From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750907AbdGZFgI (ORCPT ); Wed, 26 Jul 2017 01:36:08 -0400 Received: from mail-pg0-f54.google.com ([74.125.83.54]:37912 "EHLO mail-pg0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750711AbdGZFgH (ORCPT ); Wed, 26 Jul 2017 01:36:07 -0400 Subject: Re: [PATCH V2 4/4] arm64: disable irq between breakpoint and step exception To: Will Deacon Cc: mark.rutland@arm.com, huawei.libin@huawei.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <8f9c3073792c107e3d928e413499572ec75c5175.1499416107.git.panand@redhat.com> <20170725132551.GB28815@arm.com> From: Pratyush Anand Message-ID: Date: Wed, 26 Jul 2017 11:06:02 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170725132551.GB28815@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Will, Thanks for your review. On Tuesday 25 July 2017 06:55 PM, Will Deacon wrote: > On Fri, Jul 07, 2017 at 05:34:00PM +0530, Pratyush Anand wrote: >> If an interrupt is generated between breakpoint and step handler then >> step handler can not get correct step address. This situation can easily >> be invoked by samples/hw_breakpoint/data_breakpoint.c. It can also be >> reproduced if we insert any printk() statement or dump_stack() in perf >> overflow_handler. So, it seems that perf is working fine just luckily. >> If the CPU which is handling perf breakpoint handler receives any >> interrupt then, perf step handler will not execute sanely. >> >> This patch improves do_debug_exception() handling, which enforces now, >> that exception handler function: >> - should return 0 for any software breakpoint and hw >> breakpoint/watchpoint handler if it does not expect a single step stage >> - should return 1 if it expects single step. >> - A single step handler should always return 0. >> - All handler should return a -ve error in any other case. >> >> Now, we can know in do_debug_exception() that whether a step exception >> will be followed or not. If there will a step exception then disable >> irq. Re-enable it after single step handling. > > AFAICT, this is only a problem for kernel-mode breakpoints where we end up > stepping into the interrupt handler when trying to step over a breakpoint. I think yes. > > We'd probably be better off getting all users of kernel step (kprobes, kgdb > and perf) to run the step with irqs disabled, That should be doable. We can easily manage all of them in do_debug_exception() if individual brk handlers return correct value as per the rule mentioned in the commit log of this patch. I think, I can take care of kprobes and kgdb as well in next version of patch. > but I still have reservations > about that: So, IIUC, you have concern about faulting of a instruction being stepped. Since we will have a notion of *irq_en_needed*, so I think, if needed we can re-enable interrupt in fault handler do_mem_abort(). Whats your opinion here? > > http://lists.infradead.org/pipermail/linux-arm-kernel/2017-May/508066.html > http://lists.infradead.org/pipermail/linux-arm-kernel/2017-June/510814.html > > Wouldn't it be better to follow kprobes/kgdb and have perf run the step with > irqs disabled? -- Regards Pratyush