linux-kernel - Re: [PATCH 3/4] arm64/kasan: don't allocate extra shadow memory

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Message-ID: <20170601165205.GA8191@leverpostej>
Date:   Thu, 1 Jun 2017 17:52:06 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Alexander Potapenko <glider@...gle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] arm64/kasan: don't allocate extra shadow memory

On Thu, Jun 01, 2017 at 06:45:32PM +0200, Dmitry Vyukov wrote:
> On Thu, Jun 1, 2017 at 6:34 PM, Mark Rutland <mark.rutland@....com> wrote:
> > On Thu, Jun 01, 2017 at 07:23:37PM +0300, Andrey Ryabinin wrote:
> >> We used to read several bytes of the shadow memory in advance.
> >> Therefore additional shadow memory mapped to prevent crash if
> >> speculative load would happen near the end of the mapped shadow memory.
> >>
> >> Now we don't have such speculative loads, so we no longer need to map
> >> additional shadow memory.
> >
> > I see that patch 1 fixed up the Linux helpers for outline
> > instrumentation.
> >
> > Just to check, is it also true that the inline instrumentation never
> > performs unaligned accesses to the shadow memory?
> 
> Inline instrumentation generally accesses only a single byte.

Sorry to be a little pedantic, but does that mean we'll never access the
additional shadow, or does that mean it's very unlikely that we will?

I'm guessing/hoping it's the former!

Thanks,
Mark.