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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT 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 385D4C28CC0 for ; Wed, 29 May 2019 16:25:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 07F9723D60 for ; Wed, 29 May 2019 16:25:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726969AbfE2QZe (ORCPT ); Wed, 29 May 2019 12:25:34 -0400 Received: from foss.arm.com ([217.140.101.70]:49016 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726062AbfE2QZd (ORCPT ); Wed, 29 May 2019 12:25:33 -0400 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 6340715AD; Wed, 29 May 2019 09:25:33 -0700 (PDT) Received: from fuggles.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 8D9723F5AF; Wed, 29 May 2019 09:25:30 -0700 (PDT) Date: Wed, 29 May 2019 17:25:28 +0100 From: Will Deacon To: Peter Zijlstra Cc: Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk, mark.rutland@arm.com, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, kan.liang@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, ravi.bangoria@linux.vnet.ibm.com, mpe@ellerman.id.au, acme@redhat.com, eranian@google.com, fweisbec@gmail.com, jolsa@redhat.com Subject: Re: [PATCH] perf: Fix oops when kthread execs user process Message-ID: <20190529162528.GB12420@fuggles.cambridge.arm.com> References: <20190528153224.GE20758@fuggles.cambridge.arm.com> <20190528173228.GW2623@hirez.programming.kicks-ass.net> <20190529091733.GA4485@fuggles.cambridge.arm.com> <20190529101042.GN2623@hirez.programming.kicks-ass.net> <20190529102022.GC4485@fuggles.cambridge.arm.com> <20190529125557.GU2623@hirez.programming.kicks-ass.net> <20190529130521.GA11023@fuggles.cambridge.arm.com> <20190529132515.GW2623@hirez.programming.kicks-ass.net> <20190529143510.GA11154@fuggles.cambridge.arm.com> <20190529161955.GZ2623@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190529161955.GZ2623@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) () Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 29, 2019 at 06:19:55PM +0200, Peter Zijlstra wrote: > On Wed, May 29, 2019 at 03:35:10PM +0100, Will Deacon wrote: > > On Wed, May 29, 2019 at 03:25:15PM +0200, Peter Zijlstra wrote: > > > On Wed, May 29, 2019 at 02:05:21PM +0100, Will Deacon wrote: > > > > On Wed, May 29, 2019 at 02:55:57PM +0200, Peter Zijlstra wrote: > > > > > > > > if (user_mode(regs)) { > > > > > > > > Hmm, so it just occurred to me that Mark's observation is that the regs > > > > can be junk in some cases. In which case, should we be checking for > > > > kthreads first? > > > > > > task_pt_regs() can return garbage, but @regs is the exception (or > > > perf_arch_fetch_caller_regs()) regs, and for those user_mode() had > > > better be correct. > > > > So what should we report for the idle task? > > If an interrupt hits the idle task, @regs would be !user_mode(regs), > we'll find current->flags & PF_KTHREAD (idle not having passed through > exec()) and therefore we'll take ABI_NONE for the user regs. > > Or am I not getting it? Sorry, I'm not trying to catch you out! Just trying to understand what the semantics are supposed to be. I do find the concept of user_mode(regs) bizarre for the idle task. By the above, we definitely have a bug on arm64 (user_mode(regs) tends to be true for the idle task), and I couldn't figure out how you avoided it on x86. I guess it happens to work because the stack is zero-initialised or something? Will