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=-14.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL autolearn=no 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 B190EC433E9 for ; Mon, 1 Mar 2021 17:01:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 88736650FD for ; Mon, 1 Mar 2021 17:01:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235960AbhCARBn (ORCPT ); Mon, 1 Mar 2021 12:01:43 -0500 Received: from linux.microsoft.com ([13.77.154.182]:34560 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236027AbhCAQ7f (ORCPT ); Mon, 1 Mar 2021 11:59:35 -0500 Received: from [192.168.254.32] (unknown [47.187.194.202]) by linux.microsoft.com (Postfix) with ESMTPSA id DA81A20B57A1; Mon, 1 Mar 2021 08:58:51 -0800 (PST) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com DA81A20B57A1 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1614617932; bh=5iLiT4zCUvpdCRGwIN7OylfYUPKmTAZ9Z3KeVffXDOc=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=dS1CAaa3yJ0UPOnsU90ZhqIFUaGnM5whBFQxd4ni8FzqozwSNdhHqCBBMfo4UpdRZ yNWiIqBtGNJ7FyPngJzbZcDNLT1EqeU3tLQjuKaN0CVz+YPzSYOuTqxKLDikiqRP09 sQglsDNjAYMPV/CAe9LfgyevWmJNRu5VzfExbUXM= Subject: Re: [RFC PATCH v1 1/1] arm64: Unwinder enhancements for reliable stack trace To: Mark Rutland Cc: broonie@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210223181243.6776-1-madvenka@linux.microsoft.com> <20210223181243.6776-2-madvenka@linux.microsoft.com> <20210224121716.GE50741@C02TD0UTHF1T.local> <4a96b31d-0d16-1f12-fa64-5fdae64cd2d1@linux.microsoft.com> <20210225115825.GB37015@C02TD0UTHF1T.local> From: "Madhavan T. Venkataraman" Message-ID: <0227d386-c392-eb5a-3f52-621a637e46a8@linux.microsoft.com> Date: Mon, 1 Mar 2021 10:58:51 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210225115825.GB37015@C02TD0UTHF1T.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On 2/25/21 5:58 AM, Mark Rutland wrote: > On Wed, Feb 24, 2021 at 01:34:09PM -0600, Madhavan T. Venkataraman wrote: >> On 2/24/21 6:17 AM, Mark Rutland wrote: >>> On Tue, Feb 23, 2021 at 12:12:43PM -0600, madvenka@linux.microsoft.com wrote: >>>> From: "Madhavan T. Venkataraman" >>>> Termination >>>> =========== >>>> >>>> Currently, the unwinder terminates when both the FP (frame pointer) >>>> and the PC (return address) of a frame are 0. But a frame could get >>>> corrupted and zeroed. There needs to be a better check. >>>> >>>> The following special terminating frame and function have been >>>> defined for this purpose: >>>> >>>> const u64 arm64_last_frame[2] __attribute__ ((aligned (16))); >>>> >>>> void arm64_last_func(void) >>>> { >>>> } >>>> >>>> So, set the FP to arm64_last_frame and the PC to arm64_last_func in >>>> the bottom most frame. >>> >>> My expectation was that we'd do this per-task, creating an empty frame >>> record (i.e. with fp=NULL and lr=NULL) on the task's stack at the >>> instant it was created, and chaining this into x29. That way the address >>> is known (since it can be derived from the task), and the frame will >>> also implicitly check that the callchain terminates on the task stack >>> without loops. That also means that we can use it to detect the entry >>> code going wrong (e.g. if the SP gets corrupted), since in that case the >>> entry code would place the record at a different location. >> >> That is exactly what this is doing. arm64_last_frame[] is a marker frame >> that contains fp=0 and pc=0. > > Almost! What I meant was that rather that each task should have its own > final/marker frame record on its task task rather than sharing a common > global variable. > > That way a check for that frame record implicitly checks that a task > started at the expected location *on that stack*, which catches > additional stack corruption cases (e.g. if we left data on the stack > prior to an EL0 entry). > Ok. I will think about this. > [...] > >>> ... I reckon once we've moved the last of the exception triage out to C >>> this will be relatively simple, since all of the exception handlers will >>> look like: >>> >>> | SYM_CODE_START_LOCAL(elX_exception) >>> | kernel_entry X >>> | mov x0, sp >>> | bl elX_exception_handler >>> | kernel_exit X >>> | SYM_CODE_END(elX_exception) >>> >>> ... and so we just need to identify the set of elX_exception functions >>> (which we'll never expect to take exceptions from directly). We could be >>> strict and reject unwinding into arbitrary bits of the entry code (e.g. >>> if we took an unexpected exception), and only permit unwinding to the >>> BL. >>> >>>> It can do this if the FP and PC are also recorded elsewhere in the >>>> pt_regs for comparison. Currently, the FP is also stored in >>>> regs->regs[29]. The PC is stored in regs->pc. However, regs->pc can >>>> be changed by lower level functions. >>>> >>>> Create a new field, pt_regs->orig_pc, and record the return address >>>> PC there. With this, the unwinder can validate the exception frame >>>> and set a flag so that the caller of the unwinder can know when >>>> an exception frame is encountered. >>> >>> I don't understand the case you're trying to solve here. When is >>> regs->pc changed in a way that's problematic? >>> >> >> For instance, I used a test driver in which the driver calls a function >> pointer which is NULL. The low level fault handler sends a signal to the >> task. Looks like it changes regs->pc for this. > > I'm struggling to follow what you mean here. > > If the kernel branches to NULL, the CPU should raise an instruction > abort from the current EL, and our handling of that should terminate the > thread via die_kernel_fault(), without returning to the faulting > context, and without altering the PC in the faulting context. > > Signals are delivered to userspace and alter the userspace PC, not a > kernel PC, so this doesn't seem relevant. Do you mean an exception fixup > handler rather than a signal? > >> When I dump the stack from the low level handler, the comparison with >> regs->pc does not work. But comparison with regs->orig_pc works. > > As above, I'm struggling with this; could you share a concrete example? > Actually, my bad. I needed the orig_pc because of something that my test driver did. And, it slipped my mind entirely. Thanks for pointing it out. I will fix this in my resend. Thanks again for your comments. I am currently studying probing/tracing. As soon as I have a solution for that, I will send out the next version. I look forward to the in-depth review. Thanks, Madhavan