Re: [PATCH] kcov: fix generic Kconfig dependencies if ARCH_WANTS_NO_INSTR

[Mark Rutland <mark.rutland@arm.com>]

Hi Marco,

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.

I agree this is the right thing to do, but since GCC 12.0 isn't out yet (and
only x86 has objtool atm) this will prevent using KCOV with a released GCC on
arm64 and s390, which would be unfortunate for Syzkaller.

AFAICT the relevant GCC commit is:

   https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cec4d4a6782c9bd8d071839c50a239c49caca689

Currently we mostly get away with disabling KCOV for while compilation units,
so maybe it's worth waiting for the GCC 12.0 release, and restricting things
once that's out?

Thanks,
Mark.

> 
> 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
>  	select DEBUG_FS
>  	select GCC_PLUGIN_SANCOV if !CC_HAS_SANCOV_TRACE_PC
>  	help
> -- 
> 2.34.0.rc2.393.gf8c9666880-goog
>