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@xxxxxxxxxxxxxxx
Closes: https://github.com/ClangBuiltLinux/linux/issues/2002
Link: https://github.com/llvm/llvm-project/commit/8c2ae42b3e1c6aa7c18f873edcebff7c0b45a37e
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
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,
--
Nathan Chancellor <nathan@xxxxxxxxxx>
