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: <20191213104706.xnpqaehmtean3mkd@ltop.local>
Date:   Fri, 13 Dec 2019 11:47:06 +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 Thu, Dec 12, 2019 at 09:53:38PM +0100, Peter Zijlstra wrote:
> Now, looking at the current GCC source:
> 
>   https://github.com/gcc-mirror/gcc/blob/97d7270f894395e513667a031a0c309d1819d05e/gcc/c/c-parser.c#L3707
> 
> it seems that __typeof__() is supposed to strip all qualifiers from
> _Atomic types. That lead me to try:
> 
> 	typeof(_Atomic typeof(p)) __p = (p);
> 
> But alas, I still get the same junk you got for ool_store_release() :/

I was checking this to see if Sparse was ready to support this.
I was a bit surprised because at first sigth GCC was doing as
it claims (typeof striping const & volatile on _Atomic types)
but your exampe wasn't working. But it's working if an
intermediate var is used:
	_Atomic typeof(p) tmp;
	typeof(tmp) __p = (p);
or, uglier but probably more practical:
	typeof(({_Atomic typeof(p) tmp; })) __p = (p);

Go figure!

OTOH, at least on GCC 8.3, it seems to always do the same with
volatiles than it does with consts.

-- Luc Van Oostenryck