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 1BC34C43603 for ; Tue, 17 Dec 2019 17:56:17 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id D66E6206EC for ; Tue, 17 Dec 2019 17:56:16 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D66E6206EC 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 735CF8E0097; Tue, 17 Dec 2019 12:56:16 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6E6628E0079; Tue, 17 Dec 2019 12:56:16 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5ADB48E0097; Tue, 17 Dec 2019 12:56:16 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0184.hostedemail.com [216.40.44.184]) by kanga.kvack.org (Postfix) with ESMTP id 4583A8E0079 for ; Tue, 17 Dec 2019 12:56:16 -0500 (EST) Received: from smtpin26.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id EDE1E824999B for ; Tue, 17 Dec 2019 17:56:15 +0000 (UTC) X-FDA: 76275387510.26.coal62_36a1466382401 X-HE-Tag: coal62_36a1466382401 X-Filterd-Recvd-Size: 4151 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf49.hostedemail.com (Postfix) with ESMTP for ; Tue, 17 Dec 2019 17:56:15 +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 BC65230E; Tue, 17 Dec 2019 09:56:14 -0800 (PST) Received: from arrakis.emea.arm.com (arrakis.cambridge.arm.com [10.1.197.42]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 37C283F67D; Tue, 17 Dec 2019 09:56:13 -0800 (PST) Date: Tue, 17 Dec 2019 17:56:11 +0000 From: Catalin Marinas To: Peter Collingbourne Cc: Kevin Brodsky , Linux ARM , Will Deacon , Marc Zyngier , Vincenzo Frascino , Szabolcs Nagy , Richard Earnshaw , Andrey Konovalov , linux-mm@kvack.org, linux-arch@vger.kernel.org, Branislav Rankov Subject: Re: [PATCH 20/22] arm64: mte: Allow user control of the excluded tags via prctl() Message-ID: <20191217175610.GN5624@arrakis.emea.arm.com> References: <20191211184027.20130-1-catalin.marinas@arm.com> <20191211184027.20130-21-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) 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 Mon, Dec 16, 2019 at 09:30:36AM -0800, Peter Collingbourne wrote: > On Mon, Dec 16, 2019 at 6:20 AM Kevin Brodsky wrote: > > In this patch, the default exclusion mask remains 0 (i.e. all tags can be generated). > > After some more discussions, Branislav and I think that it would be better to start > > with the reverse, i.e. all tags but 0 excluded (mask = 0xfe or 0xff). So with mask 0xff, IRG generates only tag 0? This seems to be the case reading the pseudocode in the ARM ARM. > > This should simplify the MTE setup in the early C runtime quite a bit. Indeed, if all > > tags can be generated, doing any heap or stack tagging before the > > PR_SET_TAGGED_ADDR_CTRL prctl() is issued can cause problems, notably because tagged > > addresses could end up being passed to syscalls. Conversely, if IRG and ADDG never > > set the top byte by default, then tagging operations should be no-ops until the > > prctl() is issued. This would be particularly useful given that it may not be > > straightforward for the C runtime to issue the prctl() before doing anything else. > > > > Additionally, since the default tag checking mode is PR_MTE_TCF_NONE, it would make > > perfect sense not to generate tags by default. > > > > Any thoughts? > > This would indeed allow the early C runtime startup code to pass > tagged addresses to syscalls, but I don't think it would entirely free > the code from the burden of worrying about stack tagging. Either way, > any stack frames that are active at the point when the prctl() is > issued would need to be compiled without stack tagging, because > otherwise those stack frames may use ADDG to rematerialize a stack > object address, which may produce a different address post-prctl. > Setting the exclude mask to 0xffff would at least make it more likely > for this problem to be detected, though. > > If we change the default in this way, maybe it would be worth > considering flipping the meaning of the tag mask and have it be a mask > of tags to allow. That would be consistent with the existing behaviour > where userspace sets bits in tagged_addr_ctrl in order to enable > tagging features. Either option works for me. It's really for the libc people to decide what they need. I think an "include" rather than "exclude" mask makes sense with the default 0 meaning only generate tag 0. Thanks. -- Catalin