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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E9DC8C433EF for ; Wed, 27 Oct 2021 16:09:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CBBB660D43 for ; Wed, 27 Oct 2021 16:09:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242932AbhJ0QLl (ORCPT ); Wed, 27 Oct 2021 12:11:41 -0400 Received: from linux.microsoft.com ([13.77.154.182]:52424 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239136AbhJ0QLk (ORCPT ); Wed, 27 Oct 2021 12:11:40 -0400 Received: from [192.168.254.32] (unknown [47.187.212.181]) by linux.microsoft.com (Postfix) with ESMTPSA id 4CAA32034CB7; Wed, 27 Oct 2021 09:09:14 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 4CAA32034CB7 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1635350955; bh=lRMHS3oUIFOH6oRjnTIYOSwO1nqeQHqP48OJr6TyQNw=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=jIc1Q0dgolV8PNZzp++zlwsk6NJtUXvM+gU1c0C197ejfOYt/mtYJbmFQPS6F3VP4 ICabiJl/ce4G7d9SkRT8BtopMxxidoXzDkCnfuS4hvWWroEwmzUALgSF3c4YIKo0GZ REF0bHAvgbi5Hd+mo34KG7t7UosLRePcBawMssfU= Subject: Re: [PATCH v10 05/11] arm64: Make dump_stacktrace() use arch_stack_walk() To: Mark Rutland Cc: broonie@kernel.org, jpoimboe@redhat.com, ardb@kernel.org, nobuta.keiya@fujitsu.com, sjitindarsingh@gmail.com, catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org References: <20211015025847.17694-1-madvenka@linux.microsoft.com> <20211015025847.17694-6-madvenka@linux.microsoft.com> <20211025164925.GB2001@C02TD0UTHF1T.local> <20211026120516.GA34073@C02TD0UTHF1T.local> From: "Madhavan T. Venkataraman" Message-ID: Date: Wed, 27 Oct 2021 11:09:13 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20211026120516.GA34073@C02TD0UTHF1T.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On 10/26/21 7:05 AM, Mark Rutland wrote: > On Mon, Oct 25, 2021 at 05:49:25PM +0100, Mark Rutland wrote: >> From f3e66ca75aff3474355839f72d123276028204e1 Mon Sep 17 00:00:00 2001 >> From: Mark Rutland >> Date: Mon, 25 Oct 2021 13:23:11 +0100 >> Subject: [PATCH] arm64: ftrace: use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR >> >> When CONFIG_FUNCTION_GRAPH_TRACER is selected, and the function graph: >> tracer is in use, unwind_frame() may erroneously asscociate a traced >> function with an incorrect return address. This can happen when starting >> an unwind from a pt_regs, or when unwinding across an exception >> boundary. >> >> The underlying problem is that ftrace_graph_get_ret_stack() takes an >> index offset from the most recent entry added to the fgraph return >> stack. We start an unwind at offset 0, and increment the offset each >> time we encounter `return_to_handler`, which indicates a rewritten >> return address. This is broken in two cases: >> >> * Between creating a pt_regs and starting the unwind, function calls may >> place entries on the stack, leaving an abitrary offset which we can >> only determine by performing a full unwind from the caller of the >> unwind code. While this initial unwind is open-coded in >> dump_backtrace(), this is not performed for other unwinders such as >> perf_callchain_kernel(). >> >> * When unwinding across an exception boundary (whether continuing an >> unwind or starting a new unwind from regs), we always consume the LR >> of the interrupted context, though this may not have been live at the >> time of the exception. Where the LR was not live but happened to >> contain `return_to_handler`, we'll recover an address from the graph >> return stack and increment the current offset, leaving subsequent >> entries off-by-one. >> >> Where the LR was not live and did not contain `return_to_handler`, we >> will still report an erroneous address, but subsequent entries will be >> unaffected. > > It turns out I had this backwards, and we currently always *skip* the LR > when unwinding across regs, because: > > * The entry assembly creates a synthetic frame record with the original > FP and the ELR_EL1 value (i.e. the PC at the point of the exception), > skipping the LR. > > * In arch_stack_walk() we start the walk from regs->pc, and continue > with the frame record, skipping the LR. > > * In the existing dump_backtrace, we skip until we hit a frame record > whose FP value matches the FP in the regs (i.e. the synthetic frame > record created by the entry assembly). That'll dump the ELR_EL1 value, > then continue to the next frame record, skipping the LR. > > So case two is bogus, and only case one can happen today. This cleanup > shouldn't trigger the WARN_ON_ONCE() in unwind_frame(), and we can fix > the missing LR entry in a subsequent cleanup. > OK. Thanks. Madhavan