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Date: Tue, 10 Oct 2017 10:56:19 +0100
From: Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v2 1/3] kcov: support comparison operands collection
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To: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>, Andrew Morton <akpm@linux-foundation.org>, Alexander Popov <alex.popov@linux.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Quentin Casasnovas <quentin.casasnovas@oracle.com>, andreyknvl <andreyknvl@google.com>, Kees Cook <keescook@chromium.org>, Vegard Nossum <vegard.nossum@oracle.com>, syzkaller <syzkaller@googlegroups.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Mon, Oct 09, 2017 at 08:46:18PM +0200, 'Dmitry Vyukov' via syzkaller wrote:
> On Mon, Oct 9, 2017 at 8:37 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Mon, Oct 09, 2017 at 08:15:10PM +0200, 'Dmitry Vyukov' via syzkaller wrote:
> >> On Mon, Oct 9, 2017 at 5:46 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> >> > On Mon, Oct 09, 2017 at 05:05:19PM +0200, Alexander Potapenko wrote:
> >
> >> > ... I note that a few places in the kernel use a 128-bit type. Are
> >> > 128-bit comparisons not instrumented?
> >>
> >> Yes, they are not instrumented.
> >> How many are there? Can you give some examples?
> >
> > From a quick scan, it doesn't looks like there are currently any
> > comparisons.
> >
> > It's used as a data type in a few places under arm64:
> >
> > arch/arm64/include/asm/checksum.h:      __uint128_t tmp;
> > arch/arm64/include/asm/checksum.h:      tmp = *(const __uint128_t *)iph;
> > arch/arm64/include/asm/fpsimd.h:                        __uint128_t vregs[32];
> > arch/arm64/include/uapi/asm/ptrace.h:   __uint128_t     vregs[32];
> > arch/arm64/include/uapi/asm/sigcontext.h:       __uint128_t vregs[32];
> > arch/arm64/kernel/signal32.c:   __uint128_t     raw;
> > arch/arm64/kvm/guest.c: __uint128_t tmp;
> 
> Then I think we just continue ignoring them for now :)
> In the future we can extend kcov to trace 128-bits values. We will
> need to add a special flag and write 2 consecutive entries for them.
> Or something along these lines.

Just wanted to make sure that we weren't backing ourselves into a corner
w.r.t. ABI; that sounds fine to me.

Thanks,
Mark.

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