linux-kernel - Re: [PATCH v2 4/4] x86: Ignore stack unwinding in KCOV

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Date: Wed, 19 Jun 2024 15:10:51 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org, 
	syzkaller@...glegroups.com, elver@...gle.com, glider@...gle.com, 
	nogikh@...gle.com, tarasmadan@...gle.com
Subject: Re: [PATCH v2 4/4] x86: Ignore stack unwinding in KCOV

On Wed, 19 Jun 2024 at 13:23, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Jun 11, 2024 at 09:50:33AM +0200, Dmitry Vyukov wrote:
> > Stack unwinding produces large amounts of uninteresting coverage.
> > It's called from KASAN kmalloc/kfree hooks, fault injection, etc.
> > It's not particularly useful and is not a function of system call args.
> > Ignore that code.
>
> This stems from KCOV's purpose being guiding syzkaller as opposed to it
> being a more general coverage tool, right?
>
> Is that spelled out anywhere?

It may be used for other similar purposes as well, e.g. collecting
unit test coverage (not the whole kernel, but a single specific test
provided that even other tests can run and collect their own coverage
in parallel).

It's spelled explicitly in the docs:

https://elixir.bootlin.com/linux/latest/source/Documentation/dev-tools/kcov.rst

"""
KCOV collects and exposes kernel code coverage information in a form suitable
for coverage-guided fuzzing .... Coverage collection is enabled on a
task basis, and
thus KCOV can capture precise coverage of a single system call.

Note that KCOV does not aim to collect as much coverage as possible. It aims
to collect more or less stable coverage that is a function of syscall inputs.
To achieve this goal, it does not collect coverage in soft/hard interrupts...
"""



> Anyway,
>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thanks for your reviews, Peter!


> > Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Reviewed-by: Alexander Potapenko <glider@...gle.com>
> > Reviewed-by: Marco Elver <elver@...gle.com>
> > Cc: x86@...nel.org
> > Cc: linux-kernel@...r.kernel.org
> > Cc: syzkaller@...glegroups.com
> > ---
> >  arch/x86/kernel/Makefile | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> > index 20a0dd51700a..cd49ebfae984 100644
> > --- a/arch/x86/kernel/Makefile
> > +++ b/arch/x86/kernel/Makefile
> > @@ -39,6 +39,14 @@ KMSAN_SANITIZE_sev.o                                       := n
> >  # first second.
> >  KCOV_INSTRUMENT_head$(BITS).o                                := n
> >  KCOV_INSTRUMENT_sev.o                                        := n
> > +# These are called from save_stack_trace() on debug paths,
> > +# and produce large amounts of uninteresting coverage.
> > +KCOV_INSTRUMENT_stacktrace.o                         := n
> > +KCOV_INSTRUMENT_dumpstack.o                          := n
> > +KCOV_INSTRUMENT_dumpstack_$(BITS).o                  := n
> > +KCOV_INSTRUMENT_unwind_orc.o                         := n
> > +KCOV_INSTRUMENT_unwind_frame.o                               := n
> > +KCOV_INSTRUMENT_unwind_guess.o                               := n
> >
> >  CFLAGS_irq.o := -I $(src)/../include/asm/trace
> >
> > --
> > 2.45.2.505.gda0bf45e8d-goog
> >
>
> --
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