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 701F0C433EF for ; Wed, 27 Apr 2022 14:48:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238684AbiD0Ovl (ORCPT ); Wed, 27 Apr 2022 10:51:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46478 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238693AbiD0Oud (ORCPT ); Wed, 27 Apr 2022 10:50:33 -0400 Received: from out01.mta.xmission.com (out01.mta.xmission.com [166.70.13.231]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8BFA43B297; Wed, 27 Apr 2022 07:47:21 -0700 (PDT) Received: from in02.mta.xmission.com ([166.70.13.52]:55074) by out01.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1njiwo-00H7U3-OG; Wed, 27 Apr 2022 08:47:18 -0600 Received: from ip68-227-174-4.om.om.cox.net ([68.227.174.4]:35882 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1njiwn-00AnmM-Hj; Wed, 27 Apr 2022 08:47:18 -0600 From: "Eric W. Biederman" To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de, bigeasy@linutronix.de, Will Deacon , tj@kernel.org, linux-pm@vger.kernel.org, Peter Zijlstra , Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Chris Zankel , Max Filippov , inux-xtensa@linux-xtensa.org, Kees Cook , Jann Horn References: <878rrrh32q.fsf_-_@email.froward.int.ebiederm.org> <20220426225211.308418-6-ebiederm@xmission.com> <20220427141018.GA17421@redhat.com> <874k2ea9q4.fsf@email.froward.int.ebiederm.org> Date: Wed, 27 Apr 2022 09:47:10 -0500 In-Reply-To: <874k2ea9q4.fsf@email.froward.int.ebiederm.org> (Eric W. Biederman's message of "Wed, 27 Apr 2022 09:20:51 -0500") Message-ID: <87zgk67fdd.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1njiwn-00AnmM-Hj;;;mid=<87zgk67fdd.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.174.4;;;frm=ebiederm@xmission.com;;;spf=softfail X-XM-AID: U2FsdGVkX1/9mpdQTyplIkPxPVrXFUt3r9iizls3AWY= X-SA-Exim-Connect-IP: 68.227.174.4 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org "Eric W. Biederman" writes: > Oleg Nesterov writes: > >> On 04/26, Eric W. Biederman wrote: >>> >>> @@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk, >>> } >>> >>> sighand = parent->sighand; >>> - spin_lock_irqsave(&sighand->siglock, flags); >>> + lock = tsk->sighand != sighand; >>> + if (lock) >>> + spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING); >> >> But why is it safe? >> >> Suppose we have two tasks, they both trace each other, both call >> ptrace_stop() at the same time. Of course this is ugly, they both >> will block. >> >> But with this patch in this case we have the trivial ABBA deadlock, >> no? > > I was thinking in terms of the process tree (which is fine). > > The ptrace parental relationship definitely has the potential to be a > graph with cycles. Which as you point out is not fine. > > > The result is very nice and I don't want to give it up. I suspect > something ptrace cycles are always a problem and can simply be > forbidden. That is going to take some analsysis and some additional > code in ptrace_attach. > > I will go look at that. Hmm. If we have the following process tree. A \ B \ C Process A, B, and C are all in the same process group. Process A and B are setup to receive SIGCHILD when their process stops. Process C traces process A. When a sigstop is delivered to the group we can have: Process B takes siglock(B) siglock(A) to notify the real_parent Process C takes siglock(C) siglock(B) to notify the real_parent Process A takes siglock(A) siglock(C) to notify the tracer If they all take their local lock at the same time there is a deadlock. I don't think the restriction that you can never ptrace anyone up the process tree is going to fly. So it looks like I am back to the drawing board for this one. Eric