linux-kernel - Re: [PATCH] powerpc/boot: Use address-of operator on section symbols

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Date:   Mon, 3 Aug 2020 12:06:34 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Geoff Levand <geoff@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Paul Mackerras <paulus@...ba.org>,
        Joel Stanley <joel@....id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc/boot: Use address-of operator on section symbols

Hi Segher,

On Mon, Jul 20, 2020 at 11:03 PM Segher Boessenkool
<segher@...nel.crashing.org> wrote:
> On Sat, Jul 18, 2020 at 09:50:50AM +0200, Geert Uytterhoeven wrote:
> > On Wed, Jun 24, 2020 at 6:02 AM Nathan Chancellor
> > <natechancellor@...il.com> wrote:
> > >         /* If we have an image attached to us, it overrides anything
> > >          * supplied by the loader. */
> > > -       if (_initrd_end > _initrd_start) {
> > > +       if (&_initrd_end > &_initrd_start) {
> >
> > Are you sure that fix is correct?
> >
> >     extern char _initrd_start[];
> >     extern char _initrd_end[];
> >     extern char _esm_blob_start[];
> >     extern char _esm_blob_end[];
> >
> > Of course the result of their comparison is a constant, as the addresses
> > are constant.  If clangs warns about it, perhaps that warning should be moved
> > to W=1?
> >
> > But adding "&" is not correct, according to C.
>
> Why not?
>
> 6.5.3.2/3
> The unary & operator yields the address of its operand.  [...]
> Otherwise, the result is a pointer to the object or function designated
> by its operand.
>
> This is the same as using the name of an array without anything else,
> yes.  It is a bit clearer if it would not be declared as array, perhaps,
> but it is correct just fine like this.

Thanks, I stand corrected.

Regardless, the comparison is still a comparison between two constant
addresses, so my fear is that the compiler will start generating
warnings for that in the near or distant future, making this change
futile.

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