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=-7.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,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 83879C04AB6 for ; Tue, 28 May 2019 16:13:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5901621721 for ; Tue, 28 May 2019 16:13:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727135AbfE1QNA (ORCPT ); Tue, 28 May 2019 12:13:00 -0400 Received: from foss.arm.com ([217.140.101.70]:60330 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726546AbfE1QNA (ORCPT ); Tue, 28 May 2019 12:13:00 -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 63936341; Tue, 28 May 2019 09:12:59 -0700 (PDT) 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 2B63A3F59C; Tue, 28 May 2019 09:12:57 -0700 (PDT) Date: Tue, 28 May 2019 17:12:54 +0100 From: Mark Rutland To: Will Deacon Cc: Peter Zijlstra , Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk, 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 Subject: Re: [PATCH] perf: Fix oops when kthread execs user process Message-ID: <20190528161254.GA28492@lakrids.cambridge.arm.com> References: <1559046689-24091-1-git-send-email-92siuyang@gmail.com> <20190528140103.GT2623@hirez.programming.kicks-ass.net> <20190528153224.GE20758@fuggles.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190528153224.GE20758@fuggles.cambridge.arm.com> User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 28, 2019 at 04:32:24PM +0100, Will Deacon wrote: > On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote: > > On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote: > > > When a kthread calls call_usermodehelper() the steps are: > > > 1. allocate current->mm > > > 2. load_elf_binary() > > > 3. populate current->thread.regs > > > > > > While doing this, interrupts are not disabled. If there is a perf > > > interrupt in the middle of this process (i.e. step 1 has completed > > > but not yet reached to step 3) and if perf tries to read userspace > > > regs, kernel oops. > > This seems to be because pt_regs(current) gives NULL for kthreads on Power. I think you mean task_pt_regs(current) here. > > > Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace > > > pt_regs are not set. > > > > > > See commit bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs > > > user process") for details. > > > > Why the hell do we set current->mm before it is complete? Note that > > normally exec() builds the new mm before attaching it, see exec_mmap() > > in flush_old_exec(). > > From the initial report [1], it doesn't look like the mm isn't initialised, > but rather than we're dereferencing a NULL pt_regs pointer somehow for the > current task (see previous comment). I don't see how that can happen on > arm64, given that we put the pt_regs on the kernel stack which is allocated > during fork. > > Will > > [1] https://git.kernel.org/linus/bf05fc25f268 One caveat is that for the idle threads, the initial SP overlaps the task_pt_regs() area: * __primary_switched starts SP at init_thread_union + THREAD_SIZE. * __cpu_up() starts SP at task_stack_page(idle) + THREAD_SIZE. ... and in either case, sampling that would be bad. For both arm, I believe similar holds true. AFAICT x86 seems to reserve the regs area in its head_{64,32}.S, but I can't see what it does for other threads. Regardless, for arm, arm64, and x86, task_pt_regs(current) cannot be NULL. Thanks, Mark.