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Date:   Mon, 7 Aug 2023 12:58:03 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Pengfei Xu <pengfei.xu@intel.com>
Cc:     dvyukov@google.com, nogikh@google.com,
        linux-kernel@vger.kernel.org, Heng Su <heng.su@intel.com>,
        syzkaller@googlegroups.com, keescook@google.com, mingo@elte.hu,
        Mehta Sohil <sohil.mehta@intel.com>,
        kirill.shutemov@linux.intel.com
Subject: Re: [PATCH RESEND v1 1/1] x86/kernel: Increase kcov coverage under
 arch/x86/kernel folder
Message-Id: <20230807125803.94dfb08402939e0080505a5e@linux-foundation.org>
In-Reply-To: <06c0bb7b5f61e5884bf31180e8c122648c752010.1690771380.git.pengfei.xu@intel.com>
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        <06c0bb7b5f61e5884bf31180e8c122648c752010.1690771380.git.pengfei.xu@intel.com>
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On Mon, 31 Jul 2023 11:04:18 +0800 Pengfei Xu <pengfei.xu@intel.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
> 
> ...
>
> --- 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

This patch exposes (but probably doesn't cause) an objtool warning:

arch/x86/kernel/traps.o: warning: objtool: ibt_selftest+0x18: sibling call from callable instruction with modified stack frame

gcc-12.2.0, x86_64 allmodconfig.