linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Arnd Bergmann <arnd@arndb.de>
To: Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	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>,
	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: Mon, 16 Dec 2019 13:06:22 +0100	[thread overview]
Message-ID: <CAK8P3a0qgbqKTtRWTh6c0R2E93rehKbkBcB18TL3JX_RWHsTZA@mail.gmail.com> (raw)
In-Reply-To: <20191213144359.GA3826@willie-the-truck>

On Mon, Dec 16, 2019 at 11:28 AM Will Deacon <will@kernel.org> wrote:
> On Fri, Dec 13, 2019 at 02:17:08PM +0100, Arnd Bergmann wrote:
> > On Thu, Dec 12, 2019 at 9:50 PM Linus Torvalds
> > <torvalds@linux-foundation.org> 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.
> > >
> > > I'm not entirely sure we have READ_ONCE() with a struct. I do know we
> > > have it with 64-bit entities on 32-bit machines, but that's ok with
> > > the "0+" trick.
> >
> > I'll have my randconfig builder look for instances, so far I found one,
> > see below. My feeling is that it would be better to enforce at least
> > the size being a 1/2/4/8, to avoid cases where someone thinks
> > the access is atomic, but it falls back on a memcpy.
>
> I've been using something similar built on compiletime_assert_atomic_type()
> and I spotted another instance in the xdp code (xskq_validate_desc()) which
> tries to READ_ONCE() on a 128-bit descriptor, although a /very/ quick read
> of the code suggests that this probably can't be concurrently modified if
> the ring indexes are synchronised properly.

That's the only other one I found. I have not checked how many are structs
that are the size of a normal u8/u16/u32/u64, or if there are types that
have a lower alignment than there size, such as a __u16[2] that might
span a page boundary.

> However, enabling this for 32-bit ARM is total carnage; as Linus mentioned,
> a whole bunch of code appears to be relying on atomic 64-bit access of
> READ_ONCE(); the perf ring buffer, io_uring, the scheduler, pm_runtime,
> cpuidle, ... :(
>
> Unfortunately, at least some of these *do* look like bugs, but I can't see
> how we can fix them, not least because the first two are user ABI afaict. It
> may also be that in practice we get 2x32-bit stores, and that works out fine
> when storing a 32-bit virtual address. I'm not sure what (if anything) the
> compiler guarantees in these cases.

Would it help if 32-bit architectures use atomic64_read() and atomic64_set()
to implement a 64-bit READ_ONCE()/WRITE_ONCE(), or would that make it
worse in other ways?

On mips32, riscv32 and some minor 32-bit architectures with SMP support
(xtensa,  csky, hexagon, openrisc, parisc32, sparc32 and ppc32 AFAICT) this
ends up using a spinlock for GENERIC_ATOMIC64, but at least ARMv6+,
i586+ and most ARC should be fine.

(Side note: the ARMv7 implementation is suboptimimal for ARMv7VE+
if LPAE is disabled, I think we need to really add Kconfig options for
ARMv7VE and 32-bit ARMv8 improve this and things like integer divide).

      Arnd

  parent reply	other threads:[~2019-12-16 12:06 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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAK8P3a0qgbqKTtRWTh6c0R2E93rehKbkBcB18TL3JX_RWHsTZA@mail.gmail.com \
    --to=arnd@arndb.de \
    --cc=borntraeger@de.ibm.com \
    --cc=christophe.leroy@c-s.fr \
    --cc=dja@axtens.net \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=mark.rutland@arm.com \
    --cc=mpe@ellerman.id.au \
    --cc=peterz@infradead.org \
    --cc=segher@kernel.crashing.org \
    --cc=torvalds@linux-foundation.org \
    --cc=will@kernel.org \
    --subject='Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).