linux-kernel - Re: [RESEND PATCH v1] x86/build: add -fno-builtin flag to prevent shadowing

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Message-ID: <CAK8P3a3Fw2T0WSkxv4DiTG2wGkKcs24StPx-BG_vi=ffa9OLVA@mail.gmail.com>
Date:   Sun, 8 May 2022 12:27:49 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>
Subject: Re: [RESEND PATCH v1] x86/build: add -fno-builtin flag to prevent shadowing

On Sun, May 8, 2022 at 12:09 PM Vincent Mailhol
<mailhol.vincent@...adoo.fr> wrote:
>
> Aside of the __builtin_foo() ones, x86 does not directly rely on any
> builtin functions.
>
> However, such builtin functions are not explicitly deactivated,
> creating some collisions, concrete example being ffs() from bitops.h,
> c.f.:
>
> | ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]
> |   283 | static __always_inline int ffs(int x)
>
> This patch adds -fno-builtin to KBUILD_CFLAGS for the x86
> architectures in order to prevent shadowing of builtin functions.
>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> ---
> FYI, I tested this patch on a "make allyesconfig" for both x86_32 and
> x86_64.
>
> This is a resend. Only difference is that I dropped the RFC flag and
> added Arnd in CC because he did a similar patch to fix ffs shadow
> warnings in the past:
>
> https://lore.kernel.org/all/20201026160006.3704027-1-arnd@kernel.org/

I think this is a correct change, but unfortunately it exposes a clang bug
 with -mregparm=3. Nick should be able to provide more details, I think
he has a plan.

> ---
>  arch/x86/Makefile | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index e84cdd409b64..5ff7b6571dd2 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -53,6 +53,8 @@ export REALMODE_CFLAGS
>  # e.g.: obj-y += foo_$(BITS).o
>  export BITS
>
> +KBUILD_CFLAGS += -fno-builtin
> +
>  #
>  # Prevent GCC from generating any FP code by mistake.
>  #
> --
> 2.34.1
>