From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls Date: Wed, 29 May 2019 16:18:40 +0100 Message-ID: <20190529151839.GF28398@e103592.cambridge.arm.com> References: <00eb4c63fefc054e2c8d626e8fedfca11d7c2600.1557160186.git.andreyknvl@google.com> <20190527143719.GA59948@MBP.local> <20190528145411.GA709@e119886-lin.cambridge.arm.com> <20190528154057.GD32006@arrakis.emea.arm.com> <20190528155644.GD28398@e103592.cambridge.arm.com> <20190528163400.GE32006@arrakis.emea.arm.com> <20190529124224.GE28398@e103592.cambridge.arm.com> <20190529132341.27t3knoxpb7t7y3g@mbp> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190529132341.27t3knoxpb7t7y3g@mbp> Sender: linux-kernel-owner@vger.kernel.org To: Catalin Marinas Cc: Mark Rutland , kvm@vger.kernel.org, Christian Koenig , Szabolcs Nagy , Will Deacon , dri-devel@lists.freedesktop.org, linux-mm@kvack.org, Lee Smith , linux-kselftest@vger.kernel.org, Vincenzo Frascino , Jacob Bramley , Leon Romanovsky , linux-rdma@vger.kernel.org, amd-gfx@lists.freedesktop.org, linux-arm-kernel@lists.infradead.org, Evgeniy Stepanov , linux-media@vger.kernel.org, Kees Cook , Ruben Ayrapetyan , Andrey Konovalov , Kevin Brodsky , Alex Williamson List-Id: linux-rdma@vger.kernel.org On Wed, May 29, 2019 at 02:23:42PM +0100, Catalin Marinas wrote: > On Wed, May 29, 2019 at 01:42:25PM +0100, Dave P Martin wrote: > > On Tue, May 28, 2019 at 05:34:00PM +0100, Catalin Marinas wrote: > > > On Tue, May 28, 2019 at 04:56:45PM +0100, Dave P Martin wrote: > > > > On Tue, May 28, 2019 at 04:40:58PM +0100, Catalin Marinas wrote: > > > > > > > > [...] > > > > > > > > > My thoughts on allowing tags (quick look): > > > > > > > > > > brk - no > > > > > > > > [...] > > > > > > > > > mlock, mlock2, munlock - yes > > > > > mmap - no (we may change this with MTE but not for TBI) > > > > > > > > [...] > > > > > > > > > mprotect - yes > > > > > > > > I haven't following this discussion closely... what's the rationale for > > > > the inconsistencies here (feel free to refer me back to the discussion > > > > if it's elsewhere). > > > > > > _My_ rationale (feel free to disagree) is that mmap() by default would > > > not return a tagged address (ignoring MTE for now). If it gets passed a > > > tagged address or a "tagged NULL" (for lack of a better name) we don't > > > have clear semantics of whether the returned address should be tagged in > > > this ABI relaxation. I'd rather reserve this specific behaviour if we > > > overload the non-zero tag meaning of mmap() for MTE. Similar reasoning > > > for mremap(), at least on the new_address argument (not entirely sure > > > about old_address). > > > > > > munmap() should probably follow the mmap() rules. > > > > > > As for brk(), I don't see why the user would need to pass a tagged > > > address, we can't associate any meaning to this tag. > > > > > > For the rest, since it's likely such addresses would have been tagged by > > > malloc() in user space, we should allow tagged pointers. > > > > Those arguments seem reasonable. We should try to capture this > > somewhere when documenting the ABI. > > > > To be clear, I'm not sure that we should guarantee anywhere that a > > tagged pointer is rejected: rather the behaviour should probably be > > left unspecified. Then we can tidy it up incrementally. > > > > (The behaviour is unspecified today, in any case.) > > What is specified (or rather de-facto ABI) today is that passing a user > address above TASK_SIZE (e.g. non-zero top byte) would fail in most > cases. If we relax this with the TBI we may end up with some de-facto I may be being too picky, but "would fail in most cases" sounds like "unspecified" ? > ABI before we actually get MTE hardware. Tightening it afterwards may be > slightly more problematic, although MTE needs to be an explicit opt-in. > > IOW, I wouldn't want to unnecessarily relax the ABI if we don't need to. So long we don't block foreseeable future developments unnecessarily either -- I agree there's a balance to be struck. I guess this can be reviewed when we have nailed down the details a bit further. Cheers ---Dave