From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Eric W. Biederman" Date: Tue, 10 May 2022 15:17:32 +0000 Subject: Re: [PATCH v4 10/12] ptrace: Don't change __state Message-Id: <87ee11wh6b.fsf@email.froward.int.ebiederm.org> List-Id: References: <87a6bv6dl6.fsf_-_@email.froward.int.ebiederm.org> <20220505182645.497868-10-ebiederm@xmission.com> <20220510142202.GB23277@redhat.com> In-Reply-To: <20220510142202.GB23277@redhat.com> (Oleg Nesterov's message of "Tue, 10 May 2022 16:23:18 +0200") MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 , linux-xtensa@linux-xtensa.org, Kees Cook , Jann Horn , linux-ia64@vger.kernel.org Oleg Nesterov writes: > On 05/05, Eric W. Biederman wrote: >> >> static void ptrace_unfreeze_traced(struct task_struct *task) >> { >> - if (READ_ONCE(task->__state) != __TASK_TRACED) >> - return; >> - >> - WARN_ON(!task->ptrace || task->parent != current); >> + unsigned long flags; >> >> /* >> - * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely. >> - * Recheck state under the lock to close this race. >> + * The child may be awake and may have cleared >> + * JOBCTL_PTRACE_FROZEN (see ptrace_resume). The child will >> + * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew. >> */ >> - spin_lock_irq(&task->sighand->siglock); >> - if (READ_ONCE(task->__state) = __TASK_TRACED) { >> + if (lock_task_sighand(task, &flags)) { > > But I still think that a lockless > > if (!(task->jobctl & JOBCTL_PTRACE_FROZEN)) > return; > > check at the start of ptrace_unfreeze_traced() makes sense to avoid > lock_task_sighand() if possible. > > And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with > JOBCTL_TRACED to make this optimization work better. The same for > ptrace_signal_wake_up(). What do you have that suggests that taking siglock there is a problem? What you propose will definitely work as an incremental change, and in an incremental change we can explain why doing the stupid simple thing is not good enough. I am not really opposed on any grounds except that simplicity is good, and hard to get wrong. Eric