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=-14.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL 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 B8842C433C1 for ; Tue, 23 Mar 2021 20:25:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 807F0619D5 for ; Tue, 23 Mar 2021 20:25:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233330AbhCWUYo (ORCPT ); Tue, 23 Mar 2021 16:24:44 -0400 Received: from linux.microsoft.com ([13.77.154.182]:54414 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232991AbhCWUYQ (ORCPT ); Tue, 23 Mar 2021 16:24:16 -0400 Received: from [192.168.254.32] (unknown [47.187.194.202]) by linux.microsoft.com (Postfix) with ESMTPSA id 2B6FF20B5680; Tue, 23 Mar 2021 13:24:15 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 2B6FF20B5680 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1616531055; bh=cEj00evtMTrgg3cKg1V64dkb0xvskevU8Gc0vpIf5qA=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=HwhD4YSRjga0r3WY+zbJ7G0I+vkoRxpCZw1ynuhNCYao8TxXDoPgBhSD9yJrriCgq k68Yyk6c3xER5L3nNjCmKKCHMkIQbpwFhj+dsR/ITJ6JY6DLmMPJ6lIc3VeqZrHMuq BgqDaVK0CSsVVGJYKqH0ewDhW3rnQg4ySju9jwhc= Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable To: Mark Rutland 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 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> <20210323183053.GH98545@C02TD0UTHF1T.local> From: "Madhavan T. Venkataraman" Message-ID: <8aa50127-3f00-818d-d58c-4b3ff7235c74@linux.microsoft.com> Date: Tue, 23 Mar 2021 15:24:14 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210323183053.GH98545@C02TD0UTHF1T.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/23/21 1:30 PM, Mark Rutland wrote: > 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. > OK. Fair enough. Thanks. Madhavan