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

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Message-ID: <20200109204638.GA523773@rani.riverdale.lan>
Date:   Thu, 9 Jan 2020 15:46:41 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Zhenzhong Duan <zhenzhong.duan@...il.com>,
        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 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