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=-2.5 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT 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 6383CC282DD for ; Thu, 23 May 2019 16:13:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2AA7621773 for ; Thu, 23 May 2019 16:13:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=armh.onmicrosoft.com header.i=@armh.onmicrosoft.com header.b="TI/RCTTw" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731127AbfEWQND (ORCPT ); Thu, 23 May 2019 12:13:03 -0400 Received: from mail-eopbgr60076.outbound.protection.outlook.com ([40.107.6.76]:39480 "EHLO EUR04-DB3-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730782AbfEWQND (ORCPT ); Thu, 23 May 2019 12:13:03 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=armh.onmicrosoft.com; s=selector2-armh-onmicrosoft-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=v0+os/sdMMXz7DI3IasTQ/bX6qmv//gAQT6tXiuRggI=; b=TI/RCTTwk/erlf+r9ZDvEXAgbP0TM7Yiy18Xl/4priFSAR4PyUy/o0XjsrH88xVHBVta5YDhWLCrLLr9fS+enrukq3onMmKiamGt+bmd43SCb1N843UvlBiQmJ2y9KYmO+tvTCgyXQ/YxywrbfNjA78Jp6DhPJqjzYj0za+SWfA= Received: from VE1PR08MB4847.eurprd08.prod.outlook.com (10.255.113.87) by VE1PR08MB4847.eurprd08.prod.outlook.com (10.255.113.87) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.1900.16; Thu, 23 May 2019 16:12:59 +0000 Received: from VE1PR08MB4847.eurprd08.prod.outlook.com ([fe80::1534:1364:767d:777b]) by VE1PR08MB4847.eurprd08.prod.outlook.com ([fe80::1534:1364:767d:777b%2]) with mapi id 15.20.1900.010; Thu, 23 May 2019 16:12:59 +0000 From: Dave P Martin To: "Eric W. Biederman" CC: "linux-kernel@vger.kernel.org" , Linux Containers , Oleg Nesterov , "linux-arch@vger.kernel.org" , James Morse , Will Deacon Subject: Re: [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig_fault for SIGKILL Thread-Topic: [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig_fault for SIGKILL Thread-Index: AQHVEQAmROT4Tsc0Y02QCkja63yvnaZ4kTWAgAA7TwyAABY/AA== Date: Thu, 23 May 2019 16:12:59 +0000 Message-ID: <20190523161256.GF2019@e103592.cambridge.arm.com> References: <20190523003916.20726-1-ebiederm@xmission.com> <20190523003916.20726-4-ebiederm@xmission.com> <20190523102101.GW28398@e103592.cambridge.arm.com> <87r28pgr3h.fsf@xmission.com> In-Reply-To: <87r28pgr3h.fsf@xmission.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: user-agent: Mutt/1.5.23 (2014-03-12) x-originating-ip: [217.140.106.49] x-clientproxiedby: LO2P265CA0455.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:e::35) To VE1PR08MB4847.eurprd08.prod.outlook.com (2603:10a6:802:a6::23) authentication-results: spf=none (sender IP is ) smtp.mailfrom=Dave.Martin@arm.com; x-ms-exchange-messagesentrepresentingtype: 1 x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 5fb12db9-812e-4730-b50b-08d6df9985d1 x-ms-office365-filtering-ht: Tenant x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600141)(711020)(4605104)(4618075)(2017052603328)(7193020);SRVR:VE1PR08MB4847; x-ms-traffictypediagnostic: VE1PR08MB4847: x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:9508; x-forefront-prvs: 00462943DE x-forefront-antispam-report: SFV:NSPM;SFS:(10009020)(346002)(39860400002)(136003)(396003)(376002)(366004)(40434004)(199004)(189003)(386003)(66476007)(73956011)(7736002)(102836004)(6506007)(5024004)(14444005)(229853002)(66946007)(64756008)(305945005)(68736007)(66556008)(26005)(6916009)(186003)(316002)(81156014)(66446008)(256004)(3846002)(86362001)(2906002)(81166006)(8676002)(52116002)(6436002)(6116002)(99286004)(66066001)(58126008)(76176011)(54906003)(6486002)(33656002)(25786009)(71200400001)(71190400001)(11346002)(1076003)(72206003)(4326008)(8936002)(478600001)(53936002)(486006)(5660300002)(446003)(6512007)(14454004)(476003)(6246003);DIR:OUT;SFP:1101;SCL:1;SRVR:VE1PR08MB4847;H:VE1PR08MB4847.eurprd08.prod.outlook.com;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;A:1;MX:1; received-spf: None (protection.outlook.com: arm.com does not designate permitted sender hosts) x-ms-exchange-senderadcheck: 1 x-microsoft-antispam-message-info: qnNJ8qDxcRqGVsUixjKvKgHaLdkkdlhhuC0RTafqhEfARbN2TTpa+zn/+u/v3Mb3+jImqirypD5wrY/axlg8wcAHZw6IVw8WzpMob91VHyW0IZMHlCHoZuH4fCTtxxWlZO1UWriDqC4vkGQqkb8Sy8t2j7orts9Q6PTRUOF9EshJbH61UaEP4YO7RJxVevAxh+rPIQehcUPJEXp//pGn/U7YL+XYxiWVR9Ju2gas49e8zskZhjnm6363m5Oey2MCxg7NzoghkClzc8R62pJj+8xdeEUInLwLbGw2CvGAdTWQ+e3U9oMwZbOR0dwm79d0fAfjVSh1H7zTtmpUzBtVkg/4dbGqX/olQT89SDVZNUaDLjAoD3q+2874QcF2s9+e5V0LWWigQyztZcFhF7KKv5lGAM+3wDsSXi2IQhH3t1Y= Content-Type: text/plain; charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-Network-Message-Id: 