linux-kernel - Re: [PATCH 1/3] kcov: support comparison operands collection

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Message-ID: <CACT4Y+a85z12FdjuGTPzeJXYdYhQiNOMjykO2e0PwXEkqJUOag@mail.gmail.com>
Date:   Tue, 12 Sep 2017 19:41:58 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Victor Chibotaru <tchibo@...gle.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Kees Cook <keescook@...omium.org>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Quentin Casasnovas <quentin.casasnovas@...cle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] kcov: support comparison operands collection

On Wed, Aug 30, 2017 at 9:08 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Wed, Aug 30, 2017 at 8:23 PM, Mark Rutland <mark.rutland@....com> wrote:
>> Hi,
>>
>> On Wed, Aug 30, 2017 at 06:23:29PM +0200, Dmitry Vyukov wrote:
>>> From: Victor Chibotaru <tchibo@...gle.com>
>>>
>>> Enables kcov to collect comparison operands from instrumented code.
>>> This is done by using Clang's -fsanitize=trace-cmp instrumentation
>>> (currently not available for GCC).
>>
>> What's needed to build the kernel with Clang these days?
>>
>> I was under the impression that it still wasn't possible to build arm64
>> with clang due to a number of missing features (e.g. the %a assembler
>> output template).
>>
>>> The comparison operands help a lot in fuzz testing. E.g. they are
>>> used in Syzkaller to cover the interiors of conditional statements
>>> with way less attempts and thus make previously unreachable code
>>> reachable.
>>>
>>> To allow separate collection of coverage and comparison operands two
>>> different work modes are implemented. Mode selection is now done via
>>> a KCOV_ENABLE ioctl call with corresponding argument value.
>>>
>>> Signed-off-by: Victor Chibotaru <tchibo@...gle.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Mark Rutland <mark.rutland@....com>
>>> Cc: Alexander Popov <alex.popov@...ux.com>
>>> Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
>>> Cc: Kees Cook <keescook@...omium.org>
>>> Cc: Vegard Nossum <vegard.nossum@...cle.com>
>>> Cc: Quentin Casasnovas <quentin.casasnovas@...cle.com>
>>> Cc: syzkaller@...glegroups.com
>>> Cc: linux-mm@...ck.org
>>> Cc: linux-kernel@...r.kernel.org
>>> ---
>>> Clang instrumentation:
>>> https://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow
>>
>> How stable is this?
>>
>> The comment at the end says "This interface is a subject to change."
>
>
> The intention is that this is not subject to change anymore (since we
> are using it in kernel).
> I've mailed change to docs: https://reviews.llvm.org/D37303
>
> FWIW, there is patch in flight that adds this instrumentation to gcc:
> https://groups.google.com/forum/#!topic/syzkaller/CSLynn6nI-A
> It seems to be stalled on review phase, though.


Good news is that this is submitted to gcc in 251801.