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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 4.9 61/90] Stop the ad-hoc games with -Wno-maybe-initialized
Date:   Mon, 18 May 2020 19:36:39 +0200
Message-Id: <20200518173503.542960889@linuxfoundation.org>
In-Reply-To: <20200518173450.930655662@linuxfoundation.org>
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From: Linus Torvalds <torvalds@linux-foundation.org>

commit 78a5255ffb6a1af189a83e493d916ba1c54d8c75 upstream.

We have some rather random rules about when we accept the
"maybe-initialized" warnings, and when we don't.

For example, we consider it unreliable for gcc versions < 4.9, but also
if -O3 is enabled, or if optimizing for size.  And then various kernel
config options disabled it, because they know that they trigger that
warning by confusing gcc sufficiently (ie PROFILE_ALL_BRANCHES).

And now gcc-10 seems to be introducing a lot of those warnings too, so
it falls under the same heading as 4.9 did.

At the same time, we have a very straightforward way to _enable_ that
warning when wanted: use "W=2" to enable more warnings.

So stop playing these ad-hoc games, and just disable that warning by
default, with the known and straight-forward "if you want to work on the
extra compiler warnings, use W=123".

Would it be great to have code that is always so obvious that it never
confuses the compiler whether a variable is used initialized or not?
Yes, it would.  In a perfect world, the compilers would be smarter, and
our source code would be simpler.

That's currently not the world we live in, though.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 Makefile             |    7 +++----
 init/Kconfig         |   17 -----------------
 kernel/trace/Kconfig |    1 -
 3 files changed, 3 insertions(+), 22 deletions(-)

--- a/Makefile
+++ b/Makefile
@@ -663,10 +663,6 @@ else
 KBUILD_CFLAGS   += -O2
 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)
 
@@ -801,6 +797,9 @@ KBUILD_CFLAGS += $(call cc-disable-warni
 # disable stringop warnings in gcc 8+
 KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)
 
+# Enabled with W=2, disabled by default as noisy
+KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
+
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
 
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -16,22 +16,6 @@ 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
@@ -1349,7 +1333,6 @@ 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
@@ -342,7 +342,6 @@ 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.