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

[Michael Ellerman]

Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> writes:
> On Mon, Jul 20, 2020 at 11:03 PM Segher Boessenkool
> <segher@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx> 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