From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755220AbdCGI6K (ORCPT ); Tue, 7 Mar 2017 03:58:10 -0500 Received: from mail-pf0-f193.google.com ([209.85.192.193]:36069 "EHLO mail-pf0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754978AbdCGI6A (ORCPT ); Tue, 7 Mar 2017 03:58:00 -0500 Message-ID: <1488869455.4285.10.camel@gmail.com> Subject: Re: [PATCH v5 01/15] stacktrace/x86: add function for detecting reliable stack traces From: Balbir Singh To: Josh Poimboeuf , Jessica Yu , Jiri Kosina , Miroslav Benes , Petr Mladek Cc: 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: Tue, 07 Mar 2017 17:50:55 +1100 In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.5 (3.22.5-1.fc25) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Thanks, Balbir Singh.