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Edgecombe" , Deepak Gupta , Ard Biesheuvel , "H.J. Lu" , Paul Walmsley , Palmer Dabbelt , Albert Ou , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org Subject: Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks Message-ID: References: <20230807-arm64-gcs-v4-0-68cfa37f9069@kernel.org> <20230807-arm64-gcs-v4-3-68cfa37f9069@kernel.org> <4e215e53-c7d1-4338-8df9-3f9bf783ced9@sirena.org.uk> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <4e215e53-c7d1-4338-8df9-3f9bf783ced9@sirena.org.uk> X-ClientProxiedBy: LO4P123CA0359.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:18d::22) To PAXPR08MB7172.eurprd08.prod.outlook.com (2603:10a6:102:20a::19) Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: PAXPR08MB7172:EE_|DU0PR08MB9251:EE_|DBAEUR03FT031:EE_|PA4PR08MB6095:EE_ X-MS-Office365-Filtering-Correlation-Id: cf114416-caa5-44f6-cea0-08db99a68788 x-checkrecipientrouted: true NoDisclaimer: true X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam-Untrusted: BCL:0; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Aug 2023 13:34:32.0567 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: cf114416-caa5-44f6-cea0-08db99a68788 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: DBAEUR03FT031.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PA4PR08MB6095 The 08/10/2023 12:41, Mark Brown wrote: > On Thu, Aug 10, 2023 at 09:55:50AM +0100, Szabolcs Nagy wrote: > > The 08/09/2023 16:34, Mark Brown wrote: > > > > It's actually based on bitrot that I'd initially chosen a smaller value > > > since it's likely that functions will push at least something as you > > > suggest, the patches now just use RLIMIT_STACK. I'll fix. > > > the pcs requires 16byte aligned stack frames, with 8byte per gcs entry > > there is no need for same gcs size as stack size in userspace. > > I agree that it's going to be excessive for pretty much all > applications, I adjusted it to match x86 as part of the general effort > to avoid divergence and because I was a bit concerned about non-PCS > cases (eg, JITed code) potentially running into trouble, especially with is that even possible? 16byte alignment is not a convention but architectural: access via unaligned sp traps (at least in userspace). it is possible to use bl such that the stack is not involved e.g. if there is no bl/ret pairing, but if we base the gcs size on the stack size then i'd expect one stack frame per bl/ret pair with 16byte alignment, or is there a programming model possible that uses 8byte stack per bl? > smaller stack limits. It's not an issue I have super strong opinions on > though, as you can see I had implemented it both ways at various times.