linux-kernel - Re: [PATCH v2] kernel: add kcov code coverage

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Date:	Fri, 22 Jan 2016 12:52:11 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	syzkaller <syzkaller@...glegroups.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Tavis Ormandy <taviso@...gle.com>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Drysdale <drysdale@...gle.com>,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v2] kernel: add kcov code coverage

On Fri, Jan 22, 2016 at 01:15:27PM +0100, Dmitry Vyukov wrote:
> On Fri, Jan 22, 2016 at 12:55 PM, Mark Rutland <mark.rutland@....com> wrote:
> > On Mon, Jan 18, 2016 at 09:09:43PM +0100, Dmitry Vyukov wrote:
> >> On Mon, Jan 18, 2016 at 8:44 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >> > Regarding KASLR and dynamically loaded modules. I've looked at my
> >> > use-case and concluded
> >> > that most of the time I can work with "non-stable" PCs within a single
> >> > VM. Whenever I need to
> >> > store PCs persistently or send to another machine, I think I can
> >> > "canonicalize" PCs using
> >> > /proc/modules and /proc/kallsyms to something like (module hash,
> >> > module offset). So kernel does
> >> > not need to do this during coverage collection.
> >>
> >> On second though, maybe it's better to just always export unsigned long PCs...
> >> Need to measure how much memory coverage information consumes,
> >> and how much slower it is with uint64 PCs. Maybe I can live with large PCs,
> >> or maybe I can make syzkaller require !KASLR and compress PCs in user-space...
> >> Need to think about this more.
> >
> > I can imagine we might keep the expanded module range even in the
> > absence of full KASLR, though I don't know how realistic that thought
> > is.
> 
> The last version of the patch just exposes PCs as unsigned longs
> without any compression. So it should not be a problem (at least for
> kernel, now it's user responsibility to make sense out of the PCs).

Ah, ok. Sorry for the noise!

Mark.