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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 C1090C433B4 for ; Fri, 9 Apr 2021 22:32:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 78BCF61106 for ; Fri, 9 Apr 2021 22:32:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235065AbhDIWcx (ORCPT ); Fri, 9 Apr 2021 18:32:53 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:27872 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234602AbhDIWcw (ORCPT ); Fri, 9 Apr 2021 18:32:52 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618007558; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=yyr/6Ag1Vbeg1psngxfhYGjr6J2CEL/d4aEho0RnXho=; b=LcqrHOeBwwnbaBCqh6U9zXnjTh7cCe77GguZ2NxOnOLQXXHQMEDHhJMhZkgNpYnP0GHc9W 1SFLdGMhQab+EBbkZgRxBEX8AU3IxxjKVSbFjVJZBbY8hgUHwmELS8ioRX+cjzfKe9f1Li 6tSTY0DRdWL4wltwWUWpIK6I1ux54LY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-241-0AUWjXB6OJWe5318dHN-_w-1; Fri, 09 Apr 2021 18:32:36 -0400 X-MC-Unique: 0AUWjXB6OJWe5318dHN-_w-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E0F5F10053E7; Fri, 9 Apr 2021 22:32:34 +0000 (UTC) Received: from treble (ovpn-112-2.rdu2.redhat.com [10.10.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A29605C1D5; Fri, 9 Apr 2021 22:32:29 +0000 (UTC) Date: Fri, 9 Apr 2021 17:32:27 -0500 From: Josh Poimboeuf To: "Madhavan T. Venkataraman" Cc: Mark Rutland , broonie@kernel.org, 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, Peter Zijlstra Subject: Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks Message-ID: <20210409223227.rvf6tfhvgnpzmabn@treble> References: <705993ccb34a611c75cdae0a8cb1b40f9b218ebd> <20210405204313.21346-1-madvenka@linux.microsoft.com> <20210409120859.GA51636@C02TD0UTHF1T.local> <20210409213741.kqmwyajoppuqrkge@treble> <8c30ec5f-b51e-494f-5f6c-d2f012135f69@linux.microsoft.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <8c30ec5f-b51e-494f-5f6c-d2f012135f69@linux.microsoft.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 09, 2021 at 05:05:58PM -0500, Madhavan T. Venkataraman wrote: > > FWIW, over the years we've had zero issues with encoding the frame > > pointer on x86. After you save pt_regs, you encode the frame pointer to > > point to it. Ideally in the same macro so it's hard to overlook. > > > > I had the same opinion. In fact, in my encoding scheme, I have additional > checks to make absolutely sure that it is a true encoding and not stack > corruption. The chances of all of those values accidentally matching are, > well, null. Right, stack corruption -- which is already exceedingly rare -- would have to be combined with a miracle or two in order to come out of the whole thing marked as 'reliable' :-) And really, we already take a similar risk today by "trusting" the frame pointer value on the stack to a certain extent. > >> I think there's a lot more code that we cannot unwind, e.g. KVM > >> exception code, or almost anything marked with SYM_CODE_END(). > > > > Just a reminder that livepatch only unwinds blocked tasks (plus the > > 'current' task which calls into livepatch). So practically speaking, it > > doesn't matter whether the 'unreliable' detection has full coverage. > > The only exceptions which really matter are those which end up calling > > schedule(), e.g. preemption or page faults. > > > > Being able to consistently detect *all* possible unreliable paths would > > be nice in theory, but it's unnecessary and may not be worth the extra > > complexity. > > > > You do have a point. I tried to think of arch_stack_walk_reliable() as > something that should be implemented independent of livepatching. But > I could not really come up with a single example of where else it would > really be useful. > > So, if we assume that the reliable stack trace is solely for the purpose > of livepatching, I agree with your earlier comments as well. One thought: if folks really view this as a problem, it might help to just rename things to reduce confusion. For example, instead of calling it 'reliable', we could call it something more precise, like 'klp_reliable', to indicate that its reliable enough for live patching. Then have a comment above 'klp_reliable' and/or stack_trace_save_tsk_klp_reliable() which describes what that means. Hm, for that matter, even without renaming things, a comment above stack_trace_save_tsk_reliable() describing the meaning of "reliable" would be a good idea. -- Josh