Re: [PATCH] hardening: Remove Clang's enable flag for -ftrivial-auto-var-init=zero

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Sep 29, 2022 at 11:06:24PM -0700, Kees Cook wrote:
> Now that Clang's -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> option is no longer required, remove it from the command line. Clang 16
> and later will warn when it is used, which will cause Kconfig to think
> it can't use -ftrivial-auto-var-init=zero at all. Check for whether it
> is required and only use it when so.
> 
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: linux-kbuild@vger.kernel.org
> Cc: llvm@lists.linux.dev
> Cc: stable@vger.kernel.org
> Fixes: f02003c860d9 ("hardening: Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO")
> Signed-off-by: Kees Cook <keescook@chromium.org>

Thanks for sending this change!

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

Please consider getting this to Linus ASAP so that this can start
filtering into stable now that the LLVM change has landed, as I lost the
ability to use CONFIG_INIT_STACK_ALL_ZERO after upgrading my toolchain
over the weekend :)

Additionally, I am not sure the fixes tag is going to ensure that this
change automatically makes it back to 5.15 and 5.10, which have
commit f0fe00d4972a ("security: allow using Clang's zero initialization
for stack variables") but not commit f02003c860d9 ("hardening: Avoid
harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO"). I guess if I
am reading the stable documentation right, we could do something like:

Cc: stable@vger.kernel.org # dcb7c0b9461c + f02003c860d9
Fixes: f0fe00d4972a ("security: allow using Clang's zero initialization for stack variables")

but I am not sure. I guess we can always just send manual backports
once it is merged.

> ---
>  Makefile                   |  4 ++--
>  security/Kconfig.hardening | 14 ++++++++++----
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index c7705f749601..02c857e2243c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -831,8 +831,8 @@ endif
>  # Initialize all stack variables with a zero value.
>  ifdef CONFIG_INIT_STACK_ALL_ZERO
>  KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
> -ifdef CONFIG_CC_IS_CLANG
> -# https://bugs.llvm.org/show_bug.cgi?id=45497
> +ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
> +# https://github.com/llvm/llvm-project/issues/44842
>  KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
>  endif
>  endif
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index bd2aabb2c60f..995bc42003e6 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -22,11 +22,17 @@ menu "Memory initialization"
>  config CC_HAS_AUTO_VAR_INIT_PATTERN
>  	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
>  
> -config CC_HAS_AUTO_VAR_INIT_ZERO
> -	# GCC ignores the -enable flag, so we can test for the feature with
> -	# a single invocation using the flag, but drop it as appropriate in
> -	# the Makefile, depending on the presence of Clang.
> +config CC_HAS_AUTO_VAR_INIT_ZERO_BARE
> +	def_bool $(cc-option,-ftrivial-auto-var-init=zero)
> +
> +config CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
> +	# Clang 16 and later warn about using the -enable flag, but it
> +	# is required before then.
>  	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
> +	depends on !CC_HAS_AUTO_VAR_INIT_ZERO_BARE
> +
> +config CC_HAS_AUTO_VAR_INIT_ZERO
> +	def_bool CC_HAS_AUTO_VAR_INIT_ZERO_BARE || CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
>  
>  choice
>  	prompt "Initialize kernel stack variables at function entry"
> -- 
> 2.34.1
>