linux-kernel - Re: [PATCH 1/2] Makefile.extrawarn: move -Wcast-align to W=3

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Date:   Mon, 23 Nov 2020 20:17:46 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 1/2] Makefile.extrawarn: move -Wcast-align to W=3

On Tue, Oct 27, 2020 at 10:42 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Mon, Oct 26, 2020 at 11:03:13PM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > This warning behaves differently depending on the architecture
> > and compiler. Using x86 gcc, we get no output at all because
> > gcc knows the architecture can handle unaligned accesses.
> >
> > Using x86 clang, or gcc on an architecture that needs to
> > manually deal with unaligned accesses, the build log is
> > completely flooded with these warnings, as they are commonly
> > invoked by inline functions of networking headers, e.g.
> >
> > include/linux/skbuff.h:1426:26: warning: cast increases required alignment of target type [-Wcast-align]
> >
> > The compiler is correct to point this out, as we are dealing
> > with undefined behavior that does cause problems in practice,
> > but there is also no good way to rewrite the code in commonly
> > included headers to a safer method.
> >
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> Always sad to see a warning move further down the list but noisy headers
> are rough to deal with. This seems okay.
>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>
> > ---
> >  scripts/Makefile.extrawarn | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> > index 95e4cdb94fe9..6baee1200615 100644
> > --- a/scripts/Makefile.extrawarn
> > +++ b/scripts/Makefile.extrawarn
> > @@ -60,7 +60,6 @@ endif
> >  #
> >  ifneq ($(findstring 2, $(KBUILD_EXTRA_WARN)),)
> >
> > -KBUILD_CFLAGS += -Wcast-align
> >  KBUILD_CFLAGS += -Wdisabled-optimization
> >  KBUILD_CFLAGS += -Wnested-externs
> >  KBUILD_CFLAGS += -Wshadow
> > @@ -80,6 +79,7 @@ endif
> >  ifneq ($(findstring 3, $(KBUILD_EXTRA_WARN)),)
> >
> >  KBUILD_CFLAGS += -Wbad-function-cast
> > +KBUILD_CFLAGS += -Wcast-align
> >  KBUILD_CFLAGS += -Wcast-qual
> >  KBUILD_CFLAGS += -Wconversion
> >  KBUILD_CFLAGS += -Wpacked
> > --
> > 2.27.0
> >


Applied to linux-kbuild. Thanks.

But, I think people already tend to ignore W=2 warnings.


-- 
Best Regards
Masahiro Yamada