Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757929AbcDHI6b (ORCPT ); Fri, 8 Apr 2016 04:58:31 -0400 Received: from mail-pa0-f49.google.com ([209.85.220.49]:32772 "EHLO mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757400AbcDHI62 (ORCPT ); Fri, 8 Apr 2016 04:58:28 -0400 Date: Fri, 8 Apr 2016 14:28:24 +0530 From: Pratyush Anand To: Li Bin Cc: He Kuang , mark.rutland@arm.com, yang.shi@linaro.org, wangnan0@huawei.com, marc.zyngier@arm.com, catalin.marinas@arm.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, richard@nod.at, james.morse@arm.com, hanjun.guo@linaro.org, gregkh@linuxfoundation.org, Dave.Martin@arm.com, linux-arm-kernel@lists.infradead.org, Hanjun Guo , Ding Tianhong Subject: Re: [PATCH 2/2] arm64: Fix watchpoint recursion when single-step is wrongly triggered in irq Message-ID: <20160408085824.GC28371@dhcppc6.redhat.com> References: <1458549470-124791-1-git-send-email-hekuang@huawei.com> <1458549470-124791-2-git-send-email-hekuang@huawei.com> <20160321102423.GB15150@dhcppc6.redhat.com> <56FD1BD1.7070101@huawei.com> <20160404051714.GH28435@dhcppc0.redhat.com> <570645CD.6030400@huawei.com> <20160408051408.GA28371@dhcppc6.redhat.com> <570766B0.60100@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <570766B0.60100@huawei.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4940 Lines: 102 On 08/04/2016:04:07:12 PM, Li Bin wrote: > > > on 2016/4/8 13:14, Pratyush Anand wrote: > > Hi Li, > > > > On 07/04/2016:07:34:37 PM, Li Bin wrote: > >> Hi Pratyush, > >> > >> on 2016/4/4 13:17, Pratyush Anand wrote: > >>> Hi Li, > >>> > >>> On 31/03/2016:08:45:05 PM, Li Bin wrote: > >>>> Hi Pratyush, > >>>> > >>>> on 2016/3/21 18:24, Pratyush Anand wrote: > >>>>> On 21/03/2016:08:37:50 AM, He Kuang wrote: > >>>>>> On arm64, watchpoint handler enables single-step to bypass the next > >>>>>> instruction for not recursive enter. If an irq is triggered right > >>>>>> after the watchpoint, a single-step will be wrongly triggered in irq > >>>>>> handler, which causes the watchpoint address not stepped over and > >>>>>> system hang. > >>>>> > >>>>> Does patch [1] resolves this issue as well? I hope it should. Patch[1] has still > >>>>> not been sent for review. Your test result will be helpful. > >>>>> > >>>>> ~Pratyush > >>>>> > >>>>> [1] https://github.com/pratyushanand/linux/commit/7623c8099ac22eaa00e7e0f52430f7a4bd154652 > >>>> > >>>> This patch did not consider that, when excetpion return, the singlestep flag > >>>> should be restored, otherwise the right singlestep will not triggered. > >>>> Right? > >>> > >>> Yes, you are right, and there are other problems as well. Will Deacon pointed > >>> out [1] that kernel debugging is per-cpu rather than per-task. So, I did thought > >>> of a per-cpu implementation by introducing a new element "flags" in struct > >>> pt_regs. But even with that I see issues. For example: > >>> - While executing single step instruction, we get a data abort > >>> - In the kernel_entry of data abort we disable single stepping based on "flags" > >>> bit field > >>> - While handling data abort we receive anther interrupt, so we are again in > >>> kernel_entry (for el1_irq). Single stepping will be disabled again (although > >>> it does not matter). > >>> > >>> Now the issue is that, what condition should be verified in kernel_exit for > >>> enabling single step again? In the above scenario, kernel_exit for el1_irq > >>> should not enable single stepping, but how to prevent that elegantly? > >> > >> The condition for kernel_entry to disable the single step is that MDSCR_EL1.SS > >> has been set. And only when the corresponding kernel_entry has disabled the single > >> step, the kernel_exit should enable it, but the kernel_exit of single-step exception > >> should be handled specially, that when disable single step in single-step exception > >> handler, flag of pt_regs stored in stack should be cleard to prevent to be re-enabled > >> by kernel_exit. > > > > Nice, :-) > > I had latter on almost similar patch [1], but it did fail when I merged two of > > the tests. > > -- I inserted kprobe to an instruction in function __copy_to_user() which could > > generate data abort. > > -- In parallel I also run test case which is defined here [2] > > -- As soon as I did `cat /proc/version`, kernel crashed. > > Hi Pratyush, > > Firstly, I have test this case, and it does not trigger failture as you describing. > But it indeed may trigger problem, and it is an another issue that if an exception > triggered before single-step exception, changes the regs->pc (data abort exception will > fixup_exception), the current implemetion of kprobes does not support, for example: Yes, you are right, I missed it. All those aborts which has a fixup defined, will fail. While, I did not see any issue when running test individually, ie only hitting kprobe at __copy_to_user() instructions, because there is no fixup for them. I was able to trace instruction which was aborting. Problem occurred only when I run perf memory read to linux_proc_banner in parallel. Since I do not see failure due to fixup_exception in this test case, so I think we are missing some more pitfalls. But certainly it is going to fail in the case __get_user/__put_user etc are being traced, because there exists a fixup section for them. > 1. kprobes brk exception setup single-step, regs->pc points to the slot, MDSCR.SS=1, > SPSR_EL1.SS=1 (Inactive state) > 2. brk exception eret (Active-not-pending state) > 3. execute the slot instruction and trigger data abort exception, and this case the > return addr is also the slot instruction, so the SPSR_EL1.SS is set to 1 (Inactive state) > 4. but in the data abort exception, fixup_exception will change the regs->pc to the fixup Yes, for the instructions with fixup defined. > code > 5. data abort exception eret, going into Active-not-pending state, executing fixup code > without taking an exception, going into Active-pending state, triggering single-step > exception. But the single-step instruction is not the target instrution, so kprobe fails. > > And so this case including copy_to/from_user should be added to kprobes blacklist. > Right, or am i missing something? As of now, we are be going with blacklisting approach only. ~Pratyush