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

[Dmitry Vyukov]

On Wed, Aug 30, 2017 at 9:08 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Wed, Aug 30, 2017 at 8:23 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> Hi,
>>
>> On Wed, Aug 30, 2017 at 06:23:29PM +0200, Dmitry Vyukov wrote:
>>> From: Victor Chibotaru <tchibo@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>>> Cc: Alexander Popov <alex.popov@xxxxxxxxx>
>>> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>>> Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
>>> Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
>>> Cc: syzkaller@xxxxxxxxxxxxxxxx
>>> Cc: linux-mm@xxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> ---
>>> 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.