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

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

[Vineet Gupta]

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.

Suggested-by: Borislav Petkov <bp@alien8.de>
Link: https://lore.kernel.org/all/20261104160722.1637541-1-vineet.gupta@linux.dev/
Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
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@linux.dev/

 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

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@linux.dev> 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@google.com>

>
> Suggested-by: Borislav Petkov <bp@alien8.de>
> Link: https://lore.kernel.org/all/20261104160722.1637541-1-vineet.gupta@linux.dev/
> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
> ---
> 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@linux.dev/
>
>  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

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

[Nathan Chancellor]

On Thu, Nov 05, 2026 at 09:57:45AM -0800, Vineet Gupta 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.
> 
> Suggested-by: Borislav Petkov <bp@alien8.de>
> Link: https://lore.kernel.org/all/20261104160722.1637541-1-vineet.gupta@linux.dev/
> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>

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

This probably wants a 'Cc: stable' so that older kernels can build
warning free with GCC 17.

-- 
Cheers,
Nathan