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Wed, 23 Sep 2020 09:10:18 +0000 Date: Wed, 23 Sep 2020 10:10:08 +0100 From: Szabolcs Nagy To: Catalin Marinas Cc: Will Deacon , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Dave P Martin , Vincenzo Frascino , Kevin Brodsky , Andrey Konovalov , Peter Collingbourne , Andrew Morton , libc-alpha@sourceware.org Subject: Re: [PATCH v9 29/29] arm64: mte: Add Memory Tagging Extension documentation Message-ID: <20200923091008.GC16385@arm.com> References: <20200904103029.32083-1-catalin.marinas@arm.com> <20200904103029.32083-30-catalin.marinas@arm.com> <20200917081107.GA29031@willie-the-truck> <20200917090229.GA10662@gaia> <20200922155248.GA16385@arm.com> <20200922165529.GH15643@gaia> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200922165529.GH15643@gaia> User-Agent: Mutt/1.9.4 (2018-02-28) X-ClientProxiedBy: SN4PR0401CA0026.namprd04.prod.outlook.com (2603:10b6:803:2a::12) To VE1PR08MB5566.eurprd08.prod.outlook.com (2603:10a6:800:1a9::7) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from arm.com (217.140.106.54) by SN4PR0401CA0026.namprd04.prod.outlook.com (2603:10b6:803:2a::12) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3391.19 via Frontend Transport; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Sep 2020 09:10:26.8967 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0fb1d07e-2065-472e-7e3b-08d85fa08305 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: VE1EUR03FT060.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM6PR08MB4674 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: The 09/22/2020 17:55, Catalin Marinas wrote: > On Tue, Sep 22, 2020 at 04:52:49PM +0100, Szabolcs Nagy wrote: > > if we add a kernel level opt-in mechanism for tag checks later (e.g. > > elf marking) or if the settings are exclusively owned by early libc > > code then i think the proposed abi is ok (this is our current > > agreement and works as long as no late runtime change is needed to the > > settings). > > In the Android case, run-time changes to the tag checking mode I think > are expected (usually via signal handlers), though per-thread. ok that works, but does not help allocators or runtimes that don't own the signal handlers. > > i'm now wondering about the default tag check mode: it may be better > > to enable sync tag checks in the kernel. it's not clear to me what > > would break with that. this is probably late to discuss now and libc > > would need ways to override the default no matter what, but i'd like > > to know if somebody sees problems or risks with unconditional sync tag > > checks turned on (sorry i don't remember if we went through this > > before). i assume it would have no effect on a process that never uses > > PROT_MTE. > > I don't think it helps much. We already have a requirement that to be > able to pass tagged pointers to kernel syscalls, the user needs a > prctl(PR_TAGGED_ADDR_ENABLE) call (code already in mainline). Using > PROT_MTE without tagged pointers won't be of much use. So if we are to > set different tag check defaults, we should also enable the tagged addr > ABI automatically. > > That said, I still have a preference for MTE and tagged addr ABI to be > explicitly requested by the (human) user either via environment > variables or marked in an ELF note as "safe with/using tags". Given the > recent mremap() issue we caused in glibc, I'm worried that other things > may break with enabling the tagged addr ABI everywhere. > > Another aspect is that sync mode by default in a distro where glibc is > MTE-aware will lead to performance regressions. That's another case in > favour of the user explicitly asking for tag checking. ok this all makes sense to me. > > Anyway, I'm open to having a debate on changing the defaults. > > -- > Catalin