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

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Date:   Wed, 1 May 2019 17:13:12 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Colascione <dancol@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        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 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.

Oleg.