linux-kernel - Re: [PATCH v2] x86/boot/KASLR: Fix unused variable warning

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Message-ID: <CAFH1YnNA9qfM4tPzKKDaQD6DPxnE=tJsB7AUZQBohDTW3zm=Xg@mail.gmail.com>
Date:   Fri, 10 Jan 2020 10:27:02 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Chao Fan <fanc.fnst@...fujitsu.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v2] x86/boot/KASLR: Fix unused variable warning

On Fri, Jan 10, 2020 at 4:46 AM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> On Thu, Jan 09, 2020 at 07:40:55PM +0100, Borislav Petkov wrote:
> > On Fri, Jan 03, 2020 at 11:39:29AM +0800, Zhenzhong Duan wrote:
> > > Local variable 'i' is referenced only when CONFIG_MEMORY_HOTREMOVE and
> > > CONFIG_ACPI are defined, but definition of variable 'i' is out of guard.
> > > If any of the two macros is undefined, below warning triggers during
> > > build, fix it by moving 'i' in the guard.
> > >
> > > arch/x86/boot/compressed/kaslr.c:698:6: warning: unused variable ‘i’ [-Wunused-variable]
> >
> > How do you trigger this?
> >
> > I have:
> >
> > $  grep -E "(CONFIG_MEMORY_HOTREMOVE|CONFIG_ACPI)" .config
> > # CONFIG_ACPI is not set
> >
> > but no warning. Neither with gcc 8 nor with gcc 9.
> >
> > --
> > Regards/Gruss,
> >     Boris.
> >
> > https://people.kernel.org/tglx/notes-about-netiquette
>
> The boot/compressed Makefile resets KBUILD_CFLAGS.  Following hack and
> building with W=1 shows it, or just add -Wunused in there.
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 56aa5fa0a66b..791c0d5a952a 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -35,6 +35,9 @@ KBUILD_CFLAGS += $(cflags-y)
>  KBUILD_CFLAGS += -mno-mmx -mno-sse
>  KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
>  KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
> +
> +include scripts/Makefile.extrawarn
> +
>  KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
>  KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
>  KBUILD_CFLAGS += -Wno-pointer-sign

Yes. Will you send this formally? Not clear if there is other reason making
KBUILD_CFLAGS for arch/x86/boot/compressed different from other part.

Regards
Zhenzhong