linux-kernel - Re: [PATCH] acpica: fix -Wnull-pointer-arithmetic warnings

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Message-ID: <1563979197.11067.11.camel@lca.pw>
Date:   Wed, 24 Jul 2019 10:39:57 -0400
From:   Qian Cai <cai@....pw>
To:     "Moore, Robert" <robert.moore@...el.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "Schmauss, Erik" <erik.schmauss@...el.com>,
        "jkim@...ebsd.org" <jkim@...ebsd.org>, Len Brown <lenb@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "devel@...ica.org" <devel@...ica.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] acpica: fix -Wnull-pointer-arithmetic warnings

On Wed, 2019-07-24 at 14:17 +0000, Moore, Robert wrote:
> 
> -----Original Message-----
> From: Qian Cai [mailto:cai@....pw] 
> Sent: Wednesday, July 24, 2019 6:40 AM
> To: Moore, Robert <robert.moore@...el.com>; Nick Desaulniers <ndesaulniers@goo
> gle.com>
> Cc: Wysocki, Rafael J <rafael.j.wysocki@...el.com>; Schmauss, Erik <erik.schma
> uss@...el.com>; jkim@...ebsd.org; Len Brown <lenb@...nel.org>; linux-acpi@...r
> .kernel.org; devel@...ica.org; clang-built-linux <clang-built-linux@...glegrou
> ps.com>; LKML <linux-kernel@...r.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@....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)

Yes.

> 
> 
> > 
> > > > #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
> > 
> >