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From: Will Deacon <will@kernel.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	dja@axtens.net,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	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>
Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
Date: Wed, 18 Dec 2019 10:35:01 +0000	[thread overview]
Message-ID: <20191218103501.GA15021@willie-the-truck> (raw)
In-Reply-To: <fb4bba85-1f83-435e-b84e-ee29770d7090@de.ibm.com>

On Wed, Dec 18, 2019 at 11:22:05AM +0100, Christian Borntraeger wrote:
> On 12.12.19 21:49, Linus Torvalds wrote:
> > On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <will@kernel.org> wrote:
> >>
> >> The root of my concern in all of this, and what started me looking at it in
> >> the first place, is the interaction with 'typeof()'. Inheriting 'volatile'
> >> for a pointer means that local variables in macros declared using typeof()
> >> suddenly start generating *hideous* code, particularly when pointless stack
> >> spills get stackprotector all excited.
> > 
> > Yeah, removing volatile can be a bit annoying.
> > 
> > For the particular case of the bitops, though, it's not an issue.
> > Since you know the type there, you can just cast it.
> > 
> > And if we had the rule that READ_ONCE() was an arithmetic type, you could do
> > 
> >     typeof(0+(*p)) __var;
> > 
> > since you might as well get the integer promotion anyway (on the
> > non-volatile result).
> > 
> > But that doesn't work with structures or unions, of course.
> 
> We do have a READ_ONCE on the following union in s390 code.
> 
> union ipte_control {
>         unsigned long val;
>         struct {
>                 unsigned long k  : 1;
>                 unsigned long kh : 31;
>                 unsigned long kg : 32;
>         }; 
> };
> 
> 
> In fact this one was the original failure case why we change ACCESS_ONCE.
> 
> see arch/s390/kvm/gaccess.c

Thanks. I think we should be ok just using the 'val' field instead of the
whole union but, then again, when bitfields are involved who knows what the
compiler might do. I thought we usually shied away from using them to mirror
hardware structures like this?

Will

  reply	other threads:[~2019-12-18 10:35 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-06 12:46 [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops) Michael Ellerman
2019-12-06 13:16 ` Peter Zijlstra
2019-12-10  5:38   ` Michael Ellerman
2019-12-10 10:15     ` Peter Zijlstra
2019-12-11  0:29       ` Michael Ellerman
2019-12-12  5:42   ` READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)) Michael Ellerman
2019-12-12  8:01     ` Peter Zijlstra
2019-12-12 10:07       ` Will Deacon
2019-12-12 10:46         ` Peter Zijlstra
2019-12-12 17:04           ` Will Deacon
2019-12-12 17:16             ` Will Deacon
2019-12-12 17:41           ` Linus Torvalds
2019-12-12 17:50             ` Segher Boessenkool
2019-12-12 18:06             ` Will Deacon
2019-12-12 18:29               ` Christian Borntraeger
2019-12-12 18:43               ` Linus Torvalds
2019-12-12 19:34                 ` Will Deacon
2019-12-12 20:21                   ` Peter Zijlstra
2019-12-12 20:53                     ` Peter Zijlstra
2019-12-13 10:47                       ` Luc Van Oostenryck
2019-12-13 12:56                         ` Peter Zijlstra
2019-12-13 14:28                           ` Luc Van Oostenryck
2019-12-12 20:49                   ` Linus Torvalds
2019-12-13 13:17                     ` Arnd Bergmann
2019-12-13 21:32                       ` Arnd Bergmann
2019-12-13 22:01                         ` Linus Torvalds
2019-12-16 10:28                       ` Will Deacon
2019-12-16 11:47                         ` Peter Zijlstra
2019-12-16 12:06                         ` Arnd Bergmann
2019-12-17 17:07                     ` Will Deacon
2019-12-17 18:04                       ` Linus Torvalds
2019-12-17 18:05                         ` Linus Torvalds
2019-12-17 18:31                           ` Will Deacon
2019-12-17 18:32                         ` Linus Torvalds
2019-12-18 12:17                           ` Michael Ellerman
2019-12-19 12:11                           ` Will Deacon
2019-12-18 10:22                     ` Christian Borntraeger
2019-12-18 10:35                       ` Will Deacon [this message]
2019-12-13 12:07           ` Michael Ellerman
2019-12-13 13:53             ` Segher Boessenkool
2019-12-13 21:06               ` Michael Ellerman
2019-12-12 15:10     ` Segher Boessenkool
2019-12-06 22:15 ` [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops) pr-tracker-bot

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