Re: [PATCH] lib/iomem_copy: fix __iomem casts

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Jun 24, 2026 at 11:17:17AM +0100, Ben Dooks wrote:
> On 24/06/2026 00:47, Al Viro wrote:
> > On Mon, Jun 22, 2026 at 01:48:57PM +0100, Ben Dooks wrote:
> > > The iomem_copy.c code discards __iomem address space when using
> > > the IS_ALIGNED() macro. It would make more sense to fix this in
> > > one place by aing a PTR_ALIGNED_LONG() macro and then doing the
> > > necessary casts there before invoking IS_ALIGNED().
> > > 
> > > As part of this, also force the pointer to an unsigned long as
> > > pointers are generally not signed, although there is no warning
> > > as yet on treating pointers as signed.
> > 
> > > +#define PTR_ALIGNED_LONG(__ptr) IS_ALIGNED((__force unsigned long)__ptr, sizeof(long))
> > 
> > Casting to unsigned long is fine (indeed, casting a pointer to long had
> > been very odd in the first place), but... why __force?  Casts to unsigned long
> > (de facto uintptr_t) do *not* require __force - they are explicitly allowed,
> > unless you pass -Wcast-from-as in sparse arguments.  -Wall does not turn
> > those on; -Wsparse-all would, but kbuild doesn't pass that.
> 
> Trying to get rid of the address space warning, which requires the
> __force part.

It does not require __force - not with the type we are casting to being
unsigned long.  As the examples you've quoted demonstrate.  See
evaluate_cast() in sparse evaluate.c:
        if ((ttype == &ulong_ctype || ttype == uintptr_ctype) && !Wcast_from_as)
		tas = &bad_address_space;
with
        if (!tas && valid_as(sas))
		warning(expr->pos, "cast removes address space '%s' of expression", show_as(sas));
several lines below.

And yes, it is deliberate.