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 77347C433EF for ; Thu, 14 Oct 2021 10:27:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5595360F93 for ; Thu, 14 Oct 2021 10:27:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230299AbhJNK3O (ORCPT ); Thu, 14 Oct 2021 06:29:14 -0400 Received: from foss.arm.com ([217.140.110.172]:52742 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230179AbhJNK3M (ORCPT ); Thu, 14 Oct 2021 06:29:12 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E7D6611D4; Thu, 14 Oct 2021 03:27:07 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.73.197]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8C1D93F66F; Thu, 14 Oct 2021 03:27:05 -0700 (PDT) Date: Thu, 14 Oct 2021 11:27:02 +0100 From: Mark Rutland To: Masami Hiramatsu Cc: Steven Rostedt , "Naveen N . Rao" , Ananth N Mavinakayanahalli , Ingo Molnar , linux-kernel@vger.kernel.org, Sven Schnelle , Catalin Marinas , Will Deacon , Russell King , Nathan Chancellor , Nick Desaulniers , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe instance Message-ID: <20211014102702.GB13770@C02TD0UTHF1T.local> References: <163369609308.636038.15295764725220907794.stgit@devnote2> <163369611948.636038.11552166777773804729.stgit@devnote2> <20211013100126.GA3187@C02TD0UTHF1T.local> <20211014170405.f59d287b30086efe7dd7f4d9@kernel.org> <20211014091332.GA13770@C02TD0UTHF1T.local> <20211014190155.3fdc7cf7c42e44ee75c43a9d@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211014190155.3fdc7cf7c42e44ee75c43a9d@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 14, 2021 at 07:01:55PM +0900, Masami Hiramatsu wrote: > On Thu, 14 Oct 2021 10:13:32 +0100 > Mark Rutland wrote: > > > On Thu, Oct 14, 2021 at 05:04:05PM +0900, Masami Hiramatsu wrote: > > > On Wed, 13 Oct 2021 11:01:39 +0100 > > > Mark Rutland wrote: > > > > > > > On Fri, Oct 08, 2021 at 09:28:39PM +0900, Masami Hiramatsu wrote: > > > > > Record the frame pointer instead of stack address with kretprobe > > > > > instance as the identifier on the instance list. > > > > > Since arm64 always enable CONFIG_FRAME_POINTER, we can use the > > > > > actual frame pointer (x29). > > > > > > > > Just to check, why do we need to use the FP rather than SP? It wasn't > > > > clear to me if that's necessary later in the series, or if I'm missing > > > > something here. > > > > > > Actually, this is for finding correct return address from the per-task > > > kretprobe instruction list (suppose it as a shadow stack) when it will > > > be searched in stack-backtracing. At that point, the framepointer will > > > be a reliable key. > > > > Sure, my question was more "why isn't the SP a reliable key?", because both > > the SP and FP should be balanced at function-entry and function-return > > time. I'm asking because I think I'm missing a subtlety. > > Ah, because SP is not used while unwinding frame. For the kretprobe, > either FP or SP is OK. But for the stacktrace.c, I can not use SP > and is easy to change to use FP. :) Ah, so this is just so that stacktrace can match the address. For clarity, would you be happy to add a sentence to the commit message like: | This will allow the stacktrace code to find the original return | address from the FP alone. Thanks, Mark.