[PATCH 4.19 47/80] Stop the ad-hoc games with -Wno-maybe-initialized

From: Greg Kroah-Hartman
Date: Mon May 18 2020 - 13:52:14 EST


From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/Makefile
+++ b/Makefile
@@ -662,10 +662,6 @@ 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)

@@ -796,6 +792,9 @@ 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
@@ -26,22 +26,6 @@ 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
@@ -1099,7 +1083,6 @@ 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
@@ -370,7 +370,6 @@ 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.