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: <CAHk-=wjNLxSZZs+A0tyb-MnJ2YU-sqTAfy0-X+9vxjXfs_O4zA@mail.gmail.com>
Date:   Tue, 17 Dec 2019 10:32:35 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Daniel Axtens <dja@...ens.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <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 Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Let me think about it.

How about we just get rid of the union entirely, and just use
'unsigned long' or 'unsigned long long' depending on the size.

Something like the attached patch - it still requires that it be an
arithmetic type, but now because of the final cast.

But it might still be a cast to a volatile type, of course. Then the
result will be volatile, but at least now READ_ONCE() won't be taking
the address of a volatile variable on the stack - does that at least
fix some of the horrible code generation. Hmm?

This is untested, because I obviously still have the cases of
structures (page table entries) being accessed once..

              Linus

View attachment "patch.diff" of type "text/x-patch" (2149 bytes)