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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 56DE6C3A5A1 for ; Thu, 22 Aug 2019 16:37:30 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 04E19206B7 for ; Thu, 22 Aug 2019 16:37:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 04E19206B7 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 3E74C6B0340; Thu, 22 Aug 2019 12:37:29 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 397496B0341; Thu, 22 Aug 2019 12:37:29 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 286486B0342; Thu, 22 Aug 2019 12:37:29 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0049.hostedemail.com [216.40.44.49]) by kanga.kvack.org (Postfix) with ESMTP id 082BB6B0340 for ; Thu, 22 Aug 2019 12:37:28 -0400 (EDT) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with SMTP id A3BC3180AD7C1 for ; Thu, 22 Aug 2019 16:37:28 +0000 (UTC) X-FDA: 75850619376.27.eggs03_7b688a2bcec4b X-HE-Tag: eggs03_7b688a2bcec4b X-Filterd-Recvd-Size: 4641 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf48.hostedemail.com (Postfix) with ESMTP for ; Thu, 22 Aug 2019 16:37:27 +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 0F16E28; Thu, 22 Aug 2019 09:37:27 -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 5B9283F718; Thu, 22 Aug 2019 09:37:25 -0700 (PDT) Date: Thu, 22 Aug 2019 17:37:23 +0100 From: Dave Martin To: Catalin Marinas Cc: linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Will Deacon , linux-mm@kvack.org, Dave Hansen , Andrew Morton , Vincenzo Frascino , Will Deacon , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst Message-ID: <20190822163723.GF27757@arm.com> References: <20190821164730.47450-1-catalin.marinas@arm.com> <20190821164730.47450-4-catalin.marinas@arm.com> <20190821173352.yqfgaozi7nfhcofg@willie-the-truck> <20190821184649.GD27757@arm.com> <20190822155531.GB55798@arrakis.emea.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190822155531.GB55798@arrakis.emea.arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) 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 22, 2019 at 04:55:32PM +0100, Catalin Marinas wrote: > On Wed, Aug 21, 2019 at 07:46:51PM +0100, Dave P Martin wrote: > > 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: > > > > @@ -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. > > We should support tagged SP with the new ABI as they'll be required for > MTE. sigaltstack() and clone() are the two syscalls that come to mind > here. > > > Do we keep the ss_sp tag in the kernel, but squash it when delivering > > a signal to the alternate stack? > > We don't seem to be doing any untagging, so we just just use whatever > the caller asked for. We may need a small test to confirm. If we want to support tagged SP, then I guess we shouldn't be squashing the tag anywhere. A test for that would be sensible to have. > That said, on_sig_stack() probably needs some untagging as it does user > pointer arithmetics with potentially different tags. Good point. > > > 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... > > I would expect the norm to be the preservation of tags with a few > exceptions. The only ones I think where we won't preserve the tags are > mmap, mremap, brk (apart from the signal stuff already mentioned in the > current tagged-pointers.rst doc). > > So I can remove this paragraph altogether and add a note in part 3 of > the tagged-address-abi.rst document that mmap/mremap/brk do not preserve > the tag information. Deleting text is always a good idea ;) There are other cases like (non-)propagation of the tag to si_addr when a fault is reported via a signal, but I think we already have appropriate wording to cover that. Cheers ---Dave