Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

From: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	dja@axtens.net,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linuxppc-dev@lists.ozlabs.org,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	linux-arch <linux-arch@vger.kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
Date: Fri, 13 Dec 2019 15:28:46 +0100	[thread overview]
Message-ID: <20191213142846.ki2t2fwljnql66lt@ltop.local> (raw)
In-Reply-To: <20191213125618.GD2844@hirez.programming.kicks-ass.net>

On Fri, Dec 13, 2019 at 01:56:18PM +0100, Peter Zijlstra wrote:
> 
> Excellent! I had to change it to something like:
> 
> #define unqual_typeof(x)    typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; }))
> 
> but that does indeed work!
> 
> Now I suppose we should wrap that in a symbol that indicates our
> compiler does indeed support _Atomic, otherwise things will come apart.
> 
> That is, my gcc-4.6 doesn't seem to have it, while gcc-4.8 does, which
> is exactly the range that needs the daft READ_ONCE() construct, how
> convenient :/
> 
> Something a little like this perhaps?

Yes, this looks good to me.
Just a small nit here below.

> ---
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 7d9cc5ec4971..c389af602da8 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -75,9 +75,9 @@ static inline unsigned long array_index_mask_nospec(unsigned long idx,
>  
>  #define __smp_store_release(p, v)					\
>  do {									\
> -	typeof(p) __p = (p);						\
> -	union { typeof(*p) __val; char __c[1]; } __u =			\
> -		{ .__val = (__force typeof(*p)) (v) };			\
> +	unqual_typeof(p) __p = (p);					\
> +	union { unqual_typeof(*p) __val; char __c[1]; } __u =	\
> +		{ .__val = (__force unqual_typeof(*p)) (v) };	\

The 2 two trailing backslashes are now off by one tab.

-- Luc 