linux-kernel - Re: [PATCH v2 1/2] Add polling support to pidfd

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Date:   Thu, 2 May 2019 17:13:21 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        David Howells <dhowells@...hat.com>, kernel-team@...roid.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        KJ Tsanaktsidis <ktsanaktsidis@...desk.com>,
        linux-kselftest@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Nadav Amit <namit@...are.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Serge Hallyn <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH v2 1/2] Add polling support to pidfd

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