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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 11 Apr 2019 10:52:05 +0200
Message-ID: <CAK8P3a0cdvxkoRPOF7moxZ1W2059OEJeOGAFPfSemMH1_g=YPg@mail.gmail.com>
Subject: Re: [PATCH 2/2] s390: boot, purgatory: pass $(CLANG_FLAGS) where needed
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        clang-built-linux@googlegroups.com,
        Nathan Chancellor <natechancellor@gmail.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Philipp Rudo <prudo@linux.ibm.com>,
        Hendrik Brueckner <brueckner@linux.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Apr 11, 2019 at 12:14 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
> On Wed, Apr 10, 2019 at 1:13 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > The purgatory and boot Makefiles do not inherit the original cflags,
> > so clang falls back to the default target architecture when building it,
> > typically this would be x86 when cross-compiling.
> >
> > Add $(CLANG_FLAGS) everywhere so we pass the correct --target=s390x-linux
> > option when cross-compiling.
> >
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> >  arch/s390/Makefile           | 5 +++--
> >  arch/s390/purgatory/Makefile | 1 +
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/s390/Makefile b/arch/s390/Makefile
> > index 9c079a506325..443990791099 100644
> > --- a/arch/s390/Makefile
> > +++ b/arch/s390/Makefile
> > @@ -17,12 +17,13 @@ KBUILD_CFLAGS_MODULE += -fPIC
> >  KBUILD_AFLAGS  += -m64
> >  KBUILD_CFLAGS  += -m64
> >  aflags_dwarf   := -Wa,-gdwarf-2
> > -KBUILD_AFLAGS_DECOMPRESSOR := -m64 -D__ASSEMBLY__
> > +KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
> >  KBUILD_AFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),$(aflags_dwarf))
> > -KBUILD_CFLAGS_DECOMPRESSOR := -m64 -O2
> > +KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2
> >  KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
> >  KBUILD_CFLAGS_DECOMPRESSOR += -fno-delete-null-pointer-checks -msoft-float
> >  KBUILD_CFLAGS_DECOMPRESSOR += -fno-asynchronous-unwind-tables
>
> Thanks for the respin with Nathan's suggestion.
>
> > +KBUILD_CFLAGS_DECOMPRESSOR += $(call cc-disable-warning,pointer-sign)
>
> What's up with this ^ ?  Seems like the top level sets it (without
> cc-disable-warning :( ), but then KBUILD_CFLAGS_DECOMPRESSOR discards
> it.  Does Clang actually flag code in this arch (that GCC doesn't)?

Oops, that should have been a separate patch.

I think what happens is that clang warns more aggressively about pointer sign
bugs than gcc in some cases, and some of those cases happen in s390
header files that are included by both the kernel and the decompressor.

The full warning log without this change is rather long, see
https://pastebin.com/KG9xaTNB

I also tried patching the code to avoid the warnings, but I'm not entirely
happy with that result either, see
https://pastebin.com/pSMz5eZA

     Arnd