linux-kernel - [PATCH 4.4 57/86] kbuild: compute false-positive -Wmaybe-uninitialized cases in Kconfig

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Date:   Mon, 18 May 2020 19:36:28 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH 4.4 57/86] kbuild: compute false-positive -Wmaybe-uninitialized cases in Kconfig

From: Masahiro Yamada <yamada.masahiro@...ionext.com>

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@...ionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 Makefile             |    5 ++++-
 init/Kconfig         |   17 +++++++++++++++++
 kernel/trace/Kconfig |    1 +
 3 files changed, 22 insertions(+), 1 deletion(-)

--- a/Makefile
+++ b/Makefile
@@ -631,7 +631,6 @@ ARCH_CFLAGS :=
 include arch/$(SRCARCH)/Makefile
 
 KBUILD_CFLAGS	+= $(call cc-option,-fno-delete-null-pointer-checks,)
-KBUILD_CFLAGS	+= $(call cc-disable-warning,maybe-uninitialized,)
 KBUILD_CFLAGS	+= $(call cc-disable-warning,frame-address,)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, format-truncation)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, format-overflow)
@@ -649,6 +648,10 @@ KBUILD_CFLAGS   += -O2
 endif
 endif
 
+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
@@ -1331,6 +1347,7 @@ 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
@@ -312,6 +312,7 @@ 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.