From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B2606C432C0 for ; Fri, 29 Nov 2019 18:16:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 925712158A for ; Fri, 29 Nov 2019 18:16:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727010AbfK2SQy (ORCPT ); Fri, 29 Nov 2019 13:16:54 -0500 Received: from mx2.suse.de ([195.135.220.15]:44892 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727107AbfK2SQx (ORCPT ); Fri, 29 Nov 2019 13:16:53 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 79ED4B469; Fri, 29 Nov 2019 18:16:51 +0000 (UTC) Date: Fri, 29 Nov 2019 19:16:50 +0100 (CET) From: Miroslav Benes To: Vasily Gorbik cc: heiko.carstens@de.ibm.com, borntraeger@de.ibm.com, jpoimboe@redhat.com, joe.lawrence@redhat.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, jikos@kernel.org, pmladek@suse.com, nstange@suse.de, live-patching@vger.kernel.org Subject: Re: [PATCH v4 2/2] s390/livepatch: Implement reliable stack tracing for the consistency model In-Reply-To: Message-ID: References: <20191106095601.29986-5-mbenes@suse.cz> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org Archived-At: List-Archive: List-Post: On Fri, 29 Nov 2019, Vasily Gorbik wrote: > From: Miroslav Benes > > The livepatch consistency model requires reliable stack tracing > architecture support in order to work properly. In order to achieve > this, two main issues have to be solved. First, reliable and consistent > call chain backtracing has to be ensured. Second, the unwinder needs to > be able to detect stack corruptions and return errors. > > The "zSeries ELF Application Binary Interface Supplement" says: > > "The stack pointer points to the first word of the lowest allocated > stack frame. If the "back chain" is implemented this word will point to > the previously allocated stack frame (towards higher addresses), except > for the first stack frame, which shall have a back chain of zero (NULL). > The stack shall grow downwards, in other words towards lower addresses." > > "back chain" is optional. GCC option -mbackchain enables it. Quoting > Martin Schwidefsky [1]: > > "The compiler is called with the -mbackchain option, all normal C > function will store the backchain in the function prologue. All > functions written in assembler code should do the same, if you find one > that does not we should fix that. The end result is that a task that > *voluntarily* called schedule() should have a proper backchain at all > times. > > Dependent on the use case this may or may not be enough. Asynchronous > interrupts may stop the CPU at the beginning of a function, if kernel > preemption is enabled we can end up with a broken backchain. The > production kernels for IBM Z are all compiled *without* kernel > preemption. So yes, we might get away without the objtool support. > > On a side-note, we do have a line item to implement the ORC unwinder for > the kernel, that includes the objtool support. Once we have that we can > drop the -mbackchain option for the kernel build. That gives us a nice > little performance benefit. I hope that the change from backchain to the > ORC unwinder will not be too hard to implement in the livepatch tools." > > Since -mbackchain is enabled by default when the kernel is compiled, the > call chain backtracing should be currently ensured and objtool should > not be necessary for livepatch purposes. > > Regarding the second issue, stack corruptions and non-reliable states > have to be recognized by the unwinder. Mainly it means to detect > preemption or page faults, the end of the task stack must be reached, > return addresses must be valid text addresses and hacks like function > graph tracing and kretprobes must be properly detected. > > Unwinding a running task's stack is not a problem, because there is a > livepatch requirement that every checked task is blocked, except for the > current task. Due to that, we can consider a task's kernel/thread stack > only and skip the other stacks. > > [1] 20180912121106.31ffa97c@mschwideX1 [not archived on lore.kernel.org] > > Signed-off-by: Vasily Gorbik Signed-off-by: Miroslav Benes M