From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from out01.mta.xmission.com ([166.70.13.231]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1njiws-001rV2-UV for linux-um@lists.infradead.org; Wed, 27 Apr 2022 14:47:24 +0000 From: "Eric W. Biederman" 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> MIME-Version: 1.0 Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-um" Errors-To: linux-um-bounces+geert=linux-m68k.org@lists.infradead.org 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 "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 _______________________________________________ linux-um mailing list linux-um@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-um