linux-kernel - Re: [PATCH 01/17] kbuild: Disable -Waddress-of-packed-member for gcc 9

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Date:   Fri, 22 Mar 2019 14:58:51 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH 01/17] kbuild: Disable -Waddress-of-packed-member for gcc 9

On Thu, Mar 21, 2019 at 11:00 PM Andi Kleen <andi@...stfloor.org> wrote:
>
> From: Andi Kleen <ak@...ux.intel.com>
>
> This warning is very noisy in a default build with gcc 9.
> Move it into W=2 only.
>
> Cc: arnd@...db.de
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

I think W=2 is too aggressive. On many architectures, this finds
real bugs and the false positives tend to be trivial to fix
(by removing the bogus __packed annotation), which improves
the generated code in the process.

Moving it to W=1 for the moment is probably fine, but ideally
I think we should fix the kernel to behave according to the
C standard.

      Arnd

> ---
>  Makefile                   | 1 +
>  scripts/Makefile.extrawarn | 1 +
>  2 files changed, 2 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 71c2c482e2a2..78efbb172991 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -676,6 +676,7 @@ KBUILD_CFLAGS       += $(call cc-disable-warning,frame-address,)
>  KBUILD_CFLAGS  += $(call cc-disable-warning, format-truncation)
>  KBUILD_CFLAGS  += $(call cc-disable-warning, format-overflow)
>  KBUILD_CFLAGS  += $(call cc-disable-warning, int-in-bool-context)
> +KBUILD_CFLAGS  += $(call cc-disable-warning, address-of-packed-member)
>
>  ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
>  KBUILD_CFLAGS  += $(call cc-option,-Oz,-Os)
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 768306add591..ed974a0f4a47 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -43,6 +43,7 @@ warning-2 += $(call cc-option, -Wmissing-field-initializers)
>  warning-2 += $(call cc-option, -Wsign-compare)
>  warning-2 += $(call cc-option, -Wmaybe-uninitialized)
>  warning-2 += $(call cc-option, -Wunused-macros)
> +warning-2 += $(call cc-option, -Waddress-of-packed-member)
>
>  warning-3 := -Wbad-function-cast
>  warning-3 += -Wcast-qual
> --
> 2.20.1
>