From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vincenzo Frascino Subject: Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt Date: Thu, 13 Jun 2019 15:03:12 +0100 Message-ID: <5963d144-be9b-78d8-9130-ef92bc66b1fd@arm.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <8e3c9537-de10-0d0d-f5bb-c33bde92443f@arm.com> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Szabolcs Nagy , Catalin Marinas Cc: "linux-arch@vger.kernel.org" , "linux-doc@vger.kernel.org" , Andrey Konovalov , Will Deacon , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Alexander Viro , "linux-kselftest@vger.kernel.org" , nd , "linux-arm-kernel@lists.infradead.org" List-Id: linux-arch.vger.kernel.org On 13/06/2019 13:28, Szabolcs Nagy wrote: > On 13/06/2019 12:16, Vincenzo Frascino wrote: >> Hi Szabolcs, >> >> thank you for your review. >> >> On 13/06/2019 11:14, Szabolcs Nagy wrote: >>> On 13/06/2019 10:20, Catalin Marinas wrote: >>>> Hi Szabolcs, >>>> >>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote: >>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote: >>>>>> +2. ARM64 Tagged Address ABI >>>>>> +--------------------------- >>>>>> + >>>>>> +From the kernel syscall interface prospective, we define, for the purposes >>>>> ^^^^^^^^^^^ >>>>> perspective >>>>> >>>>>> +of this document, a "valid tagged pointer" as a pointer that either it has >>>>>> +a zero value set in the top byte or it has a non-zero value, it is in memory >>>>>> +ranges privately owned by a userspace process and it is obtained in one of >>>>>> +the following ways: >>>>>> + - mmap() done by the process itself, where either: >>>>>> + * flags = MAP_PRIVATE | MAP_ANONYMOUS >>>>>> + * flags = MAP_PRIVATE and the file descriptor refers to a regular >>>>>> + file or "/dev/zero" >>>>> >>>>> this does not make it clear if MAP_FIXED or other flags are valid >>>>> (there are many map flags i don't know, but at least fixed should work >>>>> and stack/growsdown. i'd expect anything that's not incompatible with >>>>> private|anon to work). >>>> >>>> Just to clarify, this document tries to define the memory ranges from >>>> where tagged addresses can be passed into the kernel in the context >>>> of TBI only (not MTE); that is for hwasan support. FIXED or GROWSDOWN >>>> should not affect this. >>> >>> yes, so either the text should list MAP_* flags that don't affect >>> the pointer tagging semantics or specify private|anon mapping >>> with different wording. >>> >> >> Good point. Could you please propose a wording that would be suitable for this case? > > i don't know all the MAP_ magic, but i think it's enough to change > the "flags =" to > > * flags have MAP_PRIVATE and MAP_ANONYMOUS set or > * flags have MAP_PRIVATE set and the file descriptor refers to... > > Fine by me. I will add it the next iterations. >>>>>> + - 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? >> >> I will add what you are suggesting about the heap area. >> -- Regards, Vincenzo From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.110.172]:40650 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732396AbfFMODQ (ORCPT ); Thu, 13 Jun 2019 10:03:16 -0400 Subject: Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt 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> From: Vincenzo Frascino Message-ID: <5963d144-be9b-78d8-9130-ef92bc66b1fd@arm.com> Date: Thu, 13 Jun 2019 15:03:12 +0100 MIME-Version: 1.0 In-Reply-To: <8e3c9537-de10-0d0d-f5bb-c33bde92443f@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: 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 Message-ID: <20190613140312.5TEkJyVFLrteUUXmbmWJ7RV3s3GDXMRL0wT6VN_0YPI@z> On 13/06/2019 13:28, Szabolcs Nagy wrote: > On 13/06/2019 12:16, Vincenzo Frascino wrote: >> Hi Szabolcs, >> >> thank you for your review. >> >> On 13/06/2019 11:14, Szabolcs Nagy wrote: >>> On 13/06/2019 10:20, Catalin Marinas wrote: >>>> Hi Szabolcs, >>>> >>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote: >>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote: >>>>>> +2. ARM64 Tagged Address ABI >>>>>> +--------------------------- >>>>>> + >>>>>> +From the kernel syscall interface prospective, we define, for the purposes >>>>> ^^^^^^^^^^^ >>>>> perspective >>>>> >>>>>> +of this document, a "valid tagged pointer" as a pointer that either it has >>>>>> +a zero value set in the top byte or it has a non-zero value, it is in memory >>>>>> +ranges privately owned by a userspace process and it is obtained in one of >>>>>> +the following ways: >>>>>> + - mmap() done by the process itself, where either: >>>>>> + * flags = MAP_PRIVATE | MAP_ANONYMOUS >>>>>> + * flags = MAP_PRIVATE and the file descriptor refers to a regular >>>>>> + file or "/dev/zero" >>>>> >>>>> this does not make it clear if MAP_FIXED or other flags are valid >>>>> (there are many map flags i don't know, but at least fixed should work >>>>> and stack/growsdown. i'd expect anything that's not incompatible with >>>>> private|anon to work). >>>> >>>> Just to clarify, this document tries to define the memory ranges from >>>> where tagged addresses can be passed into the kernel in the context >>>> of TBI only (not MTE); that is for hwasan support. FIXED or GROWSDOWN >>>> should not affect this. >>> >>> yes, so either the text should list MAP_* flags that don't affect >>> the pointer tagging semantics or specify private|anon mapping >>> with different wording. >>> >> >> Good point. Could you please propose a wording that would be suitable for this case? > > i don't know all the MAP_ magic, but i think it's enough to change > the "flags =" to > > * flags have MAP_PRIVATE and MAP_ANONYMOUS set or > * flags have MAP_PRIVATE set and the file descriptor refers to... > > Fine by me. I will add it the next iterations. >>>>>> + - 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? >> >> I will add what you are suggesting about the heap area. >> -- Regards, Vincenzo