rust: wrong SAFETY comments in group_leader() and pid() + questions

[Oleg Nesterov]

>From rust/kernel/task.rs:

    pub fn group_leader(&self) -> &Task {
        // SAFETY: The group leader of a task never changes after initialization, so reading this
        // field is not a data race.
        let ptr = unsafe { *ptr::addr_of!((*self.as_ptr()).group_leader) };

        // SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
        // and given that a task has a reference to its group leader, we know it must be valid for
        // the lifetime of the returned task reference.
        unsafe { &*ptr.cast() }
    }

    /// Returns the PID of the given task.
    pub fn pid(&self) -> Pid {
        // SAFETY: The pid of a task never changes after initialization, so reading this field is
        // not a data race.
        unsafe { *ptr::addr_of!((*self.as_ptr()).pid) }
    }

The comments look wrong. Unless same_thread_group(current, task) == T, task->group_leader
and/or task->pid can change if a non-leader task's sub-thread execs. This also means that
in general it is not safe to dereference group_leader, for example this C code is not safe:

	rcu_read_lock();
	task = find_task_by_vpid(vpid);
	if (task)
		get_task_struct(task);
	rcu_read_unlock();

	if (task)
		pid = task->group_leader->pid; // BUG! ->group_leader can be already freed


Now the questions. Sorry! I don't know rust.

1. Can I simply remove these misleading comments? Or SAFETY comment is mandatory?

2. I am working on the patch(es) which move ->group_leader from task_struct to
   signal_struct, so the 1st change adds the new trivial helper in preparation:

	struct task_struct *task_group_leader(struct task_struct *task)
	{
		return task->group_leader; // will be updated
	}

   Now, how can I change group_leader() to use it? I guess I need to add

	struct task_struct *rust_helper_task_group_leader(struct task_struct *task)
	{
		return task_group_leader(task);
	}

   into rust/helpers/task.c, but will something like

	pub fn group_leader(&self) -> &Task {
		unsafe { bindings::task_group_leader(self.as_ptr()) }
	}

    work? I'm afraid it won't ;)

Oleg.