[PATCH 03/12] pid: get ppid pid_t of task in init_pid_ns safely

[Oleg Nesterov]

On 08/20, Richard Guy Briggs wrote:
>
> Added the functions
>         task_ppid()
>         task_ppid_nr_ns()
>         task_ppid_nr_init_ns()
> to safely abstract the lookup of the PPID

but it is not safe.

> +static inline struct pid *task_ppid(struct task_struct *task)
> +{
> +	return task_tgid(rcu_dereference(current->real_parent));
                                         ^^^^^^^
task?

> +static inline pid_t task_ppid_nr_ns(struct task_struct *tsk,
> +					struct pid_namespace *ns)
> +{
> +	pid_t pid;
> +
> +	rcu_read_lock();
> +	pid = pid_nr_ns(task_ppid(current), ns);
                                  ^^^^^^^
again.

> +	rcu_read_unlock();

And why this is safe?

rcu_read_lock() can't help if tsk was already dead _before_ it takes
the rcu lock. ->real_parent can point the already freed/reused/unmapped
memory.

This is safe if, for example, the caller alredy holds rcu_read_lock()
and tsk was found by find_task_by*(), or tsk is current.

Richard, just in case... I am going to vacation, I will be completely
offline till Sep 10.

Oleg.