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

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Date:   Wed, 23 Aug 2017 15:11:48 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nathan Lynch <nathan_lynch@...tor.com>,
        Russell King <linux@...linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Nicolas Pitre <nico@...xnic.net>
Subject: Re: [PATCH] [RFC] ARM: vdso: work around gcc-8 warning

On Wed, Aug 23, 2017 at 03:07:04PM +0100, Ard Biesheuvel wrote:
> 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[];

FWIW, arm64 does this to solve this problem, since commit:

dbbb08f500d61463 ("arm64, vdso: Define vdso_{start,end} as array")

... and x86 has done likewise for a while.

Thanks,
Mark.