From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752478AbdCHK14 (ORCPT ); Wed, 8 Mar 2017 05:27:56 -0500 Received: from mail-pg0-f68.google.com ([74.125.83.68]:33368 "EHLO mail-pg0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752044AbdCHK1s (ORCPT ); Wed, 8 Mar 2017 05:27:48 -0500 Message-ID: <1488966437.13674.2.camel@gmail.com> Subject: Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting reliable stack traces From: Balbir Singh To: Josh Poimboeuf Cc: Jessica Yu , Jiri Kosina , Miroslav Benes , Petr Mladek , linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, Michael Ellerman , Heiko Carstens , x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, Vojtech Pavlik , Jiri Slaby , Chris J Arges , Andy Lutomirski , Ingo Molnar , Peter Zijlstra , Kamalesh Babulal Date: Wed, 08 Mar 2017 20:47:17 +1100 In-Reply-To: <20170307161230.44rvjm7ed7azgu6x@treble> References: <1488869455.4285.10.camel@gmail.com> <20170307161230.44rvjm7ed7azgu6x@treble> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.5-1ubuntu1 Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2017-03-07 at 10:12 -0600, Josh Poimboeuf wrote: > On Tue, Mar 07, 2017 at 05:50:55PM +1100, Balbir Singh wrote: > > On Mon, 2017-02-13 at 19:42 -0600, Josh Poimboeuf wrote: > > > For live patching and possibly other use cases, a stack trace is only > > > useful if it can be assured that it's completely reliable.  Add a new > > > save_stack_trace_tsk_reliable() function to achieve that. > > >  > > > Note that if the target task isn't the current task, and the target task > > > is allowed to run, then it could be writing the stack while the unwinder > > > is reading it, resulting in possible corruption.  So the caller of > > > save_stack_trace_tsk_reliable() must ensure that the task is either > > > 'current' or inactive. > > >  > > > save_stack_trace_tsk_reliable() relies on the x86 unwinder's detection > > > of pt_regs on the stack.  If the pt_regs are not user-mode registers > > > from a syscall, then they indicate an in-kernel interrupt or exception > > > (e.g. preemption or a page fault), in which case the stack is considered > > > unreliable due to the nature of frame pointers. > > >  > > > It also relies on the x86 unwinder's detection of other issues, such as: > > >  > > > - corrupted stack data > > > - stack grows the wrong way > > > - stack walk doesn't reach the bottom > > > - user didn't provide a large enough entries array > > >  > > > Such issues are reported by checking unwind_error() and !unwind_done(). > > >  > > > Also add CONFIG_HAVE_RELIABLE_STACKTRACE so arch-independent code can > > > determine at build time whether the function is implemented. > > >  > > > Signed-off-by: Josh Poimboeuf > > > --- > >  > > Could you comment on why we need a reliable trace for live-patching? Are > > we in any way reliant on the stack trace to patch something broken? >  > I tried to cover this comprehensively in patch 13/15 in > Documentation/livepatch/livepatch.txt.  Does that answer your questions? > Yes, it answers my questions Thanks, Balbir Singh.