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 0F5CDC4707A for ; Fri, 21 May 2021 19:42:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E0E73613D9 for ; Fri, 21 May 2021 19:42:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239193AbhEUTnW (ORCPT ); Fri, 21 May 2021 15:43:22 -0400 Received: from linux.microsoft.com ([13.77.154.182]:48848 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234945AbhEUTnV (ORCPT ); Fri, 21 May 2021 15:43:21 -0400 Received: from [192.168.254.32] (unknown [47.187.214.213]) by linux.microsoft.com (Postfix) with ESMTPSA id 81D2120B7188; Fri, 21 May 2021 12:41:57 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 81D2120B7188 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1621626118; bh=sVvcxbQSfAemZf/h/BWHu58ipxMHsNFFKvAp7FMao3I=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=NI/hYFDEvvsCn23V5INEa9b8dTn2BppIDjflkBxVCslto7/I7DbLEs2m9XN5og4yC oHP5is8Jke0hB6M2MISMT2cjSTuef18UEjo+FwH+/YQU1DLlSOTA032lcn4cZ4uoBC FBpmEg7f1Xy2eFtvPKyJiYl6l5sDLEOC0CYv3TPg= Subject: Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability checks in the unwinder To: Josh Poimboeuf Cc: Mark Brown , mark.rutland@arm.com, ardb@kernel.org, jthierry@redhat.com, catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org, pasha.tatashin@soleen.com, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210516040018.128105-1-madvenka@linux.microsoft.com> <20210516040018.128105-2-madvenka@linux.microsoft.com> <20210521161117.GB5825@sirena.org.uk> <20210521174242.GD5825@sirena.org.uk> <26c33633-029e-6374-16e6-e9418099da95@linux.microsoft.com> <20210521175318.GF5825@sirena.org.uk> <20210521184817.envdg232b2aeyprt@treble> <74d12457-7590-bca2-d1ce-5ff82d7ab0d8@linux.microsoft.com> <20210521191140.4aezpvm2kruztufi@treble> <20210521191608.f24sldzhpg3hyq32@treble> From: "Madhavan T. Venkataraman" Message-ID: Date: Fri, 21 May 2021 14:41:56 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210521191608.f24sldzhpg3hyq32@treble> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/21/21 2:16 PM, Josh Poimboeuf wrote: > On Fri, May 21, 2021 at 02:11:45PM -0500, Josh Poimboeuf wrote: >> On Fri, May 21, 2021 at 01:59:16PM -0500, Madhavan T. Venkataraman wrote: >>> >>> >>> On 5/21/21 1:48 PM, Josh Poimboeuf wrote: >>>> On Fri, May 21, 2021 at 06:53:18PM +0100, Mark Brown wrote: >>>>> On Fri, May 21, 2021 at 12:47:13PM -0500, Madhavan T. Venkataraman wrote: >>>>>> On 5/21/21 12:42 PM, Mark Brown wrote: >>>>> >>>>>>> Like I say we may come up with some use for the flag in error cases in >>>>>>> future so I'm not opposed to keeping the accounting there. >>>>> >>>>>> So, should I leave it the way it is now? Or should I not set reliable = false >>>>>> for errors? Which one do you prefer? >>>>> >>>>>> Josh, >>>>> >>>>>> Are you OK with not flagging reliable = false for errors in unwind_frame()? >>>>> >>>>> I think it's fine to leave it as it is. >>>> >>>> Either way works for me, but if you remove those 'reliable = false' >>>> statements for stack corruption then, IIRC, the caller would still have >>>> some confusion between the end of stack error (-ENOENT) and the other >>>> errors (-EINVAL). >>>> >>> >>> I will leave it the way it is. That is, I will do reliable = false on errors >>> like you suggested. >>> >>>> So the caller would have to know that -ENOENT really means success. >>>> Which, to me, seems kind of flaky. >>>> >>> >>> Actually, that is why -ENOENT was introduced - to indicate successful >>> stack trace termination. A return value of 0 is for continuing with >>> the stack trace. A non-zero value is for terminating the stack trace. >>> >>> So, either we return a positive value (say 1) to indicate successful >>> termination. Or, we return -ENOENT to say no more stack frames left. >>> I guess -ENOENT was chosen. >> >> I see. So it's a tri-state return value, and frame->reliable is >> intended to be a private interface not checked by the callers. > > Or is frame->reliable supposed to be checked after all? Looking at the > code again, I'm not sure. > > Either way it would be good to document the interface more clearly in a > comment above the function. > So, arch_stack_walk_reliable() would do this: start_backtrace(frame); while (...) { if (!frame->reliable) return error; consume_entry(...); ret = unwind_frame(...); if (ret) break; } if (ret == -ENOENT) return success; return error; Something like that. I will add a comment about all of this in the unwinder. Thanks! Madhavan