5fb12db9-812e-4730-b50b-08d6df9985d1 X-MS-Exchange-CrossTenant-originalarrivaltime: 23 May 2019 16:12:59.0689 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-Transport-CrossTenantHeadersStamped: VE1PR08MB4847 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote: > Dave Martin writes: > > > On Thu, May 23, 2019 at 01:38:53AM +0100, Eric W. Biederman wrote: > >> It really only matters to debuggers but the SIGKILL does not have any > >> si_codes that use the fault member of the siginfo union. Correct this > >> the simple way and call force_sig instead of force_sig_fault when the > >> signal is SIGKILL. > > > > I haven't fully understood the context for this, but why does it matter > > what's in siginfo for SIGKILL? My understanding is that userspace > > (including ptrace) never gets to see it anyway for the SIGKILL case. > > Yes. In practice I think it would take tracing or something very > exotic to notice anything going wrong because the task will be killed. > > > Here it feels like SIGKILL is logically a synchronous, thread-targeted > > fault: we must ensure that no subsequent insn in current executes (just > > like other fault signal). In this case, I thought we fall back to > > SIGKILL not because there is no fault, but because we failed to > > properly diagnose or report the type of fault that occurred. > > > > So maybe handling it consistently with other faults signals makes > > sense. The fact that delivery of this signal destroys the process > > before anyone can look at the resulting siginfo feels like a > > side-effect rather than something obviously wrong. > > > > The siginfo is potentially useful diagnostic information, that we could > > subsequently provide a means to access post-mortem. > > > > I just dived in on this single patch, so I may be missing something mor= e > > fundamental, or just being pedantic... > > Not really. I was working on another cleanup and this usage of SIGKILL > came up. > > A synchronous thread synchronous fault gets us as far as the forc_sig > family of functions. That only leaves the question of which union > member in struct siginfo we are using. The union members are _kill, > _fault, _timer, _rt, _sigchld, _sigfault, _sigpoll, and _sigsys. > > As it has prove quite error prone for people to fill out struct siginfo > in the past by hand, I have provided a couple of helper functions for > the common cases that come up such as: force_sig_fault, > force_sig_mceerr, force_sig_bnderr, force_sig_pkuerr. Each of those > helper functions takes the information needed to fill out the union > member of struct siginfo that kind of fault corresponds to. > > For the SIGKILL case the only si_code I see being passed SI_KERNEL. > The SI_KERNEL si_code corresponds to the _kill union member while > force_sig_fault fills in fields for the _fault union member. > > Because of the mismatch of which union member SIGKILL should be using > and the union member force_sig_fault applies alarm bells ring in my head > when I read the current arm64 kernel code. Somewhat doubly so because > the other fields in passed to force_sig_fault appear to be somewhat > random when SIGKILL is the signal. > > So I figured let's preserve the usage of SIGKILL as a synchronous > exception. That seems legitimate and other folks do that as well but > let's use force_sig instead of force_sig_fault instead. I don't know if > userspace will notice but at the very least we won't be providing a bad > example for other kernel code to follow and we won't wind up be making > assumptions that are true today and false tomorrow when some > implementation detail changes. > > For imformation on what signals and si_codes correspond to which > union members you can look at siginfo_layout. That function > is the keeper of the magic decoder key. Currently the only two > si_codes defined for SIGKILL are SI_KERNEL and SI_USER both of which > correspond to a _kill union member. I see. Assuming we cannot have a dummy internal si_code for this special case (probably a bad idea), I think Will's suggestion of at least pushing the special case handling down into arm64_force_sig_fault() is probably a bit cleaner here, expecially if other callers of that function may pass in SIGKILL (I haven't looked though). Cheers ---Dave IMPORTANT NOTICE: The contents of this email and any attachments are confid= ential and may also be privileged. If you are not the intended recipient, p= lease notify the sender immediately and do not disclose the contents to any= other person, use it for any purpose, or store or copy the information in = any medium. Thank you.