Re: [PATCH v2] kbuild: move gcc's -Wconstant-logical-operand to W=2

[Nick Desaulniers]

On Thu, Nov 5, 2026 at 9:58 AM Vineet Gupta <vineet.gupta@xxxxxxxxx> wrote:
>
> GCC 17 adds -Wconstant-logical-operand to -Wall. It warns when an operand
> of && or || folds to a constant other than 0 or 1. Unlike clang's version,
> it also checks constant left operands and operators that come from a
> macro expansion. With CONFIG_WERROR=y, an x86 defconfig build fails in
> prepare0:
>
>   arch/x86/include/asm/cpufeature.h:72:36: error: use of logical '&&' with constant operand '4194304' [-Werror=constant-logical-operand]
>
> The warning is meant to catch && written for &. Kernel history has no
> such catch. Of the 13 earlier kernel fixes for clang's version, only
> commit dd7fe5d9fd6a ("rtc: max31335: Fix comparison in
> max31335_volatile_reg()") fixed a bug that the warning found. Commit
> 394b52462020 ("drm/i915/gem: fix bitwise and logical AND mixup") fixed a
> & written for &&, which the warning cannot see. The other fixes rewrote
> correct code. In GCC's own tree, the warning found two sites since it
> was added, and both were false positives.
>
> Disable the warning for GCC unless W=2 is given. Keep clang's version,
> which skips macros and constant left operands.

Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

>
> Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
> Link: https://lore.kernel.org/all/20261104160722.1637541-1-vineet.gupta@xxxxxxxxx/
> Signed-off-by: Vineet Gupta <vineet.gupta@xxxxxxxxx>
> ---
> v1 -> v2:
>  - Replace the four per-site fixes with this one change (Borislav, Peter)
>  - Link to v1: https://lore.kernel.org/all/20261104160722.1637541-1-vineet.gupta@xxxxxxxxx/
>
>  scripts/Kconfig.toolchain | 3 +++
>  scripts/Makefile.warn     | 1 +
>  2 files changed, 4 insertions(+)
>
> diff --git a/scripts/Kconfig.toolchain b/scripts/Kconfig.toolchain
> index 2acbd40a38f1..1d68075f0469 100644
> --- a/scripts/Kconfig.toolchain
> +++ b/scripts/Kconfig.toolchain
> @@ -233,6 +233,9 @@ config CC_OPT_WNO_FORMAT_OVERFLOW
>  config CC_OPT_WNO_STRINGOP_TRUNCATION
>         def_string "$(cc-option-str,-Wno-stringop-truncation)"
>
> +config CC_OPT_WNO_CONSTANT_LOGICAL_OPERAND
> +       def_string "$(cc-option-str,-Wno-constant-logical-operand)"
> +
>  config CC_HAS_COUNTED_BY
>         bool
>         # clang needs to be at least 20.1.0 to avoid potential crashes
> diff --git a/scripts/Makefile.warn b/scripts/Makefile.warn
> index 556fa249ca07..03b677bdbb0a 100644
> --- a/scripts/Makefile.warn
> +++ b/scripts/Makefile.warn
> @@ -167,6 +167,7 @@ endif
>
>  ifdef CONFIG_CC_IS_GCC
>  KBUILD_CFLAGS += -Wno-maybe-uninitialized
> +KBUILD_CFLAGS += $(CONFIG_CC_OPT_WNO_CONSTANT_LOGICAL_OPERAND)
>  endif
>
>  endif
> --
> 2.53.0-Meta
>


-- 
Thanks,
~Nick Desaulniers