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 53A2E7D2F0 for ; Wed, 21 Aug 2019 18:46:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727959AbfHUSq4 (ORCPT ); Wed, 21 Aug 2019 14:46:56 -0400 Received: from foss.arm.com ([217.140.110.172]:35182 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727719AbfHUSq4 (ORCPT ); Wed, 21 Aug 2019 14:46:56 -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 B8135344; Wed, 21 Aug 2019 11:46:55 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 12BF53F706; Wed, 21 Aug 2019 11:46:53 -0700 (PDT) Date: Wed, 21 Aug 2019 19:46:51 +0100 From: Dave Martin To: Will Deacon Cc: Catalin Marinas , linux-arch@vger.kernel.org, Dave Hansen , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , linux-doc@vger.kernel.org, Will Deacon , linux-mm@kvack.org, Andrew Morton , Vincenzo Frascino , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst Message-ID: <20190821184649.GD27757@arm.com> References: <20190821164730.47450-1-catalin.marinas@arm.com> <20190821164730.47450-4-catalin.marinas@arm.com> <20190821173352.yqfgaozi7nfhcofg@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190821173352.yqfgaozi7nfhcofg@willie-the-truck> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Wed, Aug 21, 2019 at 06:33:53PM +0100, Will Deacon wrote: > On Wed, Aug 21, 2019 at 05:47:30PM +0100, 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. 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: Szabolcs Nagy > > Cc: Kevin Brodsky > > Acked-by: Andrey Konovalov > > 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..04f2ba9b779e 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()``). sigaltstack() is interesting, since we don't support tagged stacks. Do we keep the ss_sp tag in the kernel, but squash it when delivering a signal to the alternate stack? (I can't remember whether this would be compatible with the architectural tag checking semantics...) > Hmm. I can see the need to provide this guarantee for things like > set_robust_list(), but the problem is that the statement above is too broad > and isn't strictly true: for example, mmap() doesn't propagate the tag of > its address parameter into the VMA. > > So I think we need to nail this down a bit more, but I'm having a really > hard time coming up with some wording :( Time for some creative vagueness? We can write a statement of our overall intent, along with examples of a few cases where the tag should and should not be expected to emerge intact. There is no foolproof rule, unless we can rewrite history... Cheers ---Dave