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Date:   Thu, 19 Dec 2019 12:11:16 +0000
From:   Will Deacon <will@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Daniel Axtens <dja@axtens.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <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))
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On Tue, Dec 17, 2019 at 10:32:35AM -0800, Linus Torvalds wrote:
> On Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds
> <torvalds@linux-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?

Sounds like it according to mpe, but I'll confirm too for arm64.

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

>  include/linux/compiler.h | 33 +++++++++++++++++----------------
>  1 file changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 5e88e7e33abe..8b4282194f16 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -179,18 +179,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  
>  #include <uapi/linux/types.h>
>  
> -#define __READ_ONCE_SIZE						\
> -({									\
> -	switch (size) {							\
> -	case 1: *(__u8 *)res = *(volatile __u8 *)p; break;		\
> -	case 2: *(__u16 *)res = *(volatile __u16 *)p; break;		\
> -	case 4: *(__u32 *)res = *(volatile __u32 *)p; break;		\
> -	case 8: *(__u64 *)res = *(volatile __u64 *)p; break;		\
> -	default:							\
> -		barrier();						\
> -		__builtin_memcpy((void *)res, (const void *)p, size);	\
> -		barrier();						\
> -	}								\
> +/* "unsigned long" or "unsigned long long" - make it fit in a register if possible */
> +#define __READ_ONCE_TYPE(size) \
> +	__typeof__(__builtin_choose_expr(size > sizeof(0UL), 0ULL, 0UL))

Ha, I wondered when '__builtin_choose_expr()' would make an appearance in
this thread! Nice trick. I'll try integrating this with what I have and see
what I run into next.

Back down the rabbit hole...

Will