Re: [PATCH v4 7/9] rust: file: add `Kuid` wrapper

[Valentin Obst <kernel@valentinobst.de>]

> > +    /// Returns the given task's pid in the current pid namespace.
> > +    pub fn pid_in_current_ns(&self) -> Pid {
> > +        let current = Task::current_raw();
> > +        // SAFETY: Calling `task_active_pid_ns` with the current task is always safe.
> > +        let namespace = unsafe { bindings::task_active_pid_ns(current) };
> > +        // SAFETY: We know that `self.0.get()` is valid by the type invariant, and the namespace
> > +        // pointer is not dangling since it points at this task's namespace.
> > +        unsafe { bindings::task_tgid_nr_ns(self.0.get(), namespace) }
> > +    }
>
> pids are reference counted in the kernel, how does this deal with that?
> Are they just ignored somehow?  Where is the reference count given back?
>
> thanks,
>
> greg k-h

As far as I can see, neither `task_active_pid_ns` nor `task_active_pid_ns`
return with an incremented refcount. However, looking at the above code,
it looks like it could be simplified to:

```rust
pub fn pid_in_current_ns(&self) -> Pid {
    // SAFETY: We know that `self.0.get()` is valid by the type invariant.
    // Passing a null pointer in the second argument defaults to the current ns.
    unsafe { bindings::task_tgid_nr_ns(self.0.get(), ptr::null_mut()) }
}
```

Since:

```C
static inline pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
{
	return __task_pid_nr_ns(tsk, PIDTYPE_TGID, ns);
}
...
pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
			struct pid_namespace *ns)
{
	pid_t nr = 0;

	rcu_read_lock();
	if (!ns)
		ns = task_active_pid_ns(current);
	nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
	rcu_read_unlock();

	return nr;
}
EXPORT_SYMBOL(__task_pid_nr_ns);
```

anyway defaults to current's pid ns (plus some RCU lock protection, not sure if
that is relevant here).

	- Best Valentin