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[217.140.110.172]) by mx.google.com with ESMTP id y47si2640094edb.94.2019.06.13.08.35.38 for ; Thu, 13 Jun 2019 08:35:39 -0700 (PDT) Received-SPF: pass (google.com: domain of vincenzo.frascino@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: domain of vincenzo.frascino@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=vincenzo.frascino@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 42C42A78; Thu, 13 Jun 2019 08:35:38 -0700 (PDT) Received: from [10.1.196.72] (e119884-lin.cambridge.arm.com [10.1.196.72]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 458113F718; Thu, 13 Jun 2019 08:35:36 -0700 (PDT) Subject: Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt To: Szabolcs Nagy , Catalin Marinas Cc: nd , "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 References: <20190612142111.28161-1-vincenzo.frascino@arm.com> <20190612142111.28161-2-vincenzo.frascino@arm.com> <20190613092054.GO28951@C02TF0J2HF1T.local> <6ebbda37-5dd9-d0d5-d9cb-286c7a5b7f8e@arm.com> <8e3c9537-de10-0d0d-f5bb-c33bde92443f@arm.com> <5963d144-be9b-78d8-9130-ef92bc66b1fd@arm.com> From: Vincenzo Frascino Message-ID: Date: Thu, 13 Jun 2019 16:35:35 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 13/06/2019 16:32, Szabolcs Nagy wrote: > On 13/06/2019 15:03, Vincenzo Frascino wrote: >> On 13/06/2019 13:28, Szabolcs Nagy wrote: >>> On 13/06/2019 12:16, Vincenzo Frascino wrote: >>>> On 13/06/2019 11:14, Szabolcs Nagy wrote: >>>>> On 13/06/2019 10:20, Catalin Marinas wrote: >>>>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote: >>>>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote: >>>>>>>> + - a mapping below sbrk(0) done by the process itself >>>>>>> >>>>>>> doesn't the mmap rule cover this? >>>>>> >>>>>> IIUC it doesn't cover it as that's memory mapped by the kernel >>>>>> automatically on access vs a pointer returned by mmap(). The statement >>>>>> above talks about how the address is obtained by the user. >>>>> >>>>> ok i read 'mapping below sbrk' as an mmap (possibly MAP_FIXED) >>>>> that happens to be below the heap area. >>>>> >>>>> i think "below sbrk(0)" is not the best term to use: there >>>>> may be address range below the heap area that can be mmapped >>>>> and thus below sbrk(0) and sbrk is a posix api not a linux >>>>> syscall, the libc can implement it with mmap or whatever. >>>>> >>>>> i'm not sure what the right term for 'heap area' is >>>>> (the address range between syscall(__NR_brk,0) at >>>>> program startup and its current value?) >>>>> >>>> >>>> I used sbrk(0) with the meaning of "end of the process's data segment" not >>>> implying that this is a syscall, but just as a useful way to identify the mapping. >>>> I agree that it is a posix function implemented by libc but when it is used with >>>> 0 finds the current location of the program break, which can be changed by brk() >>>> and depending on the new address passed to this syscall can have the effect of >>>> allocating or deallocating memory. >>>> >>>> Will changing sbrk(0) with "end of the process's data segment" make it more clear? >>> >>> i don't understand what's the relevance of the *end* >>> of the data segment. >>> >>> i'd expect the text to say something about the address >>> range of the data segment. >>> >>> i can do >>> >>> mmap((void*)65536, 65536, PROT_READ|PROT_WRITE, MAP_FIXED|MAP_SHARED|MAP_ANON, -1, 0); >>> >>> and it will be below the end of the data segment. >>> >> >> As far as I understand the data segment "lives" below the program break, hence >> it is a way of describing the range from which the user can obtain a valid >> tagged pointer.> >> Said that, I am not really sure on how do you want me to document this (my aim >> is for this to be clear to the userspace developers). Could you please propose >> something? > > [...], it is in the memory ranges privately owned by a > userspace process and it is obtained in one of the > following ways: > > - mmap done by the process itself, [...] > > - brk syscall done by the process itself. > (i.e. the heap area between the initial location > of the program break at process creation and its > current location.) > > - any memory mapped by the kernel [...] > > the data segment that's part of the process image is > already covered by the last point. > Thanks Szabolcs, I will update the document accordingly. -- Regards, Vincenzo