Date:   Thu, 2 Dec 2021 15:50:31 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, kasan-dev@googlegroups.com,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] kcov: fix generic Kconfig dependencies if
 ARCH_WANTS_NO_INSTR
Message-ID: <YajdN5T8vi2ZzP3D@hirez.programming.kicks-ass.net>
References: <20211201152604.3984495-1-elver@google.com>
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In-Reply-To: <20211201152604.3984495-1-elver@google.com>
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On Wed, Dec 01, 2021 at 04:26:04PM +0100, Marco Elver wrote:
> Until recent versions of GCC and Clang, it was not possible to disable
> KCOV instrumentation via a function attribute. The relevant function
> attribute was introduced in 540540d06e9d9 ("kcov: add
> __no_sanitize_coverage to fix noinstr for all architectures").
> 
> x86 was the first architecture to want a working noinstr, and at the
> time no compiler support for the attribute existed yet. Therefore,
> 0f1441b44e823 ("objtool: Fix noinstr vs KCOV") introduced the ability to
> NOP __sanitizer_cov_*() calls in .noinstr.text.
> 
> However, this doesn't work for other architectures like arm64 and s390
> that want a working noinstr per ARCH_WANTS_NO_INSTR.
> 
> At the time of 0f1441b44e823, we didn't yet have ARCH_WANTS_NO_INSTR,
> but now we can move the Kconfig dependency checks to the generic KCOV
> option. KCOV will be available if:
> 
> 	- architecture does not care about noinstr, OR
> 	- we have objtool support (like on x86), OR
> 	- GCC is 12.0 or newer, OR
> 	- Clang is 13.0 or newer.
> 
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  arch/x86/Kconfig  | 2 +-
>  lib/Kconfig.debug | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 95dd1ee01546..c030b2ee93b3 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -78,7 +78,7 @@ config X86
>  	select ARCH_HAS_FILTER_PGPROT
>  	select ARCH_HAS_FORTIFY_SOURCE
>  	select ARCH_HAS_GCOV_PROFILE_ALL
> -	select ARCH_HAS_KCOV			if X86_64 && STACK_VALIDATION
> +	select ARCH_HAS_KCOV			if X86_64
>  	select ARCH_HAS_MEM_ENCRYPT
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>  	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 9ef7ce18b4f5..589c8aaa2d5b 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1977,6 +1977,8 @@ config KCOV
>  	bool "Code coverage for fuzzing"
>  	depends on ARCH_HAS_KCOV
>  	depends on CC_HAS_SANCOV_TRACE_PC || GCC_PLUGINS
> +	depends on !ARCH_WANTS_NO_INSTR || STACK_VALIDATION || \
> +		   GCC_VERSION >= 120000 || CLANG_VERSION >= 130000

Can we write that as something like:

	$(cc-attribute,__no_sanitize_coverage)

instead? Other than that, yes totally.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>