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

From: Guenter Roeck <linux@roeck-us.net>
To: Arnd Bergmann <arnd@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	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 05:42:30 -0700	[thread overview]
Message-ID: <23b3a9ab-7205-9f4e-9425-17506aec3170@roeck-us.net> (raw)
In-Reply-To: <CAK8P3a3yJHvJaFHUh2+5GPm2n_g9gSfX2rFbrSLzDt6yC4eDog@mail.gmail.com>

On 9/8/21 2:50 AM, Arnd Bergmann wrote:
> On Wed, Sep 8, 2021 at 9:49 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> On Wed, Sep 8, 2021 at 7:16 AM Guenter Roeck <linux@roeck-us.net> wrote:
>>> On 9/7/21 9:48 PM, Al Viro wrote:
>>>> On Tue, Sep 07, 2021 at 09:28:38PM -0700, Guenter Roeck wrote:
>>>>>       memcpy(eth_addr, sanitize_address((void *) 0xfffc1f2c), ETH_ALEN);
>>>>>
>>>>> but that just seems weird. Is there a better solution ?
>>>>
>>>> (char (*)[ETH_ALEN])?  Said that, shouldn't that be doing something like
>>>> ioremap(), rather than casting explicit constants?
>>>
>>> Typecasts or even assigning the address to a variable does not help.
>>> The sanitizer function can not be static either.
>>
>> So it can only be fixed by obfuscating the constant address in a
>> chain of out-of-line functions...
>> How is this compiler to be used for bare-metal programming?
> 
> I reported this as a gcc bug when I first saw it back in March:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
> 
> Martin Sebor suggested marking the pointer as 'volatile' as a workaround,
> which is probably fine for bare-metal programming, but I would consider
> that bad style for the kernel boot arguments. The RELOC_HIDE trick is probably
> fine here, as there are only a couple of instances, and for the network
> driver, using volatile is probably appropriate as well.
> 
> I still hope this can be fixed in a future gcc-11.x release. Maybe we should
> add further instances of the problem on the gcc bug to boost the priority?
> 
>>> I don't know the hardware, so I can not answer the ioremap() question.
>>
>> Yes it should.  But this driver dates back to 2.1.110, when only
>> half of the architectures already had ioremap().
> 
> How does  mvme16x even create the mapping? Is this a virtual address
> that is hardwired to the bus or do you have a static mapping somewhere?
> I see two other drivers accessing the nvram here
> 
> arch/m68k/mvme16x/config.c:static MK48T08ptr_t volatile rtc =
> (MK48T08ptr_t)MVME_RTC_BASE;

Is that even correct ? I am always shaky with qualifiers, but doesn't
that mean that the pointer is volatile, not the object it points to ?

> arch/m68k/mvme16x/rtc.c:        volatile MK48T08ptr_t rtc =
> (MK48T08ptr_t)MVME_RTC_BASE;
> 
> The same trick should work here, just create a local variable with a
> volatile pointer and read from that.
> 

I had tried that; it doesn't work because then the compiler complains
that the 'volatile' qualifier is discarded when passing the argument.

drivers/net/ethernet/i825xx/82596.c: In function 'i82596_probe':
drivers/net/ethernet/i825xx/82596.c:1147:34: error:
	passing argument 2 of '__builtin_memcpy' discards 'volatile' qualifier from pointer target type

Oddly enough, a memcpy on the 'rtc' variable doesn't fail,
neither with nor without volatile. Something else is going on.

Guenter