Re: [PATCH] Enable '-Werror' by default for all kernel builds

From: Guenter Roeck <linux@roeck-us.net>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"David S. Miller" <davem@davemloft.net>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Christian Koenig <christian.koenig@amd.com>,
	Huang Rui <ray.huang@amd.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-sparc <sparclinux@vger.kernel.org>,
	Martin Sebor <msebor@gcc.gnu.org>
Subject: Re: [PATCH] Enable '-Werror' by default for all kernel builds
Date: Wed, 8 Sep 2021 06:54:10 -0700	[thread overview]
Message-ID: <1c1c54c4-efd9-e3da-022f-3396471d72d2@roeck-us.net> (raw)
In-Reply-To: <YTi4RdEFfa7m/ufD@zeniv-ca.linux.org.uk>

On 9/8/21 6:19 AM, Al Viro wrote:
> On Wed, Sep 08, 2021 at 05:42:30AM -0700, Guenter Roeck wrote:
> 
>> Oddly enough, a memcpy on the 'rtc' variable doesn't fail,
>> neither with nor without volatile. Something else is going on.
> 
> While we are at it, would memcpy_fromio() complain?  Seeing that
> this is what's really intended there...
> 

It doesn't make a difference on m68k.

#define memcpy_fromio memcpy_fromio
static inline void memcpy_fromio(void *dst, const volatile void __iomem *src,
                                  int count)
{
         __builtin_memcpy(dst, (void __force *) src, count);
}

It boils down to the use of __builtin_memcpy(). m68k implements its own version
of memcpy(). If that is used, everything works fine. However, if a file includes
<linux/string.h>, memcpy is replaced with __builtin_memcpy:

#define __HAVE_ARCH_MEMCPY
extern void *memcpy(void *, const void *, __kernel_size_t);
#define memcpy(d, s, n) __builtin_memcpy(d, s, n)

and the compilation fails.

That also explains why only some architectures/files are affected.
Presumably those are the architectures which use __builtin_memcpy().

Guenter