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

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Message-ID: <20191213142846.ki2t2fwljnql66lt@ltop.local>
Date:   Fri, 13 Dec 2019 15:28:46 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>, dja@...ens.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Christophe Leroy <christophe.leroy@....fr>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Borntraeger <borntraeger@...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))

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