From: Arnd Bergmann <firstname.lastname@example.org> To: Will Deacon <email@example.com> Cc: Linus Torvalds <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Michael Ellerman <firstname.lastname@example.org>, Daniel Axtens <email@example.com>, Linux Kernel Mailing List <firstname.lastname@example.org>, linuxppc-dev <email@example.com>, Christophe Leroy <firstname.lastname@example.org>, linux-arch <email@example.com>, Mark Rutland <firstname.lastname@example.org>, Segher Boessenkool <email@example.com>, Christian Borntraeger <firstname.lastname@example.org> 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 <email@example.com> 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 > > <firstname.lastname@example.org> wrote: > > > On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <email@example.com> 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 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
next prev 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
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