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=-4.0 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 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 B0531C433DB for ; Tue, 23 Mar 2021 18:32:47 +0000 (UTC) Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 5071A619BB for ; Tue, 23 Mar 2021 18:32:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5071A619BB 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=desiato.20200630; 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=5WbHI12NNS1YpRk4YQ4HmkBlYRX/EmNp3aZnImORDBA=; b=q9/NQbifYneTHl7kYKO/QlQNq pkH+Arvt0hc9VwhqAQIZ956Lh704DRmhhu3Gxb2Q5KyeJe3guTAvKso+F0C8j5It34oqaHDN6S2x1 CDYMVUe0InS42CVhY2R+Ul42tXC2Dc4T5C/iUoe951mM1bln4JauMVmSxfS5PUOmVGwBWl6Qmls7A 98uS0f8boMwOeEsjJ8J37XpH/eU0wQ4ZA2YojyA6z97Xrrmg8N5K7e120kDfWwHTm4njRVHnZOBOH LLkUV9giWSLe0B6tfbWRgBE7rcSDXmPAbF2G7Fy9LnvlnTOxoqRRGjwqhGnGut8TaESO5DO5FVO8o UGzp9CDYg==; Received: from localhost ([::1] helo=desiato.infradead.org) by desiato.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1lOlo0-00FWXm-SG; Tue, 23 Mar 2021 18:31:05 +0000 Received: from foss.arm.com ([217.140.110.172]) by desiato.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1lOlnw-00FWXI-I0 for linux-arm-kernel@lists.infradead.org; Tue, 23 Mar 2021 18:31:02 +0000 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 47C1A1042; Tue, 23 Mar 2021 11:30:58 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.24.204]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 03D8C3F718; Tue, 23 Mar 2021 11:30:55 -0700 (PDT) Date: Tue, 23 Mar 2021 18:30:53 +0000 From: Mark Rutland To: "Madhavan T. Venkataraman" Cc: broonie@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable Message-ID: <20210323183053.GH98545@C02TD0UTHF1T.local> References: <20210323105118.GE95840@C02TD0UTHF1T.local> <2167f3c5-e7d0-40c8-99e3-ae89ceb2d60e@linux.microsoft.com> <20210323133611.GB98545@C02TD0UTHF1T.local> <20210323145734.GD98545@C02TD0UTHF1T.local> <20210323170236.GF98545@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210323_183100_909369_685D961D X-CRM114-Status: GOOD ( 24.88 ) 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 Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote: > On 3/23/21 12:02 PM, Mark Rutland wrote: [...] > I think that I did a bad job of explaining what I wanted to do. It is not > for any additional protection at all. > > So, let us say we create a field in the task structure: > > u64 unreliable_stack; > > Whenever an EL1 exception is entered or FTRACE is entered and pt_regs get > set up and pt_regs->stackframe gets chained, increment unreliable_stack. > On exiting the above, decrement unreliable_stack. > > In arch_stack_walk_reliable(), simply do this check upfront: > > if (task->unreliable_stack) > return -EINVAL; > > This way, the function does not even bother unwinding the stack to find > exception frames or checking for different return addresses or anything. > We also don't have to worry about code being reorganized, functions > being renamed, etc. It also may help in debugging to know if a task is > experiencing an exception and the level of nesting, etc. As in my other reply, since this is an optimization that is not necessary for functional correctness, I would prefer to avoid this for now. We can reconsider that in future if we encounter performance problems. Even with this there will be cases where we have to identify non-unwindable functions explicitly (e.g. the patchable-function-entry trampolines, where the real return address is in x9), and I'd prefer that we use one mechanism consistently. I suspect that in the future we'll need to unwind across exception boundaries using metadata, and we can treat the non-unwindable metadata in the same way. [...] > > 3. Figure out exception boundary handling. I'm currently working to > > simplify the entry assembly down to a uniform set of stubs, and I'd > > prefer to get that sorted before we teach the unwinder about > > exception boundaries, as it'll be significantly simpler to reason > > about and won't end up clashing with the rework. > > So, here is where I still have a question. Is it necessary for the unwinder > to know the exception boundaries? Is it not enough if it knows if there are > exceptions present? For instance, using something like num_special_frames > I suggested above? I agree that it would be legitimate to bail out early if we knew there was going to be an exception somewhere in the trace. Regardless, I think it's simpler overall to identify non-unwindability during the trace, and doing that during the trace aligns more closely with the structure that we'll need to permit unwinding across these boundaries in future, so I'd prefer we do that rather than trying to optimize for early returns today. Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel