From: Dave Martin <Dave.Martin@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Jason Wang <jasowang@redhat.com>,
virtualization@lists.linux-foundation.org,
Arnd Bergmann <arnd@arndb.de>,
Alan Stern <stern@rowland.harvard.edu>,
Sami Tolvanen <samitolvanen@google.com>,
Matt Turner <mattst88@gmail.com>,
kernel-team@android.com, Marco Elver <elver@google.com>,
Kees Cook <keescook@chromium.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
Josh Triplett <josh@joshtriplett.org>,
Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
linux-arm-kernel@lists.infradead.org,
Richard Henderson <rth@twiddle.net>,
Nick Desaulniers <ndesaulniers@google.com>,
linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y
Date: Mon, 6 Jul 2020 17:00:23 +0100 [thread overview]
Message-ID: <20200706160023.GB10992@arm.com> (raw)
In-Reply-To: <20200702072301.GA15963@willie-the-truck>
On Thu, Jul 02, 2020 at 08:23:02AM +0100, Will Deacon wrote:
> On Wed, Jul 01, 2020 at 06:07:25PM +0100, Dave P Martin wrote:
> > On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote:
> > > When building with LTO, there is an increased risk of the compiler
> > > converting an address dependency headed by a READ_ONCE() invocation
> > > into a control dependency and consequently allowing for harmful
> > > reordering by the CPU.
> > >
> > > Ensure that such transformations are harmless by overriding the generic
> > > READ_ONCE() definition with one that provides acquire semantics when
> > > building with LTO.
> > >
> > > Signed-off-by: Will Deacon <will@kernel.org>
> > > ---
> > > arch/arm64/include/asm/rwonce.h | 63 +++++++++++++++++++++++++++++++
> > > arch/arm64/kernel/vdso/Makefile | 2 +-
> > > arch/arm64/kernel/vdso32/Makefile | 2 +-
> > > 3 files changed, 65 insertions(+), 2 deletions(-)
> > > create mode 100644 arch/arm64/include/asm/rwonce.h
> > >
> > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> > > new file mode 100644
> > > index 000000000000..515e360b01a1
> > > --- /dev/null
> > > +++ b/arch/arm64/include/asm/rwonce.h
> > > @@ -0,0 +1,63 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +/*
> > > + * Copyright (C) 2020 Google LLC.
> > > + */
> > > +#ifndef __ASM_RWONCE_H
> > > +#define __ASM_RWONCE_H
> > > +
> > > +#ifdef CONFIG_CLANG_LTO
> >
> > Don't we have a generic option for LTO that's not specific to Clang.
>
> /me looks at the LTO series some more
>
> Oh yeah, there's CONFIG_LTO which is selected by CONFIG_LTO_CLANG, which is
> the non-typoed version of the above. I can switch this to CONFIG_LTO.
>
> > Also, can you illustrate code that can only be unsafe with Clang LTO?
>
> I don't have a concrete example, but it's an ongoing concern over on the LTO
> thread [1], so I cooked this to show one way we could deal with it. The main
> concern is that the whole-program optimisations enabled by LTO may allow the
> compiler to enumerate possible values for a pointer at link time and replace
> an address dependency between two loads with a control dependency instead,
> defeating the dependency ordering within the CPU.
Why can't that happen without LTO?
> We likely won't realise if/when this goes wrong, other than impossible to
> debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be
> able to detect this sort of thing happening at build time, and perhaps
> even prevent it with compiler options or annotations, but none of that is
> close to being available and I'm keen to progress the LTO patches in the
> meantime because they are a requirement for CFI.
My concern was not so much why LTO makes things dangerous, as why !LTO
makes things safe...
Cheers
---Dave
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next prev parent reply other threads:[~2020-07-06 16:01 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-30 17:37 [PATCH 00/18] Allow architectures to override __READ_ONCE() Will Deacon
2020-06-30 17:37 ` [PATCH 01/18] tools: bpf: Use local copy of headers including uapi/linux/filter.h Will Deacon
2020-07-01 16:38 ` Alexei Starovoitov
2020-06-30 17:37 ` [PATCH 02/18] compiler.h: Split {READ, WRITE}_ONCE definitions out into rwonce.h Will Deacon
2020-06-30 19:11 ` Arnd Bergmann
2020-07-01 10:16 ` [PATCH 02/18] compiler.h: Split {READ,WRITE}_ONCE " Will Deacon
2020-07-01 11:33 ` [PATCH 02/18] compiler.h: Split {READ, WRITE}_ONCE " Arnd Bergmann
2020-06-30 17:37 ` [PATCH 03/18] asm/rwonce: Allow __READ_ONCE to be overridden by the architecture Will Deacon
2020-06-30 17:37 ` [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation Will Deacon
2020-07-02 9:32 ` Mark Rutland
2020-07-02 9:48 ` Will Deacon
2020-07-02 10:08 ` Arnd Bergmann
2020-07-02 11:18 ` Will Deacon
2020-07-02 11:39 ` Arnd Bergmann
2020-07-02 14:43 ` Joel Fernandes
2020-07-02 14:55 ` Will Deacon
2020-07-02 15:07 ` Joel Fernandes
2020-06-30 17:37 ` [PATCH 05/18] asm/rwonce: Remove smp_read_barrier_depends() invocation Will Deacon
2020-06-30 17:37 ` [PATCH 06/18] vhost: Remove redundant use of read_barrier_depends() barrier Will Deacon
2020-06-30 17:37 ` [PATCH 07/18] alpha: Replace smp_read_barrier_depends() usage with smp_[r]mb() Will Deacon
2020-06-30 17:37 ` [PATCH 08/18] locking/barriers: Remove definitions for [smp_]read_barrier_depends() Will Deacon
2020-06-30 17:37 ` [PATCH 09/18] Documentation/barriers: Remove references to [smp_]read_barrier_depends() Will Deacon
2020-06-30 17:37 ` [PATCH 10/18] Documentation/barriers/kokr: " Will Deacon
2020-06-30 17:37 ` [PATCH 11/18] tools/memory-model: Remove smp_read_barrier_depends() from informal doc Will Deacon
2020-06-30 17:37 ` [PATCH 12/18] include/linux: Remove smp_read_barrier_depends() from comments Will Deacon
2020-06-30 17:37 ` [PATCH 13/18] checkpatch: Remove checks relating to [smp_]read_barrier_depends() Will Deacon
2020-06-30 17:37 ` [PATCH 14/18] arm64: Reduce the number of header files pulled into vmlinux.lds.S Will Deacon
2020-06-30 17:37 ` [PATCH 15/18] arm64: alternatives: Split up alternative.h Will Deacon
2020-06-30 17:37 ` [PATCH 16/18] arm64: cpufeatures: Add capability for LDAPR instruction Will Deacon
2020-06-30 17:37 ` [PATCH 17/18] arm64: alternatives: Remove READ_ONCE() usage during patch operation Will Deacon
2020-06-30 17:37 ` [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Will Deacon
2020-06-30 19:25 ` Arnd Bergmann
2020-07-01 10:19 ` Will Deacon
2020-07-01 10:59 ` Arnd Bergmann
2020-06-30 19:47 ` Marco Elver
2020-06-30 20:20 ` Peter Zijlstra
2020-06-30 22:57 ` Sami Tolvanen
2020-07-01 10:25 ` Will Deacon
2020-07-01 10:24 ` Will Deacon
2020-07-01 17:07 ` Dave P Martin
2020-07-02 7:23 ` Will Deacon
2020-07-06 16:00 ` Dave Martin [this message]
2020-07-06 16:34 ` Paul E. McKenney
2020-07-06 17:05 ` Dave Martin
2020-07-06 17:36 ` Paul E. McKenney
2020-07-07 10:29 ` Dave Martin
2020-07-07 22:51 ` Paul E. McKenney
2020-07-07 23:01 ` Nick Desaulniers
2020-07-08 7:15 ` Marco Elver
2020-07-08 9:16 ` Peter Zijlstra
2020-07-08 18:20 ` Paul E. McKenney
2020-07-06 18:35 ` Will Deacon
2020-07-06 19:23 ` Marco Elver
2020-07-06 19:42 ` Paul E. McKenney
2020-07-06 16:08 ` Dave Martin
2020-07-06 18:35 ` Will Deacon
2020-07-07 10:10 ` Dave Martin
2020-07-01 7:38 ` [PATCH 00/18] Allow architectures to override __READ_ONCE() Josh Triplett
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