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

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Message-ID: <87zh7cyoi7.fsf@mpe.ellerman.id.au>
Date:   Mon, 03 Aug 2020 21:09:36 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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

Geert Uytterhoeven <geert@...ux-m68k.org> writes:
> 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.

They're not constant at compile time though. So I don't think the
compiler could (sensibly) warn about that? (surely!)

cheers