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Date:   Wed, 27 Feb 2019 21:44:11 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] kbuild: compute false-positive -Wmaybe-uninitialized
 cases in Kconfig

On Fri, Feb 22, 2019 at 9:53 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Wed, Feb 20, 2019 at 8:14 PM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> >
> > 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.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>
> I verified that mainline+this patch did not regression arm64
> allyesconfig clang build.
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Thanks for the patch! Would you mind adding a link tag to:
> Link: https://github.com/ClangBuiltLinux/linux/issues/350

I added the tags, and applied to linux-kbuild.



> > ---
> >
> >  Makefile             | 10 +++-------
> >  init/Kconfig         | 17 +++++++++++++++++
> >  kernel/trace/Kconfig |  1 +
> >  3 files changed, 21 insertions(+), 7 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 1bb0535..b21aa2e3 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -656,17 +656,13 @@ KBUILD_CFLAGS     += $(call cc-disable-warning, int-in-bool-context)
> >
> >  ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
> >  KBUILD_CFLAGS  += $(call cc-option,-Oz,-Os)
> > -KBUILD_CFLAGS  += $(call cc-disable-warning,maybe-uninitialized,)
> > -else
> > -ifdef CONFIG_PROFILE_ALL_BRANCHES
> > -KBUILD_CFLAGS  += -O2 $(call cc-disable-warning,maybe-uninitialized,)
> >  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)
> > diff --git a/init/Kconfig b/init/Kconfig
> > index c9386a3..1f05a88 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -26,6 +26,22 @@ 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
> > @@ -1113,6 +1129,7 @@ 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.
> > diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> > index fa8b1fe..8bd1d6d 100644
> > --- a/kernel/trace/Kconfig
> > +++ b/kernel/trace/Kconfig
> > @@ -370,6 +370,7 @@ 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.
> > --
> > 2.7.4
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Best Regards
Masahiro Yamada

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