Clang enables -Wenum-enum-conversion and -Wenum-compare-conditional under -Wenum-conversion. A recent change in Clang strengthened these warnings and they appear frequently in common builds, primarily due to several instances in common headers but there are quite a few drivers that have individual instances as well.
include/linux/vmstat.h:508:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion] 508 | return vmstat_text[NR_VM_ZONE_STAT_ITEMS + | ~~~~~~~~~~~~~~~~~~~~~ ^ 509 | item]; | ~~~~
drivers/net/wireless/intel/iwlwifi/mvm/mac-ctxt.c:955:24: warning: conditional expression between different enumeration types ('enum iwl_mac_beacon_flags' and 'enum iwl_mac_beacon_flags_v1') [-Wenum-compare-conditional] 955 | flags |= is_new_rate ? IWL_MAC_BEACON_CCK | ^ ~~~~~~~~~~~~~~~~~~ 956 | : IWL_MAC_BEACON_CCK_V1; | ~~~~~~~~~~~~~~~~~~~~~ drivers/net/wireless/intel/iwlwifi/mvm/mac-ctxt.c:1120:21: warning: conditional expression between different enumeration types ('enum iwl_mac_beacon_flags' and 'enum iwl_mac_beacon_flags_v1') [-Wenum-compare-conditional] 1120 | 0) > 10 ? | ^ 1121 | IWL_MAC_BEACON_FILS : | ~~~~~~~~~~~~~~~~~~~ 1122 | IWL_MAC_BEACON_FILS_V1; | ~~~~~~~~~~~~~~~~~~~~~~
While doing arithmetic with different types of enums may be potentially problematic, inspecting several instances of the warning does not reveal any obvious problems. To silence the warnings at the source level, an integral cast must be added to each mismatched enum (which is incredibly ugly when done frequently) or the value must moved out of the enum to a macro, which can remove the type safety offered by enums in other places, such as assignments that would trigger -Wenum-conversion.
As the warnings do not appear to have a high signal to noise ratio and the source level silencing options are not sustainable, disable the warnings unconditionally, as they will be enabled with -Wenum-conversion and are supported in all versions of clang that can build the kernel.
Cc: stable@vger.kernel.org Closes: https://github.com/ClangBuiltLinux/linux/issues/2002 Link: https://github.com/llvm/llvm-project/commit/8c2ae42b3e1c6aa7c18f873edcebff7c... Signed-off-by: Nathan Chancellor nathan@kernel.org --- scripts/Makefile.extrawarn | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn index a9e552a1e910..6053aa22b8f5 100644 --- a/scripts/Makefile.extrawarn +++ b/scripts/Makefile.extrawarn @@ -81,6 +81,14 @@ KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init)
# Warn if there is an enum types mismatch KBUILD_CFLAGS += $(call cc-option,-Wenum-conversion) +ifdef CONFIG_CC_IS_CLANG +# Clang enables these extra warnings under -Wenum-conversion but the kernel +# performs arithmetic using or has conditionals returning enums of different +# types in several different places, which is rarely a bug in the kernel's +# case, so disable the warnings. +KBUILD_CFLAGS += -Wno-enum-compare-conditional +KBUILD_CFLAGS += -Wno-enum-enum-conversion +endif
# # W=1 - warnings which may be relevant and do not occur too often
--- base-commit: 90d35da658da8cff0d4ecbb5113f5fac9d00eb72 change-id: 20240304-disable-extra-clang-enum-warnings-bf574c7c99fd
Best regards,