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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Date:   Thu, 12 Dec 2019 09:41:32 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.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 2:46 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> +#ifdef GCC_VERSION < 40800

Where does that 4.8 version check come from, and why?

Yeah, I know, but this really wants a comment. Sadly it looks like gcc
bugzilla is down, so

   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

currently gives an "Internal Server Error" for me.

[ Delete the horrid code we have because of gcc bugs ]

> +#else /* GCC_VERSION < 40800 */
> +
> +#define READ_ONCE_NOCHECK(x)                                           \
> +({                                                                     \
> +       typeof(x) __x = *(volatile typeof(x))&(x);                      \

I think we can/should just do this unconditionally if it helps th eissue.

Maybe add a warning about how gcc < 4.8 might mis-compile the kernel -
those versions are getting close to being unacceptable for kernel
builds anyway.

We could also look at being stricter for the normal READ/WRITE_ONCE(),
and require that they are

 (a) regular integer types

 (b) fit in an atomic word

We actually did (b) for a while, until we noticed that we do it on
loff_t's etc and relaxed the rules. But maybe we could have a
"non-atomic" version of READ/WRITE_ONCE() that is used for the
questionable cases?

              Linus