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[217.140.110.172]) by mx.google.com with ESMTP id l13si2008619ejb.383.2019.06.21.08.16.45 for ; Fri, 21 Jun 2019 08:16:45 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of catalin.marinas@arm.com designates 217.140.110.172 as permitted sender) client-ip=217.140.110.172; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of catalin.marinas@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=catalin.marinas@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C4CF5344; Fri, 21 Jun 2019 08:16:44 -0700 (PDT) Received: from arrakis.emea.arm.com (arrakis.cambridge.arm.com [10.1.196.78]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 558C63F575; Fri, 21 Jun 2019 08:16:43 -0700 (PDT) Date: Fri, 21 Jun 2019 16:16:41 +0100 From: Catalin Marinas To: Kevin Brodsky Cc: Vincenzo Frascino , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Will Deacon , Andrey Konovalov , Alexander Viro Subject: Re: [PATCH v5 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt Message-ID: <20190621151640.GI18954@arrakis.emea.arm.com> References: <20190613155137.47675-1-vincenzo.frascino@arm.com> <20190613155137.47675-2-vincenzo.frascino@arm.com> <1c55a610-9aa5-4675-f7de-79a1661a660d@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1c55a610-9aa5-4675-f7de-79a1661a660d@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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: On Tue, Jun 18, 2019 at 02:13:01PM +0100, Kevin Brodsky wrote: > On 13/06/2019 16:51, Vincenzo Frascino wrote: > > +The ARM64 Tagged Address ABI is an opt-in feature, and an application can > > +control it using the following: > > + - /proc/sys/abi/tagged_addr: a new sysctl interface that can be used to > > + prevent the applications from enabling the relaxed ABI. > > + The sysctl is meant also for testing purposes in order to provide a > > + simple way for the userspace to verify the return error checking of > > + the prctl() commands without having to reconfigure the kernel. > > + The sysctl supports the following configuration options: > > + - 0: Disable ARM64 Tagged Address ABI for all the applications. > > + - 1 (Default): Enable ARM64 Tagged Address ABI for all the > > + applications. > > I find this very confusing, because it suggests that the default value of > PR_GET_TAGGED_ADDR_CTRL for new processes will be set to the value of this > sysctl, when in fact this sysctl is about restricting the *availability* of > the new ABI. Instead of disabling the ABI, I would talk about disabling > access to the new ABI here. This bullet point needs to be re-written. The sysctl is meant to disable opting in to the ABI. I'd also drop the "meant for testing" part. I put it in my commit log as justification but I don't think it should be part of the ABI document. > > + - prctl()s: > > + - PR_SET_TAGGED_ADDR_CTRL: can be used to enable or disable the Tagged > > + Address ABI. > > + The (unsigned int) arg2 argument is a bit mask describing the > > + control mode used: > > + - PR_TAGGED_ADDR_ENABLE: Enable ARM64 Tagged Address ABI. > > + The arguments arg3, arg4, and arg5 are ignored. > > Have we definitely decided that arg{3,4,5} are ignored? Catalin? I don't have a strong preference either way. If it's simpler for the user to ignore them, fine by me. I can see in the current prctl commands a mix if ignore vs forced zero. > > +the ABI guarantees the following behaviours: > > + > > + - Every current or newly introduced syscall can accept any valid tagged > > + pointers. > "pointer". Also, is it really useful to talk about newly introduced syscall? > New from which point of view? I think we should drop this guarantee. It would have made sense if we allowed tagged pointers everywhere but we already have some exceptions. > > +3. ARM64 Tagged Address ABI Exceptions > > +-------------------------------------- > > + > > +The behaviours described in section 2, with particular reference to the > > +acceptance by the syscalls of any valid tagged pointer are not applicable > > +to the following cases: > > + - mmap() addr parameter. > > + - mremap() new_address parameter. > > + - prctl_set_mm() struct prctl_map fields. > > + - prctl_set_mm_map() struct prctl_map fields. > > prctl_set_mm() and prctl_set_mm_map() are internal kernel functions, not > syscall names. IIUC, we don't want to allow any address field settable via > the PR_SET_MM prctl() to be tagged. Catalin, is that correct? I think this > needs rephrasing. I fully agree. It should talk about PR_SET_MM, PR_SET_MM_MAP, PR_SET_MM_MAP_SIZE. -- Catalin