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

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Date:   Wed, 8 Sep 2021 12:21:44 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-sparc <sparclinux@...r.kernel.org>,
        Martin Sebor <msebor@....gnu.org>
Subject: Re: [PATCH] Enable '-Werror' by default for all kernel builds

On Wed, Sep 8, 2021 at 11:50 AM Arnd Bergmann <arnd@...nel.org> wrote:
> On Wed, Sep 8, 2021 at 9:49 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Wed, Sep 8, 2021 at 7:16 AM Guenter Roeck <linux@...ck-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.

A related one, I guess, is:

    arch/m68k/include/asm/string.h:72:25: error: argument 2 null where
non-null expected [-Werror=nonnull]
       72 | #define memcpy(d, s, n) __builtin_memcpy(d, s, n)
          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
    drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c:387:4:
note: in expansion of macro ‘memcpy’
      387 |    memcpy((char *)kmap(pages[0]) +
          |    ^~~~~~

Seen with my sun3-allmodconfig build.
As NO_DMA=y, dmam_alloc_coherent() returns NULL, and the compiler
discovers that g_fragments_base is never assigned to and thus must
be NULL.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds