From: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
commit b303c6df80c9f8f13785aa83a0471fca7e38b24d upstream.
Since -Wmaybe-uninitialized was introduced by GCC 4.7, we have patched
various false positives:
- commit e74fc973b6e5 ("Turn off -Wmaybe-uninitialized when building
with -Os") turned off this option for -Os.
- commit 815eb71e7149 ("Kbuild: disable 'maybe-uninitialized' warning
for CONFIG_PROFILE_ALL_BRANCHES") turned off this option for
CONFIG_PROFILE_ALL_BRANCHES
- commit a76bcf557ef4 ("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: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
Makefile | 11 ++++-------
init/Kconfig | 17 +++++++++++++++++
kernel/trace/Kconfig | 1 +
3 files changed, 22 insertions(+), 7 deletions(-)
--- a/Makefile
+++ b/Makefile
@@ -658,17 +658,14 @@ KBUILD_CFLAGS += $(call cc-option,-fdata
endif
ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
-KBUILD_CFLAGS += -Os $(call cc-disable-warning,maybe-uninitialized,)
-else
-ifdef CONFIG_PROFILE_ALL_BRANCHES
-KBUILD_CFLAGS += -O2 $(call cc-disable-warning,maybe-uninitialized,)
+KBUILD_CFLAGS += -Os
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 += $(call cc-option,--param=allow-store-data-races=0)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -16,6 +16,22 @@ config DEFCONFIG_LIST
default "$ARCH_DEFCONFIG"
default "arch/$ARCH/defconfig"
+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
@@ -1333,6 +1349,7 @@ 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.
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -342,6 +342,7 @@ config PROFILE_ANNOTATED_BRANCHES
config PROFILE_ALL_BRANCHES
bool "Profile all if conditionals"
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.
