From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-x243.google.com (mail-pf0-x243.google.com [IPv6:2607:f8b0:400e:c00::243]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3vcnP55rWwzDq8g for ; Tue, 7 Mar 2017 17:51:05 +1100 (AEDT) Received: by mail-pf0-x243.google.com with SMTP id j5so20116669pfb.3 for ; Mon, 06 Mar 2017 22:51:05 -0800 (PST) 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" Mime-Version: 1.0 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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.