RE: [PATCH] acpica: fix -Wnull-pointer-arithmetic warnings

["Moore, Robert" <robert.moore@intel.com>]

-----Original Message-----
From: Qian Cai [mailto:cai@lca.pw] 
Sent: Wednesday, July 24, 2019 6:40 AM
To: Moore, Robert <robert.moore@intel.com>; Nick Desaulniers <ndesaulniers@google.com>
Cc: Wysocki, Rafael J <rafael.j.wysocki@intel.com>; Schmauss, Erik <erik.schmauss@intel.com>; jkim@freebsd.org; Len Brown <lenb@kernel.org>; linux-acpi@vger.kernel.org; devel@acpica.org; clang-built-linux <clang-built-linux@googlegroups.com>; LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] acpica: fix -Wnull-pointer-arithmetic warnings

On Tue, 2019-07-23 at 20:49 +0000, Moore, Robert wrote:
> > > Signed-off-by: Qian Cai <cai@lca.pw>
> > > ---
> > > include/acpi/actypes.h | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/acpi/actypes.h b/include/acpi/actypes.h index
> > > ad6892a24015..25b4a32da177 100644
> > > --- a/include/acpi/actypes.h
> > > +++ b/include/acpi/actypes.h
> > > @@ -500,13 +500,13 @@ typedef u64 acpi_integer;
> > > 
> > > #define ACPI_CAST_PTR(t, p)             ((t *) (acpi_uintptr_t) 
> > > (p)) #define ACPI_CAST_INDIRECT_PTR(t, p)    ((t **) 
> > > (acpi_uintptr_t) (p)) -#define ACPI_ADD_PTR(t, a, b)           
> > > ACPI_CAST_PTR (t, (ACPI_CAST_PTR (u8, (a)) + (acpi_size)(b)))
> > > +#define ACPI_ADD_PTR(t, a, b)           ACPI_CAST_PTR (t, (a) +
> > > (acpi_size)(b))
> 
> We have some questions concerning this change. If (a) is not cast to a 
> u8, the addition will be in whatever units are appropriate for (a) 
> i.e., the type of (a). However, we want ACPI_ADD_PTR (And 
> ACPI_SUB_PTR) to simply perform a byte addition or subtraction - thus 
> the cast to u8. I believe that is the original thinking behind the macros.

I posted a v2 a while ago, and should clear this concern.

OK then this change only affects ACPI_TO_POINTER?

+#define ACPI_TO_POINTER(i)              ACPI_CAST_PTR (void, i)


> 
> > > #define ACPI_SUB_PTR(t, a, b)           ACPI_CAST_PTR (t, 
> > > (ACPI_CAST_PTR (u8, (a)) - (acpi_size)(b))) #define 
> > > ACPI_PTR_DIFF(a, b)             ((acpi_size) (ACPI_CAST_PTR (u8,
> > > (a)) - ACPI_CAST_PTR (u8, (b))))
> > > 
> > > /* Pointer/Integer type conversions */
> > > 
> > > -#define ACPI_TO_POINTER(i)              ACPI_ADD_PTR (void, (void 
> > > *) 0,
> > > (acpi_size) (i))
> > > +#define ACPI_TO_POINTER(i)              ACPI_ADD_PTR (void, 0,
> > > (acpi_size) (i))
> > 
> > IIUC, these are adding `i` to NULL (or (void*)0)? X + 0 == X ?
> > --
> > Thanks,
> > ~Nick Desaulniers
> 
>