
> > section with __attribute__((section ...)) and check the start/end symbol 
> > before reporting. That's how kprobes solves similar issues. It also has the 
> > advantage that it stops inlining.
> 
> Yes, it might be better. Although, because of broken -fconserve-stack, this may 
> not work in some cases - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63533 
> Function splitter may split original function into two parts and it always puts 
> one split part in default .text section.

We do a _ton_ of such section tricks in the kernel (all of exception handling is 
based on that) - if that's broken by -fconserve-stack then the kernel is broken 
much more widely.

So unless KASAN wants to do something special here you can rely on sections just 
fine.

Thanks,

	Ingo
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