From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.7 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 4DEBD7DC63 for ; Thu, 15 Aug 2019 15:44:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730865AbfHOPoS (ORCPT ); Thu, 15 Aug 2019 11:44:18 -0400 Received: from foss.arm.com ([217.140.110.172]:45776 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729975AbfHOPoR (ORCPT ); Thu, 15 Aug 2019 11:44:17 -0400 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 647AF360; Thu, 15 Aug 2019 08:44:17 -0700 (PDT) Received: from arrakis.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id B3B1E3F706; Thu, 15 Aug 2019 08:44:15 -0700 (PDT) From: Catalin Marinas To: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Cc: Andrew Morton , Vincenzo Frascino , Will Deacon , Andrey Konovalov , Szabolcs Nagy , Kevin Brodsky , Dave P Martin , Dave Hansen , linux-doc@vger.kernel.org, linux-arch@vger.kernel.org Subject: [PATCH v8 5/5] arm64: Relax Documentation/arm64/tagged-pointers.rst Date: Thu, 15 Aug 2019 16:44:03 +0100 Message-Id: <20190815154403.16473-6-catalin.marinas@arm.com> X-Mailer: git-send-email 2.23.0.rc0 In-Reply-To: <20190815154403.16473-1-catalin.marinas@arm.com> References: <20190815154403.16473-1-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org 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. However, such pointers were not allowed at the user-kernel syscall ABI boundary. With the Tagged Address ABI patchset, it is now possible to pass tagged pointers to the syscalls. Relax the requirements described in tagged-pointers.rst to be compliant with the behaviours guaranteed by the AArch64 Tagged Address ABI. 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 --- Documentation/arm64/tagged-pointers.rst | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst index 2acdec3ebbeb..fd5306019e91 100644 --- a/Documentation/arm64/tagged-pointers.rst +++ b/Documentation/arm64/tagged-pointers.rst @@ -20,7 +20,9 @@ Passing tagged addresses to the kernel -------------------------------------- All interpretation of userspace memory addresses by the kernel assumes -an address tag of 0x00. +an address tag of 0x00, unless the application enables the AArch64 +Tagged Address ABI explicitly +(Documentation/arm64/tagged-address-abi.rst). This includes, but is not limited to, addresses found in: @@ -33,13 +35,15 @@ This includes, but is not limited to, addresses found in: - the frame pointer (x29) and frame records, e.g. when interpreting them to generate a backtrace or call graph. -Using non-zero address tags in any of these locations may result in an -error code being returned, a (fatal) signal being raised, or other modes -of failure. +Using non-zero address tags in any of these locations when the +userspace application did not enable the AArch64 Tagged Address ABI may +result in an error code being returned, a (fatal) signal being raised, +or other modes of failure. -For these reasons, passing non-zero address tags to the kernel via -system calls is forbidden, and using a non-zero address tag for sp is -strongly discouraged. +For these reasons, when the AArch64 Tagged Address ABI is disabled, +passing non-zero address tags to the kernel via system calls is +forbidden, and using a non-zero address tag for sp is strongly +discouraged. Programs maintaining a frame pointer and frame records that use non-zero address tags may suffer impaired or inaccurate debug and profiling @@ -59,6 +63,11 @@ be preserved. The architecture prevents the use of a tagged PC, so the upper byte will be set to a sign-extension of bit 55 on exception return. +This behaviour is maintained when the AArch64 Tagged Address ABI is +enabled. In addition, with the exceptions above, the kernel will +preserve any non-zero tags passed by the user via syscalls and stored in +kernel data structures (e.g. set_robust_list(), sigaltstack()). + Other considerations --------------------