Commit b303c6df authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

kbuild: compute false-positive -Wmaybe-uninitialized cases in Kconfig 

Since -Wmaybe-uninitialized was introduced by GCC 4.7, we have patched
various false positives:

 - commit e74fc973 ("Turn off -Wmaybe-uninitialized when building
   with -Os") turned off this option for -Os.

 - commit 815eb71e ("Kbuild: disable 'maybe-uninitialized' warning
   for CONFIG_PROFILE_ALL_BRANCHES") turned off this option for
   CONFIG_PROFILE_ALL_BRANCHES

 - commit a76bcf55 ("Kbuild: enable -Wmaybe-uninitialized warning
   for "make W=1"") turned off this option for GCC < 4.9
   Arnd provided more explanation in https://lkml.org/lkml/2017/3/14/903

I think this looks better by shifting the logic from Makefile to Kconfig.

Link: https://github.com/ClangBuiltLinux/linux/issues/350


Signed-off-by: default avatarMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: default avatarNathan Chancellor <natechancellor@gmail.com>
Tested-by: default avatarNick Desaulniers <ndesaulniers@google.com>
parent bd55f96f

Makefile

+3 −7
Original line number Diff line number Diff line
@@ -660,17 +660,13 @@ KBUILD_CFLAGS += $(call cc-disable-warning, int-in-bool-context) 


ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE

KBUILD_CFLAGS	+= $(call cc-option,-Oz,-Os)

KBUILD_CFLAGS	+= $(call cc-disable-warning,maybe-uninitialized,)

else

ifdef CONFIG_PROFILE_ALL_BRANCHES

KBUILD_CFLAGS	+= -O2 $(call cc-disable-warning,maybe-uninitialized,)

else

KBUILD_CFLAGS   += -O2

endif

endif



KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \

			$(call cc-disable-warning,maybe-uninitialized,))

ifdef CONFIG_CC_DISABLE_WARN_MAYBE_UNINITIALIZED

KBUILD_CFLAGS   += -Wno-maybe-uninitialized

endif



# Tell gcc to never replace conditional load with a non-conditional one

KBUILD_CFLAGS	+= $(call cc-option,--param=allow-store-data-races=0)

init/Kconfig

+17 −0
Original line number Diff line number Diff line
@@ -26,6 +26,22 @@ config CLANG_VERSION 
config CC_HAS_ASM_GOTO

	def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))



config CC_HAS_WARN_MAYBE_UNINITIALIZED

	def_bool $(cc-option,-Wmaybe-uninitialized)

	help

	  GCC >= 4.7 supports this option.



config CC_DISABLE_WARN_MAYBE_UNINITIALIZED

	bool

	depends on CC_HAS_WARN_MAYBE_UNINITIALIZED

	default CC_IS_GCC && GCC_VERSION < 40900  # unreliable for GCC < 4.9

	help

	  GCC's -Wmaybe-uninitialized is not reliable by definition.

	  Lots of false positive warnings are produced in some cases.



	  If this option is enabled, -Wno-maybe-uninitialzed is passed

	  to the compiler to suppress maybe-uninitialized warnings.



config CONSTRUCTORS

	bool

	depends on !UML
@@ -1102,6 +1118,7 @@ config CC_OPTIMIZE_FOR_PERFORMANCE 


config CC_OPTIMIZE_FOR_SIZE

	bool "Optimize for size"

	imply CC_DISABLE_WARN_MAYBE_UNINITIALIZED  # avoid false positives

	help

	  Enabling this option will pass "-Os" instead of "-O2" to

	  your compiler resulting in a smaller kernel.

kernel/trace/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -370,6 +370,7 @@ config PROFILE_ANNOTATED_BRANCHES 
config PROFILE_ALL_BRANCHES

	bool "Profile all if conditionals" if !FORTIFY_SOURCE

	select TRACE_BRANCH_PROFILING

	imply CC_DISABLE_WARN_MAYBE_UNINITIALIZED  # avoid false positives

	help

	  This tracer profiles all branch conditions. Every if ()

	  taken in the kernel is recorded whether it hit or miss.

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