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 97E47C4707C for ; Fri, 21 May 2021 17:32:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 78EDB611AB for ; Fri, 21 May 2021 17:32:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234953AbhEUReS (ORCPT ); Fri, 21 May 2021 13:34:18 -0400 Received: from linux.microsoft.com ([13.77.154.182]:59448 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233220AbhEUReR (ORCPT ); Fri, 21 May 2021 13:34:17 -0400 Received: from [192.168.254.32] (unknown [47.187.214.213]) by linux.microsoft.com (Postfix) with ESMTPSA id 8C68120B7188; Fri, 21 May 2021 10:32:53 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 8C68120B7188 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1621618374; bh=kx/ub+OL0PzoQ/hT2KpbKriy7vxNbuz0bztaoHmRMxA=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=pY1l80FOiOGwYYaK4/exjyN+IR65hI3sJUmLEcnLo+vTzCDtaGAdxamRYNs8HlR6F 0lc0twNVrQJvOfgDoSx03OxrZ90ak643peCTp1xyrmDNHB2farqYT5Oaly2vFev1hH 4gMM8lSxEOHmv1ITHi/rDmUXK9laQMDpP2W/K0Uo= Subject: Re: [RFC PATCH v4 0/2] arm64: Stack trace reliability checks in the unwinder To: Mark Brown Cc: mark.rutland@arm.com, jpoimboe@redhat.com, ardb@kernel.org, jthierry@redhat.com, catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org, pasha.tatashin@soleen.com, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org References: <68eeda61b3e9579d65698a884b26c8632025e503> <20210516040018.128105-1-madvenka@linux.microsoft.com> <20210521171808.GC5825@sirena.org.uk> From: "Madhavan T. Venkataraman" Message-ID: <654dde25-e6a2-a1e7-c2d7-e2692bc11528@linux.microsoft.com> Date: Fri, 21 May 2021 12:32:52 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210521171808.GC5825@sirena.org.uk> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/21/21 12:18 PM, Mark Brown wrote: > On Sat, May 15, 2021 at 11:00:16PM -0500, madvenka@linux.microsoft.com wrote: > >> Special cases >> ============= >> >> Some special cases need to be mentioned: > > I think it'd be good if more of this cover letter, especially sections > like this which cover the tricky bits, ended up in the code somehow - > it's recorded here and will be in the list archive but that's not the > most discoverable place so increases the maintainance burden. It'd be > great to be able to compare the code directly with the reliable > stacktrace requirements document and see everything getting ticked off, > actually going all the way there might be too much and loose the code in > the comments but I think we can get closer to it than we are. Given > that a lot of this stuff rests on the denylist perhaps some comments > just before it's called would be a good place to start? > I will add more comments in the code to make it clear. >> - EL1 interrupt and exception handlers end up in sym_code_ranges[]. >> So, all EL1 interrupt and exception stack traces will be considered >> unreliable. This the correct behavior as interrupts and exceptions > > This stuff about exceptions and preemption is a big one, rejecting any > exceptions makes a whole host of things easier (eg, Mark Rutland raised > interactions between non-AAPCS code and PLTs as being an issue but if > we're able to reliably reject stacks featuring any kind of preemption > anyway that should sidestep the issue). > Yes. I will include this in the code comments. >> Performance >> =========== > >> Currently, unwinder_blacklisted() does a linear search through >> sym_code_functions[]. If reviewers prefer, I could sort the >> sym_code_functions[] array and perform a binary search for better >> performance. There are about 80 entries in the array. > > If people are trying to live patch a very busy/big system then this > could be an issue, equally there's probably more people focused on > getting boot times as fast as possible than live patching. Deferring > the initialisation to first use would help boot times with or without > sorting, without numbers I don't actually know that sorting is worth the > effort or needs doing immediately - obvious correctness is also a > benefit! My instinct is that for now it's probably OK leaving it as a > linear scan and then revisiting if it's not adequately performant, but > I'd defer to actual users there. I have followed the example in the Kprobe deny list. I place the section in initdata so it can be unloaded during boot. This means that I need to copy the information before that in early_initcall(). If the initialization must be performed on first use, I probably have to move SYM_CODE_FUNCTIONS from initdata to some other place where it will be retained. If you prefer this, I could do it this way. Thanks! Madhavan