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=-5.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 119A9C47082 for ; Mon, 7 Jun 2021 18:14:09 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id CC0EC6100B for ; Mon, 7 Jun 2021 18:14:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CC0EC6100B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=Hh+wu0Ui6YdfRS/IIYBKeCkTgqZfDx7swX3mH6kFBz8=; b=Y7Vi3F37ytF0BB xLjqYeyWSseUU0m7ty8OZFQDgtxKpFqDGSIMObaKcey8dHZJPq9xdvOmwhiQFj8D6vJU1VfJTkGx1 J7knMpMSTrw/9DGEXrLAXy8ydThRHn50+fR+l/xMXsXwASiSIoMnIw8LIaHaDWfxQz2A2gau+VVBX 14+vFmWAic7jvO3leak/5EheebYrl3wv5tlqWK7bA40QkYrbeF0hpNX+mIFbZlMTTTi3lAAVI5muo 84UYCj0a7ES3DCkj/mcten642dVcuwSvgye6qPwI2n26It+AwuLTrBcQ5ZzdLP0zN4GnaxmKgkYaQ Jl7rmG039iq0iOudHIXw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1lqJja-004z9L-4Q; Mon, 07 Jun 2021 18:12:22 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1lqJjW-004z8j-Ci for linux-arm-kernel@lists.infradead.org; Mon, 07 Jun 2021 18:12:19 +0000 Received: by mail.kernel.org (Postfix) with ESMTPSA id C2E2261003; Mon, 7 Jun 2021 18:12:15 +0000 (UTC) Date: Mon, 7 Jun 2021 19:12:13 +0100 From: Catalin Marinas To: Dave Martin Cc: Mark Brown , linux-arch@vger.kernel.org, libc-alpha@sourceware.org, Szabolcs Nagy , Jeremy Linton , Will Deacon , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter Message-ID: <20210607181212.GD17957@arm.com> References: <20210521144621.9306-1-broonie@kernel.org> <20210521144621.9306-3-broonie@kernel.org> <20210603154034.GH4187@arm.com> <20210603165134.GF4257@sirena.org.uk> <20210603180429.GI20338@arm.com> <20210607112536.GI4187@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20210607112536.GI4187@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210607_111218_470472_11A77BCA X-CRM114-Status: GOOD ( 29.97 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Jun 07, 2021 at 12:25:38PM +0100, Dave P Martin wrote: > On Thu, Jun 03, 2021 at 07:04:31PM +0100, Catalin Marinas via Libc-alpha wrote: > > On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote: > > > On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote: > > > > Do we know how libcs will detect that they don't need to do the > > > > mprotect() calls? Do we need a detection mechanism at all? > > > > > > > > Ignoring certain errors from mprotect() when ld.so is trying to set > > > > PROT_BTI on the main executable's code pages is probably a reasonable, > > > > backwards-compatible compromise here, but it seems a bit wasteful. > > > > > > I think the theory was that they would just do the mprotect() calls and > > > ignore any errors as they currently do, or declare that they depend on a > > > new enough kernel version I guess (not an option for glibc but might be > > > for others which didn't do BTI yet). > > > > I think we discussed the possibility of an AT_FLAGS bit. Until recently, > > this field was 0 but it gained a new bit now. If we are to expose this > > to arch-specific things, it may need some reservations. Anyway, that's > > an optimisation that can be added subsequently. > > I suppose so, but AT_FLAGS doesn't seem appropriate somehow. > > I wonder why we suddenly start considering adding a flag to AT_FLAGS > every few months, when it had sat empty for decades. This may say > something about the current health of the kernel ABI, but I'm not sure > exactly what. > > I think having mprotect() fail in a predictable way may be preferable > for now: glibc still only needs to probe with a single call and could > cache the knowledge after that. Code outside libc / ld.so seems quite > unlikely to care about this. I think that's the expected approach for now. If anyone complains about an extra syscall, we can look into options but I wouldn't rush on doing something. > Any ideas on how we would document this behaviour? The kernel and libc > behaviour are 100% clear: you _are_ allowed to twiddle PROT_BTI on > executable mappings, and there is no legitimate (or even useful) reason > to disallow this. It's only systemd deliberately breaking the API that > causes the behaviour seem by "userspace" to vary. I don't think we can document all the filters that can be added on top various syscalls, so I'd leave it undocumented (or part of the systemd documentation). It was a user space program (systemd) breaking another user space program (well, anything with a new enough glibc). The kernel ABI was still valid when /sbin/init started ;). -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel