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=-14.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 36E89C3A5A0 for ; Mon, 19 Aug 2019 15:51:12 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id CC4072070B for ; Mon, 19 Aug 2019 15:51:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="PFK+Euq4" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CC4072070B Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 763726B0273; Mon, 19 Aug 2019 11:51:11 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7140D6B0274; Mon, 19 Aug 2019 11:51:11 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6039F6B0275; Mon, 19 Aug 2019 11:51:11 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0079.hostedemail.com [216.40.44.79]) by kanga.kvack.org (Postfix) with ESMTP id 3F7936B0273 for ; Mon, 19 Aug 2019 11:51:11 -0400 (EDT) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id B13C2181AC9AE for ; Mon, 19 Aug 2019 15:51:10 +0000 (UTC) X-FDA: 75839616300.03.drain66_7e9efe0c8f861 X-HE-Tag: drain66_7e9efe0c8f861 X-Filterd-Recvd-Size: 10196 Received: from mail-pg1-f195.google.com (mail-pg1-f195.google.com [209.85.215.195]) by imf17.hostedemail.com (Postfix) with ESMTP for ; Mon, 19 Aug 2019 15:51:09 +0000 (UTC) Received: by mail-pg1-f195.google.com with SMTP id p3so1444688pgb.9 for ; Mon, 19 Aug 2019 08:51:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=3aT4hIoGoOxTITnPRxvZVZEP0ZMplvO3FsPS6VTLrIE=; b=PFK+Euq4eAv4stee6fofZ6gDmqlftrBH/YMhCED0VWVhe5SooEqYb0YDDhkOtZtnZ7 JLrxQc5VrC3cql3m2lqkw7PtsRM2xPQp4TDduiB6uTA1yixI9WRHt6pwiUM/J/L7dSBG XIft5dFKclnG+cguBASUE2SfAWFUgRIkjbPSBUjIVPozrNnTE3KiaYqmdk66vRsW/Dgz 3mAKmVjZIcwcVZcprRTiqT7aoarPwN2ROPgQOoaj8KtoebC6zUruPAjuaz/oqJvH9xng ndiSeIKWtl73t5T8Vy0zFIQneF3vpVaibiO23HLMJKsr40xCM00mBJNYOW1WVePtCpoU eLBg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=3aT4hIoGoOxTITnPRxvZVZEP0ZMplvO3FsPS6VTLrIE=; b=pOigKxl2Zckn83YjmWF+UfSLcFMO690KNCpyiRmuYJyOlZ4uekjZOKFmMd9adqMYj7 qNhNtDL8Ac9BuCfKFfdbz7DCa1H4ot/LFjR2Vv8jXiXDolwe+6VCGTjH2seO5NnG0waY FMTTOhVf1AXLH+63jv6WtlfQZKC220JbnlvbXYxfv3KPBJV9B8//NqleTBJtbI3ZLvj+ 5wve6gXIrFG0+mkgUlsZiFWPHUWv6gaFc37HIuAKWJtqQLFLLSPE7hemMsrxPK2E2rzO jK9H3XPMSsm0fo69+IofHOGvV5bWZWb6wf0+3eywKVl0Hd0nDNT3UJnrEb2VdximqPdS eBnA== X-Gm-Message-State: APjAAAV7aXrdCa/MSXYv6E/nmS50obkJNmADjsaEBqEoIDIUAOTbuLRx wlYEfVWbHjYOYGa63sCI1joqp6Jr6K1fHQC03PeQ/g== X-Google-Smtp-Source: APXvYqzpbwkdUPa164NFyhgPqd+w4X5msjPIvE7rxGK6/5zEmTXY5YFlDgIHgUQ8fIpsWQbEJbgIB1DEIMJXUsWPBDA= X-Received: by 2002:aa7:9e9a:: with SMTP id p26mr25307646pfq.25.1566229868631; Mon, 19 Aug 2019 08:51:08 -0700 (PDT) MIME-Version: 1.0 References: <20190815154403.16473-1-catalin.marinas@arm.com> <20190815154403.16473-5-catalin.marinas@arm.com> In-Reply-To: <20190815154403.16473-5-catalin.marinas@arm.com> From: Andrey Konovalov Date: Mon, 19 Aug 2019 17:50:57 +0200 Message-ID: Subject: Re: [PATCH v8 4/5] arm64: Define Documentation/arm64/tagged-address-abi.rst To: Catalin Marinas Cc: Linux ARM , Linux Memory Management List , Andrew Morton , Vincenzo Frascino , Will Deacon , Szabolcs Nagy , Kevin Brodsky , Dave P Martin , Dave Hansen , "open list:DOCUMENTATION" , linux-arch Content-Type: text/plain; charset="UTF-8" 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 Thu, Aug 15, 2019 at 5:44 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 > --- > Documentation/arm64/tagged-address-abi.rst | 155 +++++++++++++++++++++ > 1 file changed, 155 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..8808337775d6 > --- /dev/null > +++ b/Documentation/arm64/tagged-address-abi.rst > @@ -0,0 +1,155 @@ > +========================== > +AArch64 TAGGED ADDRESS ABI > +========================== > + > +Authors: Vincenzo Frascino > + Catalin Marinas > + > +Date: 15 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() done by the process itself (or its parent), where either: > + > + - flags have the **MAP_ANONYMOUS** bit set > + - the file descriptor refers to a regular file (including those > + returned by memfd_create()) or **/dev/zero** > + > +- brk() system call 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 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. > + > +- A valid tagged pointer has the same semantics as 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 arguments pointing to user structures to be > + accessed by the kernel. > + > +- ioctl() other than arguments pointing to user structures 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; > + }