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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 639A5C4332F for ; Sun, 29 Oct 2023 22:01:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230169AbjJ2WB6 convert rfc822-to-8bit (ORCPT ); Sun, 29 Oct 2023 18:01:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56604 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230338AbjJ2WBz (ORCPT ); Sun, 29 Oct 2023 18:01:55 -0400 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 003E9B7 for ; Sun, 29 Oct 2023 15:01:49 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-170-8fg6BVyCOq2NOYcIuwnfBQ-1; Sun, 29 Oct 2023 22:01:47 +0000 X-MC-Unique: 8fg6BVyCOq2NOYcIuwnfBQ-1 Received: from AcuMS.Aculab.com (10.202.163.4) by AcuMS.aculab.com (10.202.163.4) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Sun, 29 Oct 2023 22:01:58 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Sun, 29 Oct 2023 22:01:58 +0000 From: David Laight To: 'Peter Zijlstra' , Jens Axboe CC: Ingo Molnar , LKML Subject: RE: lockdep: holding locks across syscall boundaries Thread-Topic: lockdep: holding locks across syscall boundaries Thread-Index: AQHaCO6j4ltnKj0zt0q1hvBGfTxP87BhVBWw Date: Sun, 29 Oct 2023 22:01:58 +0000 Message-ID: <1543b5a7ec2d4524bf8c081061048a97@AcuMS.aculab.com> References: <20231027155949.GA26550@noisy.programming.kicks-ass.net> In-Reply-To: <20231027155949.GA26550@noisy.programming.kicks-ass.net> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Peter Zijlstra > Sent: 27 October 2023 17:00 > > On Fri, Oct 27, 2023 at 09:14:53AM -0600, Jens Axboe wrote: > > Hi, > > > > Normally we'd expect locking state to be clean and consistent across > > syscall entry and exit, as that is always the case for sync syscalls. > > > We currently have a work-around for holding a lock from aio, see > > kiocb_start_write(), which pretends to drop the lock from lockdeps > > perspective, as it's held from submission to until kiocb_end_write() is > > called at completion time. > > I was not aware of this, the only such hack I knew about was the > filesystem freezer thing. > > The problem with holding locks past the end of a syscall is that you'll > nest whatever random lock hierarchies possibly by every other syscall > under that lock. > ... > > Suppose syscall-a returns with your kiocb thing held, call it lock A > Suppose syscall-b returns with your inode thing held, call it lock B > > Then userspace does: > > syscall-a > syscall-b > > while it also does: > > syscall-b > syscall-a > > and we're up a creek, no? Isn't it also open to a massive denial-of-service attack? syscall-a sleep(infinity) assuming you actually catch: syscall-a _exit() David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)