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=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT autolearn=ham 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 428A0C32789 for ; Tue, 6 Nov 2018 12:29:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 11E392085B for ; Tue, 6 Nov 2018 12:29:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 11E392085B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730497AbeKFVyg (ORCPT ); Tue, 6 Nov 2018 16:54:36 -0500 Received: from foss.arm.com ([217.140.101.70]:60444 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729711AbeKFVyg (ORCPT ); Tue, 6 Nov 2018 16:54:36 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 1AE92A78; Tue, 6 Nov 2018 04:29:37 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 95F7F3F5CF; Tue, 6 Nov 2018 04:29:35 -0800 (PST) Date: Tue, 6 Nov 2018 12:29:33 +0000 From: Mark Rutland To: Dave P Martin Cc: Daniel Thompson , Zhaoyang Huang , Catalin Marinas , Will Deacon , Michael Weiser , James Morse , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] arch/arm64 : fix error in dump_backtrace Message-ID: <20181106122932.652iuh3c4qhe63gl@lakrids.cambridge.arm.com> References: <1541488775-29610-1-git-send-email-huangzhaoyang@gmail.com> <20181106083901.erezwtcomiijvdrk@salmiak> <20181106085751.hrp7qkp53cftgew6@holly.lan> <20181106110019.36ps3tyakvocwst4@lakrids.cambridge.arm.com> <20181106113247.GF3500@e103592.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181106113247.GF3500@e103592.cambridge.arm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 06, 2018 at 11:32:50AM +0000, Dave P Martin wrote: > On Tue, Nov 06, 2018 at 11:00:19AM +0000, Mark Rutland wrote: > > On Tue, Nov 06, 2018 at 08:57:51AM +0000, Daniel Thompson wrote: > > > On Tue, Nov 06, 2018 at 08:39:01AM +0000, Mark Rutland wrote: > > > > On Tue, Nov 06, 2018 at 03:19:35PM +0800, Zhaoyang Huang wrote: > > > > > From: Zhaoyang Huang > > > > > > > > > > In some cases, the instruction of "bl foo1" will be the last one of the > > > > > foo2[1], which will cause the lr be the first instruction of the adjacent > > > > > foo3[2]. Hence, the backtrace will show the weird result as bellow[3]. > > > > > The patch will fix it by miner 4 of the lr when dump_backtrace > > > > > > > > This has come up in the past (and a similar patch has been applied, then > > > > reverted). > > > > > > > > In general, we don't know that a function call was made via BL, and therefore > > > > cannot know that LR - 4 is the address of the caller. The caller could set up > > > > the LR as it likes, then B or BR to the callee, and depending on how the basic > > > > blocks get laid out in memory, LR - 4 might point at something completely > > > > different. > > > > > > > > More ideally, the compiler wouldn't end a function with a BL. When does that > > > > happen, and is there some way we could arrange for that to not happen? e.g. > > > > somehow pad a NOP after the BL. > > > > > > It's a consequence of having __noreturn isn't it? __noreturn frees the > > > compiler from the burden of having to produce a valid return stack... so > > > it doesn't and unwinding becomes hard. > > > > In that case, the compiler could equally just use B rather than BL, > > which this patch doesn't solve. > > > > The documentation for the GCC noreturn attribute [1] says: > > > > | In order to preserve backtraces, GCC will never turn calls to noreturn > > | functions into tail calls. > > > > ... so clearly it's not intended to mess up backtracing. > > Which is a bit odd, since every call to a noreturn function is a tail- > call by definition, and other tail-calls are typically optimised to a B > (thus interfering with backtracing in all except the noreturn case). > > Avoiding this would require a change to the compiler, and since there's > no obvious correct answer anyway, I guess we shouldn't hold our breath. > > > IIUC we mostly use noreturn to prevent warings about uninitialised > > variables and such after a call to a noreturn function. I think > > optimization is a secondary concern. > > Agreed. > > > We could ask the GCC folk if they can ensure that a noreturn function > > call leave thes LR pointing into the caller, e.g. by padding with a NOP: > > > > BL > > NOP > > > > That seems cheap enough, and would keep backtraces reliable. > > -fpatchable-function-entry=1,1 does almost the right thing, by > inserting 1 NOP at the start of each function, and putting the function > entry point after that (1) NOP. Neat hack, but unfortunately insufficient in general since: * The next function may be notrace or asm, and hence will not have a preceding NOP. * There may not be a next function (e.g. for the final instruction in .text), and the LR value may point at some data symbol. * This relies on the preceding NOP being accounted as part of the previous function, which feels like a bug given we should have the function size somewhere. Generally, I think that trying to bodge around the exiting behaviour is going to cause just as many problems as it solves, and worse, makes it harder to consistently analyse a backtrace. IMO, we shouldn't change the kernel here. Thanks, Mark.