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    Subject[PATCH 4.14 072/114] kbuild: compute false-positive -Wmaybe-uninitialized cases in Kconfig
    Date
    From: Masahiro Yamada <yamada.masahiro@socionext.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@socionext.com>
    Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
    Tested-by: Nick Desaulniers <ndesaulniers@google.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    Makefile | 11 ++++-------
    init/Kconfig | 17 +++++++++++++++++
    kernel/trace/Kconfig | 1 +
    3 files changed, 22 insertions(+), 7 deletions(-)

    --- a/Makefile
    +++ b/Makefile
    @@ -656,17 +656,14 @@ KBUILD_CFLAGS += $(call cc-disable-warni
    KBUILD_CFLAGS += $(call cc-disable-warning, attribute-alias)

    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
    @@ -1044,6 +1060,7 @@ 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
    @@ -345,6 +345,7 @@ 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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