Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 2 May 2019 17:13:21 +0200
From:   Christian Brauner <christian@brauner.io>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Colascione <dancol@google.com>,
        Jann Horn <jannh@google.com>,
        Tim Murray <timmurray@google.com>,
        Jonathan Kowalski <bl0pbl33p@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        David Howells <dhowells@redhat.com>, kernel-team@android.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        KJ Tsanaktsidis <ktsanaktsidis@zendesk.com>,
        linux-kselftest@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
        Nadav Amit <namit@vmware.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Serge Hallyn <serge@hallyn.com>, Shuah Khan <shuah@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tycho Andersen <tycho@tycho.ws>
Subject: Re: [PATCH v2 1/2] Add polling support to pidfd
Message-ID: <20190502151320.cvc6uc3b4bmww23k@brauner.io>
References: <20190430162154.61314-1-joel@joelfernandes.org>
 <20190501151312.GA30235@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20190501151312.GA30235@redhat.com>
User-Agent: NeoMutt/20180716
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, May 01, 2019 at 05:13:12PM +0200, Oleg Nesterov wrote:
> On 04/30, Joel Fernandes (Google) wrote:
> >
> > +static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts)
> > +{
> > +	struct task_struct *task;
> > +	struct pid *pid = file->private_data;
> > +	int poll_flags = 0;
> > +
> > +	poll_wait(file, &pid->wait_pidfd, pts);
> > +
> > +	rcu_read_lock();
> > +	task = pid_task(pid, PIDTYPE_PID);
> > +	WARN_ON_ONCE(task && !thread_group_leader(task));
>                              ^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> Ah, this is not right, we can race with de_thread() which changes the leader,
> in particular it does leader->exit_signal = -1 to indicate that this thread is
> no longer a group leader, but pid_task() can return the old leader.
> 
> We are going to check thread_group_empty() below, it won't be true in this case,
> so this race should not make any harm.
> 
> Just remove this WARN_ON(). We can't use has_group_leader_pid(), it can return
> false if pid_task() returns the new leader.
> 
> Otherwise I see no problems.

I'll remove the WARN_ON() check when applying this. Can I get your
Acked/Review, Oleg?

Christian