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From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	dja@axtens.net, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, christophe.leroy@c-s.fr,
	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))
Date: Thu, 12 Dec 2019 17:16:31 +0000	[thread overview]
Message-ID: <20191212171630.GC16364@willie-the-truck> (raw)
In-Reply-To: <20191212170427.GA16364@willie-the-truck>

On Thu, Dec 12, 2019 at 05:04:27PM +0000, Will Deacon wrote:
> On Thu, Dec 12, 2019 at 11:46:10AM +0100, Peter Zijlstra wrote:
> > On Thu, Dec 12, 2019 at 10:07:56AM +0000, Will Deacon wrote:
> > 
> > > > So your proposed change _should_ be fine. Will, I'm assuming you never
> > > > saw this on your ARGH64 builds when you did this code ?
> > > 
> > > I did see it, but (a) looking at the code out-of-line makes it look a lot
> > > worse than it actually is (so the ext4 example is really helpful -- thanks
> > > Michael!) and (b) I chalked it up to a crappy compiler.
> > > 
> > > However, see this comment from Arnd on my READ_ONCE series from the other
> > > day:
> > > 
> > > https://lore.kernel.org/lkml/CAK8P3a0f=WvSQSBQ4t0FmEkcFE_mC3oARxaeTviTSkSa-D2qhg@mail.gmail.com
> > > 
> > > In which case, I'm thinking that we should be doing better in READ_ONCE()
> > > for non-buggy compilers which would also keep the KCSAN folks happy for this
> > > code (and would help with [1] too).
> > 
> > So something like this then? Although I suppose that should be moved
> > into compiler-gcc.h and then guarded by #ifndef READ_ONCE or so.
> 
> Ah wait, I think we've been looking at this wrong. The volatile pointer
> argument is actually the problem here, not READ_ONCE()! The use of typeof()
> means that the temporary variable to which __READ_ONCE_SIZE writes ends up
> being a volatile store, so it can't be optimised away. This is why we get
> a stack access and why stack protector then wrecks the codegen for us.

Hmm, it's actually probably the volatile read which is causing the problem,
since __READ_ONCE_SIZE has casted that away and just uses "void *", but you
get the idea.

Will

  reply	other threads:[~2019-12-12 17:16 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 [this message]
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
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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