From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from out02.mta.xmission.com ([166.70.13.232]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1njqFg-003nCR-JI for linux-um@lists.infradead.org; Wed, 27 Apr 2022 22:35:18 +0000 From: "Eric W. Biederman" References: <878rrrh32q.fsf_-_@email.froward.int.ebiederm.org> <20220426225211.308418-9-ebiederm@xmission.com> <20220427154158.GG17421@redhat.com> Date: Wed, 27 Apr 2022 17:35:05 -0500 In-Reply-To: <20220427154158.GG17421@redhat.com> (Oleg Nesterov's message of "Wed, 27 Apr 2022 17:41:59 +0200") Message-ID: <87mtg62m06.fsf@email.froward.int.ebiederm.org> MIME-Version: 1.0 Subject: Re: [PATCH 9/9] ptrace: Don't change __state 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 Oleg Nesterov writes: 2> On 04/26, Eric W. Biederman wrote: >> >> static void ptrace_unfreeze_traced(struct task_struct *task) >> { >> - if (READ_ONCE(task->__state) != __TASK_TRACED) >> + if (!(READ_ONCE(task->jobctl) & JOBCTL_DELAY_WAKEKILL)) >> return; >> >> WARN_ON(!task->ptrace || task->parent != current); >> @@ -213,11 +213,10 @@ static void ptrace_unfreeze_traced(struct task_struct *task) >> * Recheck state under the lock to close this race. >> */ >> spin_lock_irq(&task->sighand->siglock); > > Now that we do not check __state = __TASK_TRACED, we need lock_task_sighand(). > The tracee can be already woken up by ptrace_resume(), but it is possible that > it didn't clear DELAY_WAKEKILL yet. Yes. The subtle differences in when __TASK_TRACED and JOBCTL_DELAY_WAKEKILL are cleared are causing me some minor issues. This "WARN_ON(!task->ptrace || task->parent != current);" also now needs to be inside siglock, because the __TASK_TRACED is insufficient. > Now, before we take ->siglock, the tracee can exit and another thread can do > wait() and reap this task. > > Also, I think the comment above should be updated. I agree, it makes sense to > re-check JOBCTL_DELAY_WAKEKILL under siglock just for clarity, but we no longer > need to do this to close the race; jobctl &= ~JOBCTL_DELAY_WAKEKILL and > wake_up_state() are safe even if JOBCTL_DELAY_WAKEKILL was already > cleared. I think you are right about it being safe, but I am having a hard time convincing myself that is true. I want to be very careful sending __TASK_TRACED wake_ups as ptrace_stop fundamentally can't handle spurious wake_ups. So I think adding task_is_traced to the test to verify the task is still frozen. static void ptrace_unfreeze_traced(struct task_struct *task) { unsigned long flags; /* * Verify the task is still frozen before unfreezing it, * ptrace_resume could have unfrozen us. */ if (lock_task_sighand(task, &flags)) { if ((task->jobctl & JOBCTL_DELAY_WAKEKILL) && task_is_traced(task)) { task->jobctl &= ~JOBCTL_DELAY_WAKEKILL; if (__fatal_signal_pending(task)) wake_up_state(task, __TASK_TRACED); } unlock_task_sighand(task, &flags); } } >> @@ -2307,6 +2307,7 @@ static int ptrace_stop(int exit_code, int why, int clear_code, >> >> /* LISTENING can be set only during STOP traps, clear it */ >> current->jobctl &= ~JOBCTL_LISTENING; >> + current->jobctl &= ~JOBCTL_DELAY_WAKEKILL; > > minor, but > > current->jobctl &= ~(JOBCTL_LISTENING | JOBCTL_DELAY_WAKEKILL); > > looks better. Yes. Eric _______________________________________________ linux-um mailing list linux-um@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-um