From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 94D5BC3A59D for ; Thu, 22 Aug 2019 09:38:45 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 50BD121655 for ; Thu, 22 Aug 2019 09:38:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 50BD121655 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 01A616B02EF; Thu, 22 Aug 2019 05:38:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id F0DC96B02F0; Thu, 22 Aug 2019 05:38:43 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DFC0A6B02F1; Thu, 22 Aug 2019 05:38:43 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0249.hostedemail.com [216.40.44.249]) by kanga.kvack.org (Postfix) with ESMTP id BE5476B02EF for ; Thu, 22 Aug 2019 05:38:43 -0400 (EDT) Received: from smtpin16.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id 8168E1F848 for ; Thu, 22 Aug 2019 09:38:43 +0000 (UTC) X-FDA: 75849564126.16.sea79_6924f42a25834 X-HE-Tag: sea79_6924f42a25834 X-Filterd-Recvd-Size: 9387 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf32.hostedemail.com (Postfix) with ESMTP for ; Thu, 22 Aug 2019 09:38:42 +0000 (UTC) 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 6B42A1596; Thu, 22 Aug 2019 02:38:41 -0700 (PDT) Received: from [10.1.194.48] (e123572-lin.cambridge.arm.com [10.1.194.48]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CBCE93F246; Thu, 22 Aug 2019 02:38:39 -0700 (PDT) Subject: Re: [PATCH v9 2/3] arm64: Define Documentation/arm64/tagged-address-abi.rst To: Andrey Konovalov , Catalin Marinas Cc: Linux ARM , Linux Memory Management List , Andrew Morton , Vincenzo Frascino , Will Deacon , Szabolcs Nagy , Dave P Martin , Dave Hansen , "open list:DOCUMENTATION" , linux-arch , Will Deacon References: <20190821164730.47450-1-catalin.marinas@arm.com> <20190821164730.47450-3-catalin.marinas@arm.com> From: Kevin Brodsky Message-ID: Date: Thu, 22 Aug 2019 10:38:37 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-GB 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 21/08/2019 17:57, Andrey Konovalov wrote: > On Wed, Aug 21, 2019 at 6:47 PM Catalin Marinas wrote: >> From: Vincenzo Frascino >> >> On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace >> (EL0) to perform memory accesses through 64-bit pointers with a non-zero >> top byte. Introduce the document describing the relaxation of the >> syscall ABI that allows userspace to pass certain tagged pointers to >> kernel syscalls. >> >> Cc: Will Deacon >> Cc: Andrey Konovalov >> Cc: Szabolcs Nagy >> Cc: Kevin Brodsky >> Signed-off-by: Vincenzo Frascino >> Co-developed-by: Catalin Marinas >> Signed-off-by: Catalin Marinas > Acked-by: Andrey Konovalov Acked-by: Kevin Brodsky >> --- >> Documentation/arm64/tagged-address-abi.rst | 156 +++++++++++++++++++++ >> 1 file changed, 156 insertions(+) >> create mode 100644 Documentation/arm64/tagged-address-abi.rst >> >> diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst >> new file mode 100644 >> index 000000000000..d4a85d535bf9 >> --- /dev/null >> +++ b/Documentation/arm64/tagged-address-abi.rst >> @@ -0,0 +1,156 @@ >> +========================== >> +AArch64 TAGGED ADDRESS ABI >> +========================== >> + >> +Authors: Vincenzo Frascino >> + Catalin Marinas >> + >> +Date: 21 August 2019 >> + >> +This document describes the usage and semantics of the Tagged Address >> +ABI on AArch64 Linux. >> + >> +1. Introduction >> +--------------- >> + >> +On AArch64 the ``TCR_EL1.TBI0`` bit is set by default, allowing >> +userspace (EL0) to perform memory accesses through 64-bit pointers with >> +a non-zero top byte. This document describes the relaxation of the >> +syscall ABI that allows userspace to pass certain tagged pointers to >> +kernel syscalls. >> + >> +2. AArch64 Tagged Address ABI >> +----------------------------- >> + >> +From the kernel syscall interface perspective and for the purposes of >> +this document, a "valid tagged pointer" is a pointer with a potentially >> +non-zero top-byte that references an address in the user process address >> +space obtained in one of the following ways: >> + >> +- ``mmap()`` syscall where either: >> + >> + - flags have the ``MAP_ANONYMOUS`` bit set or >> + - the file descriptor refers to a regular file (including those >> + returned by ``memfd_create()``) or ``/dev/zero`` >> + >> +- ``brk()`` syscall (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 in the address space of the process >> + during creation and with the same restrictions as for ``mmap()`` above >> + (e.g. data, bss, stack). >> + >> +The AArch64 Tagged Address ABI has two stages of relaxation depending >> +how the user addresses are used by the kernel: >> + >> +1. User addresses not accessed by the kernel but used for address space >> + management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use >> + of valid tagged pointers in this context is always allowed. >> + >> +2. User addresses accessed by the kernel (e.g. ``write()``). This ABI >> + relaxation is disabled by default and the application thread needs to >> + explicitly enable it via ``prctl()`` as follows: >> + >> + - ``PR_SET_TAGGED_ADDR_CTRL``: enable or disable the AArch64 Tagged >> + Address ABI for the calling thread. >> + >> + The ``(unsigned int) arg2`` argument is a bit mask describing the >> + control mode used: >> + >> + - ``PR_TAGGED_ADDR_ENABLE``: enable AArch64 Tagged Address ABI. >> + Default status is disabled. >> + >> + Arguments ``arg3``, ``arg4``, and ``arg5`` must be 0. >> + >> + - ``PR_GET_TAGGED_ADDR_CTRL``: get the status of the AArch64 Tagged >> + Address ABI for the calling thread. >> + >> + Arguments ``arg2``, ``arg3``, ``arg4``, and ``arg5`` must be 0. >> + >> + The ABI properties described above are thread-scoped, inherited on >> + clone() and fork() and cleared on exec(). >> + >> + Calling ``prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)`` >> + returns ``-EINVAL`` if the AArch64 Tagged Address ABI is globally >> + disabled by ``sysctl abi.tagged_addr_disabled=1``. The default >> + ``sysctl abi.tagged_addr_disabled`` configuration is 0. >> + >> +When the AArch64 Tagged Address ABI is enabled for a thread, the >> +following behaviours are guaranteed: >> + >> +- All syscalls except the cases mentioned in section 3 can accept any >> + valid tagged pointer. >> + >> +- The syscall behaviour is undefined for invalid tagged pointers: it may >> + result in an error code being returned, a (fatal) signal being raised, >> + or other modes of failure. >> + >> +- The syscall behaviour for a valid tagged pointer is the same as for >> + the corresponding untagged pointer. >> + >> + >> +A definition of the meaning of tagged pointers on AArch64 can be found >> +in Documentation/arm64/tagged-pointers.rst. >> + >> +3. AArch64 Tagged Address ABI Exceptions >> +----------------------------------------- >> + >> +The following system call parameters must be untagged regardless of the >> +ABI relaxation: >> + >> +- ``prctl()`` other than pointers to user data either passed directly or >> + indirectly as arguments to be accessed by the kernel. >> + >> +- ``ioctl()`` other than pointers to user data either passed directly or >> + indirectly as arguments to be accessed by the kernel. >> + >> +- ``shmat()`` and ``shmdt()``. >> + >> +Any attempt to use non-zero tagged pointers may result in an error code >> +being returned, a (fatal) signal being raised, or other modes of >> +failure. >> + >> +4. Example of correct usage >> +--------------------------- >> +.. code-block:: c >> + >> + #include >> + #include >> + #include >> + #include >> + #include >> + >> + #define PR_SET_TAGGED_ADDR_CTRL 55 >> + #define PR_TAGGED_ADDR_ENABLE (1UL << 0) >> + >> + #define TAG_SHIFT 56 >> + >> + int main(void) >> + { >> + int tbi_enabled = 0; >> + unsigned long tag = 0; >> + char *ptr; >> + >> + /* check/enable the tagged address ABI */ >> + if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)) >> + tbi_enabled = 1; >> + >> + /* memory allocation */ >> + ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE, >> + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); >> + if (ptr == MAP_FAILED) >> + return 1; >> + >> + /* set a non-zero tag if the ABI is available */ >> + if (tbi_enabled) >> + tag = rand() & 0xff; >> + ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT)); >> + >> + /* memory access to a tagged address */ >> + strcpy(ptr, "tagged pointer\n"); >> + >> + /* syscall with a tagged pointer */ >> + write(1, ptr, strlen(ptr)); >> + >> + return 0; >> + }