[PATCH] arm64: fix clang warning about TRAMP_VALIAS

[PATCH] arm64: fix clang warning about TRAMP_VALIAS

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

The newly introduced TRAMP_VALIAS definition causes a build warning
with clang-14:

arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
                return (char *)TRAMP_VALIAS + SZ_2K * slot;

Change the addition to something clang does not complain about.

Fixes: bd09128d16fa ("arm64: Add percpu vectors for EL1")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/arm64/include/asm/vectors.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

I see this warning on 5.17-rc8, but did not test it on linux-next,
which may already have a fix.

diff --git a/arch/arm64/include/asm/vectors.h b/arch/arm64/include/asm/vectors.h
index f64613a96d53..bc9a2145f419 100644
--- a/arch/arm64/include/asm/vectors.h
+++ b/arch/arm64/include/asm/vectors.h
@@ -56,14 +56,14 @@ enum arm64_bp_harden_el1_vectors {
 DECLARE_PER_CPU_READ_MOSTLY(const char *, this_cpu_vector);
 
 #ifndef CONFIG_UNMAP_KERNEL_AT_EL0
-#define TRAMP_VALIAS	0
+#define TRAMP_VALIAS	0ul
 #endif
 
 static inline const char *
 arm64_get_bp_hardening_vector(enum arm64_bp_harden_el1_vectors slot)
 {
 	if (arm64_kernel_unmapped_at_el0())
-		return (char *)TRAMP_VALIAS + SZ_2K * slot;
+		return (char *)(TRAMP_VALIAS + SZ_2K * slot);
 
 	WARN_ON_ONCE(slot == EL1_VECTOR_KPTI);
 
-- 
2.29.2

Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS

[James Morse]

Hi Arnd,

On 3/16/22 6:38 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The newly introduced TRAMP_VALIAS definition causes a build warning
> with clang-14:
> 
> arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
>                  return (char *)TRAMP_VALIAS + SZ_2K * slot;

It is? Good to know!
(clang 11 doesn't seem to be so fussy)

Just for some more background:
TRAMP_VALIAS existed before, but it wasn't defined if KPTI is disabled at compile time, you can't
do that without CONFIG_EXPERT.
I thought these dummy definitions would be cleaner than #ifdef'ing the code.


> Change the addition to something clang does not complain about.

Acked-by: James Morse <james.morse@arm.com>


Thanks!

James



> I see this warning on 5.17-rc8, but did not test it on linux-next,
> which may already have a fix.
> 
> diff --git a/arch/arm64/include/asm/vectors.h b/arch/arm64/include/asm/vectors.h
> index f64613a96d53..bc9a2145f419 100644
> --- a/arch/arm64/include/asm/vectors.h
> +++ b/arch/arm64/include/asm/vectors.h
> @@ -56,14 +56,14 @@ enum arm64_bp_harden_el1_vectors {
>   DECLARE_PER_CPU_READ_MOSTLY(const char *, this_cpu_vector);
>   
>   #ifndef CONFIG_UNMAP_KERNEL_AT_EL0
> -#define TRAMP_VALIAS	0
> +#define TRAMP_VALIAS	0ul
>   #endif
>   
>   static inline const char *
>   arm64_get_bp_hardening_vector(enum arm64_bp_harden_el1_vectors slot)
>   {
>   	if (arm64_kernel_unmapped_at_el0())
> -		return (char *)TRAMP_VALIAS + SZ_2K * slot;
> +		return (char *)(TRAMP_VALIAS + SZ_2K * slot);
>   
>   	WARN_ON_ONCE(slot == EL1_VECTOR_KPTI);
>   
> 

Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS

[Catalin Marinas]

On Wed, 16 Mar 2022 19:38:18 +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The newly introduced TRAMP_VALIAS definition causes a build warning
> with clang-14:
> 
> arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
>                 return (char *)TRAMP_VALIAS + SZ_2K * slot;
> 
> [...]

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64: fix clang warning about TRAMP_VALIAS
      https://git.kernel.org/arm64/c/7f34b43e07cb

-- 
Catalin

Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS

[Robin Murphy]

On 2022-03-16 18:38, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The newly introduced TRAMP_VALIAS definition causes a build warning
> with clang-14:
> 
> arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]

Tangentially, all the other GNU extensions we depend on throughout Linux 
are also GNU extensions. Do we know what's special about this one?

Robin.

