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

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Message-ID: <20200720210252.GO30544@gate.crashing.org>
Date:   Mon, 20 Jul 2020 16:02:52 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.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!

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.


Segher