linux-kernel - Re: [PATCH v1 1/1] x86/kernel: Increase kcov coverage under arch/x86/kernel folder

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Date:   Wed, 5 Jul 2023 06:28:10 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Pengfei Xu <pengfei.xu@...el.com>
Cc:     nogikh@...gle.com, linux-kernel@...r.kernel.org,
        Heng Su <heng.su@...el.com>, syzkaller@...glegroups.com,
        keescook@...gle.com, Mehta Sohil <sohil.mehta@...el.com>,
        kirill.shutemov@...ux.intel.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v1 1/1] x86/kernel: Increase kcov coverage under
 arch/x86/kernel folder

On Tue, 4 Jul 2023 at 10:58, Pengfei Xu <pengfei.xu@...el.com> wrote:
>
> Currently kcov instrument is disabled for object files under arch/x86/kernel
> folder.
> For object files under arch/x86/kernel, actually just disabling the kcov
> instrument of files:"head32.o or head64.o and sev.o" could achieve
> successful booting and provide kcov coverage for object files that do not
> disable kcov instrument.
> The additional kcov coverage collected from arch/x86/kernel folder helps
> kernel fuzzing efforts to find bugs.
>
> Link to related improvement discussion is below:
> https://groups.google.com/g/syzkaller/c/Dsl-RYGCqs8/m/x-tfpTyFBAAJ
> Related ticket is as follow:
> https://bugzilla.kernel.org/show_bug.cgi?id=198443
>
> Signed-off-by: Pengfei Xu <pengfei.xu@...el.com>

Reviewed-and-tested-by: Dmitry Vyukov <dvyukov@...gle.com>

I've run a local syzkaller instance with this and I don't see any
additional kernel bugs caused by this and I see coverage in
arch/x86/kernel/* now, in particular, dumpstack.c, perf_regs.c,
signal.c.


> ---
>  arch/x86/kernel/Makefile | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 4070a01c11b7..00df34c263cc 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -33,11 +33,10 @@ KCSAN_SANITIZE := n
>  KMSAN_SANITIZE_head$(BITS).o                           := n
>  KMSAN_SANITIZE_nmi.o                                   := n
>
> -# If instrumentation of this dir is enabled, boot hangs during first second.
> -# Probably could be more selective here, but note that files related to irqs,
> -# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
> -# non-deterministic coverage.
> -KCOV_INSTRUMENT                := n
> +# If instrumentation of the following files is enabled, boot hangs during
> +# first second.
> +KCOV_INSTRUMENT_head$(BITS).o                          := n
> +KCOV_INSTRUMENT_sev.o                                  := n
>
>  CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
>
> --
> 2.31.1
>