linux-kernel - Re: [PATCH] [RFC] ARM: vdso: work around gcc-8 warning

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Message-ID: <CAKv+Gu-jCzN0K+fhe9GsEwbwuaK6VJrfdLoPnPXnGaQ9qt3WOg@mail.gmail.com>
Date:   Wed, 23 Aug 2017 15:07:04 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King <linux@...linux.org.uk>,
        Nicolas Pitre <nico@...xnic.net>,
        Nathan Lynch <nathan_lynch@...tor.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC] ARM: vdso: work around gcc-8 warning

On 23 August 2017 at 15:05, Arnd Bergmann <arnd@...db.de> wrote:
> gcc-8 correctly points out that reading four bytes from a pointer to a
> 'char' variable is wrong
>
> arch/arm/kernel/vdso.c: In function 'vdso_init':
> arch/arm/kernel/vdso.c:200:6: error: '__builtin_memcmp_eq' reading 4 bytes from a region of size 1 [-Werror=stringop-overflow=]
>
> However, in this case the variable just stands for the beginning of the
> vdso and is not actually a 'char', so the code is doing what it is meant
> to do.
>
> Not sure what the best solution for this is, changing the hack to
> declare the variable as 'int' instead makes the warning go away.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/arm/include/asm/vdso.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/include/asm/vdso.h b/arch/arm/include/asm/vdso.h
> index d0295f1dd1a3..eff7d3a1d1ce 100644
> --- a/arch/arm/include/asm/vdso.h
> +++ b/arch/arm/include/asm/vdso.h
> @@ -11,7 +11,7 @@ struct mm_struct;
>
>  void arm_install_vdso(struct mm_struct *mm, unsigned long addr);
>
> -extern char vdso_start, vdso_end;
> +extern int vdso_start, vdso_end;
>
>  extern unsigned int vdso_total_pages;
>

May I suggest

extern char vdso_start[], vdso_end[];