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<20191213144359.GA3826@willie-the-truck> In-Reply-To: <20191213144359.GA3826@willie-the-truck> From: Arnd Bergmann Date: Mon, 16 Dec 2019 13:06:22 +0100 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)) To: Will Deacon Content-Type: text/plain; charset="UTF-8" X-Provags-ID: V03:K1:TSDQJJn4NG66d3QBWWRBaMDee1S38OtP8sBuHD9JgLCqJ+Hbt34 USfRMNLz2rabXnk/+Ns0lo1G50XrHYxPrFz0AoSKdYW23B0zsUPKhTQG55Le89VgUpUUsJl 73n/QGPYT4qeeeNH+OQ9aK86T9uAkM/NAyeXzi2I5owK1ZfNibHTj3UncnUpNb22oZ37hMg JBmZnf2Os8gkG8ZOB59aw== X-UI-Out-Filterresults: notjunk:1;V03:K0:iWkM2cbPCac=:RJuAOR6Av+94EkDGAE/r9j f5G2t8VkaWiqMJr4cR69P3z00Sq5xmvdUojD9Ae4m1OKTuhoRaiD0yMjZR9eAdB3WACosciPS Mz2mFD+IxmGX4sCT8KqCYSWKyfkW/eDoS6KgPA1O5RrArFcerveuOz/htw+Og9QBmbdYZXFz4 6+HXjRu+Z+AAxzU9lOQOVw97it+P+QO859Wv+P39OtltdAW44ZIZjotXevk09c7KN0O04eyR1 +7MjAdPlop/vpT4bMxeS7w6pD743oE0pBrKFkQ+3v0q/3Ly7cysqLaQRJUqkEVrnSfQdiVjoy yoflkz8ulHZkVXAyRY0xIQUMw/DIgDA7A9aUEFjof+T1OD7In4/hryrq0e18tlcObn6YPnXGB ELCMnPrpw9CdLPdgIQ6PqKwk1BZ2IEuDTLVpHxrO71yIxgwdynIOvnfYfR70VqER1iqJFvGYb 2F/92ZERuUo7wxY0C+rqCL0C599tUI8vnD6V6yAgU1Nyan/WSa1spD7VcexIWJ/jg1xKQdXBm FP5gZJm6AeF1+Xy7GupCDqrrsENAV4ewPAf33qkrRlEK5th5aN5zd9It8AQSmqwngRvJzL9SR ib74CXj1bN8x2LvlWQe02BiDPrKc01+nJLh0W12gACv+bN5gkjjeiEL/Q+3lp63G+K1E+iOF7 av9VE5NufbhxgKy6SPNv3BpWUfI/WtCzEonf1pDVW3FKh6TUz1i7ScbSdFG4I/GpYJXxWJ7Sg FyzP7TGRq58svX373aOiwUDuABx+BMxOK/IfDs6kTV4rSV54A7WJIb080ILs2i+oFuLOeKqBy uiy53wDm9115OjPcEUEaWsZRzV73vARY0ReWa5D6ak9qLmBdGC2XLQCDwbzDHCRcPeIFw2Ixr eNQtER6f0sH8Go6BTUWQ== X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch , Christian Borntraeger , Peter Zijlstra , Linux Kernel Mailing List , Linus Torvalds , Mark Rutland , linuxppc-dev , Daniel Axtens Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, Dec 16, 2019 at 11:28 AM Will Deacon 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 > > wrote: > > > On Thu, Dec 12, 2019 at 11:34 AM Will Deacon 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