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Thu, 10 Aug 2023 13:34:18 +0000 Date: Thu, 10 Aug 2023 14:34:04 +0100 From: Szabolcs Nagy To: Mark Brown Cc: Catalin Marinas , Will Deacon , Jonathan Corbet , Andrew Morton , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Arnd Bergmann , Oleg Nesterov , Eric Biederman , Kees Cook , Shuah Khan , "Rick P. 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) MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: PAXPR08MB7172:EE_|DU0PR08MB9251:EE_|AM7EUR03FT040:EE_|AS2PR08MB10109:EE_ X-MS-Office365-Filtering-Correlation-Id: b38cd82d-58fd-4cf6-d227-08db99a6879f 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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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.