>                  return (char *)TRAMP_VALIAS + SZ_2K * slot;
> 
> Change the addition to something clang does not complain about.
> 
> Fixes: bd09128d16fa ("arm64: Add percpu vectors for EL1")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   arch/arm64/include/asm/vectors.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> I see this warning on 5.17-rc8, but did not test it on linux-next,
> which may already have a fix.
> 
> diff --git a/arch/arm64/include/asm/vectors.h b/arch/arm64/include/asm/vectors.h
> index f64613a96d53..bc9a2145f419 100644
> --- a/arch/arm64/include/asm/vectors.h
> +++ b/arch/arm64/include/asm/vectors.h
> @@ -56,14 +56,14 @@ enum arm64_bp_harden_el1_vectors {
>   DECLARE_PER_CPU_READ_MOSTLY(const char *, this_cpu_vector);
>   
>   #ifndef CONFIG_UNMAP_KERNEL_AT_EL0
> -#define TRAMP_VALIAS	0
> +#define TRAMP_VALIAS	0ul
>   #endif
>   
>   static inline const char *
>   arm64_get_bp_hardening_vector(enum arm64_bp_harden_el1_vectors slot)
>   {
>   	if (arm64_kernel_unmapped_at_el0())
> -		return (char *)TRAMP_VALIAS + SZ_2K * slot;
> +		return (char *)(TRAMP_VALIAS + SZ_2K * slot);
>   
>   	WARN_ON_ONCE(slot == EL1_VECTOR_KPTI);
>   

Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS

[Nick Desaulniers]

On Fri, Mar 18, 2022 at 12:11 PM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2022-03-16 18:38, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@arndb.de>
> >
> > The newly introduced TRAMP_VALIAS definition causes a build warning
> > with clang-14:
> >
> > arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
>
> Tangentially, all the other GNU extensions we depend on throughout Linux
> are also GNU extensions. Do we know what's special about this one?

My first thought is that -Wnull-pointer-arithmetic should be in the
-Wgnu flag group, such that -Wno-gnu (as set by Kbuild) implies
-Wno-null-pointer-arithmetic.

One issue seems to be that -Wnull-pointer-arithmetic seems to control
at least 2 different checks in clang.

https://godbolt.org/z/hdYYejj3d

So it's likely these two checks should have 2 distinct flags, such
that the warning we're observing can be grouped under -Wgnu.

Filed: https://github.com/llvm/llvm-project/issues/54444

>
> Robin.
>
> >                  return (char *)TRAMP_VALIAS + SZ_2K * slot;
> >
> > Change the addition to something clang does not complain about.
> >
> > Fixes: bd09128d16fa ("arm64: Add percpu vectors for EL1")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> >   arch/arm64/include/asm/vectors.h | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > I see this warning on 5.17-rc8, but did not test it on linux-next,
> > which may already have a fix.
> >
> > diff --git a/arch/arm64/include/asm/vectors.h b/arch/arm64/include/asm/vectors.h
> > index f64613a96d53..bc9a2145f419 100644
> > --- a/arch/arm64/include/asm/vectors.h
> > +++ b/arch/arm64/include/asm/vectors.h
> > @@ -56,14 +56,14 @@ enum arm64_bp_harden_el1_vectors {
> >   DECLARE_PER_CPU_READ_MOSTLY(const char *, this_cpu_vector);
> >
> >   #ifndef CONFIG_UNMAP_KERNEL_AT_EL0
> > -#define TRAMP_VALIAS 0
> > +#define TRAMP_VALIAS 0ul
> >   #endif
> >
> >   static inline const char *
> >   arm64_get_bp_hardening_vector(enum arm64_bp_harden_el1_vectors slot)
> >   {
> >       if (arm64_kernel_unmapped_at_el0())
> > -             return (char *)TRAMP_VALIAS + SZ_2K * slot;
> > +             return (char *)(TRAMP_VALIAS + SZ_2K * slot);
> >
> >       WARN_ON_ONCE(slot == EL1_VECTOR_KPTI);
> >



-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS

[Nick Desaulniers]

On Fri, Mar 18, 2022 at 12:52 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Mar 18, 2022 at 12:11 PM Robin Murphy <robin.murphy@arm.com> wrote:
> >
> > On 2022-03-16 18:38, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > The newly introduced TRAMP_VALIAS definition causes a build warning
> > > with clang-14:
> > >
> > > arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
> >
> > Tangentially, all the other GNU extensions we depend on throughout Linux
> > are also GNU extensions. Do we know what's special about this one?
>
> My first thought is that -Wnull-pointer-arithmetic should be in the
> -Wgnu flag group, such that -Wno-gnu (as set by Kbuild) implies
> -Wno-null-pointer-arithmetic.
>
> One issue seems to be that -Wnull-pointer-arithmetic seems to control
> at least 2 different checks in clang.
>
> https://godbolt.org/z/hdYYejj3d
>
> So it's likely these two checks should have 2 distinct flags, such
> that the warning we're observing can be grouped under -Wgnu.
>
> Filed: https://github.com/llvm/llvm-project/issues/54444

Fix should be landing shortly:
https://reviews.llvm.org/D122224

-- 
Thanks,
~Nick Desaulniers