Re: [PATCH 2/3] ubsan/overflow: Enable pattern exclusions

[Justin Stitt]

Hi,

On Thu, Mar 06, 2025 at 08:19:10PM -0800, Kees Cook wrote:
> To make integer wrap-around mitigation actually useful, the associated
> sanitizers must not instrument cases where the wrap-around is explicitly
> defined (e.g. "-2UL"), being tested for (e.g. "if (a + b < a)"), or
> where it has no impact on code flow (e.g. "while (var--)"). Enable
> pattern exclusions for the integer wrap sanitizers.
> 
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
> Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Nicolas Schier <nicolas@xxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: linux-kbuild@xxxxxxxxxxxxxxx
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> ---
>  lib/Kconfig.ubsan      | 1 +
>  scripts/Makefile.ubsan | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> index 63e5622010e0..888c2e72c586 100644
> --- a/lib/Kconfig.ubsan
> +++ b/lib/Kconfig.ubsan
> @@ -120,6 +120,7 @@ config UBSAN_INTEGER_WRAP
>  	bool "Perform checking for integer arithmetic wrap-around"
>  	default UBSAN
>  	depends on !COMPILE_TEST
> +	depends on $(cc-option,-fsanitize-undefined-ignore-overflow-pattern=all)

This option group "all" may be expanded in the future, e.g., negations
of unsigned integers (not just unsigned integer literals). As these are
deliberately designed for the kernel, I think we will want them anyways.
So, all is good.

>  	depends on $(cc-option,-fsanitize=signed-integer-overflow)
>  	depends on $(cc-option,-fsanitize=unsigned-integer-overflow)
>  	depends on $(cc-option,-fsanitize=implicit-signed-integer-truncation)
> diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
> index 4fad9afed24c..233379c193a7 100644
> --- a/scripts/Makefile.ubsan
> +++ b/scripts/Makefile.ubsan
> @@ -15,6 +15,7 @@ ubsan-cflags-$(CONFIG_UBSAN_TRAP)		+= $(call cc-option,-fsanitize-trap=undefined
>  export CFLAGS_UBSAN := $(ubsan-cflags-y)
>  
>  ubsan-integer-wrap-cflags-$(CONFIG_UBSAN_INTEGER_WRAP)     +=	\
> +	-fsanitize-undefined-ignore-overflow-pattern=all	\
>  	-fsanitize=signed-integer-overflow			\
>  	-fsanitize=unsigned-integer-overflow			\
>  	-fsanitize=implicit-signed-integer-truncation		\
> -- 
> 2.34.1
> 

Reviewed-by: Justin Stitt <justinstitt@xxxxxxxxxx>

Justin