linux-kernel - Re: [PATCH 3/4] ubsan: Force -Wno-maybe-uninitialized only for GCC

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Date:   Sun, 4 Oct 2020 00:16:14 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        George Popescu <georgepope@...roid.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        clang-built-linux@...glegroups.com, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] ubsan: Force -Wno-maybe-uninitialized only for GCC

On Fri, Oct 02, 2020 at 03:15:26PM -0700, Kees Cook wrote:
> Clang handles 'maybe-uninitialized' better in the face of using UBSAN,
> so do not make this universally disabled for UBSAN builds.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Well this patch is not strictly necessary because Clang does not support
-Wmaybe-uninitialized anyways :) its flags are -Wuninitialized and
-Wsometimes-uninitialized so the warning stays enabled for UBSAN as it
stands.

However, something like this could still worthwhile because it would
save us one call to cc-disable-warning (yay micro optimizations).

Maybe it just does not need to have a whole new symbol, just make it

ubsan-cflags-$(CONFIG_CC_IS_GCC)

instead of

ubsan-cflags-$(CONFIG_UBSAN)

No strong opinions either way though.

> ---
>  lib/Kconfig.ubsan      | 6 ++++++
>  scripts/Makefile.ubsan | 6 +++---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> index aeb2cdea0b94..1fc07f936e06 100644
> --- a/lib/Kconfig.ubsan
> +++ b/lib/Kconfig.ubsan
> @@ -36,6 +36,12 @@ config UBSAN_KCOV_BROKEN
>  	  See https://bugs.llvm.org/show_bug.cgi?id=45831 for the status
>  	  in newer releases.
>  
> +config UBSAN_DISABLE_MAYBE_UNINITIALIZED
> +	def_bool CC_IS_GCC
> +	help
> +	  -fsanitize=* options makes GCC less smart than usual and
> +	  increases the number of 'maybe-uninitialized' false-positives.
> +
>  config CC_HAS_UBSAN_BOUNDS
>  	def_bool $(cc-option,-fsanitize=bounds)
>  
> diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
> index 72862da47baf..c5ef6bac09d4 100644
> --- a/scripts/Makefile.ubsan
> +++ b/scripts/Makefile.ubsan
> @@ -1,8 +1,8 @@
>  # SPDX-License-Identifier: GPL-2.0
>  
> -# -fsanitize=* options makes GCC less smart than usual and
> -# increases the number of 'maybe-uninitialized' false-positives.
> -ubsan-cflags-$(CONFIG_UBSAN) += $(call cc-disable-warning, maybe-uninitialized)
> +# The "maybe-uninitialized" warning can be very noisy.
> +ubsan-cflags-$(CONFIG_UBSAN_DISABLE_MAYBE_UNINITIALIZED) += \
> +						$(call cc-disable-warning, maybe-uninitialized)
>  
>  # Enable available and selected UBSAN features.
>  ubsan-cflags-$(CONFIG_UBSAN_ALIGNMENT)		+= -fsanitize=alignment
> -- 
> 2.25.1