pid: implement ns_of_pid

A current problem with the pid namespace is that it is easy to do pid
related work after exit_task_namespaces which drops the nsproxy pointer.

However if we are doing pid namespace related work we are always operating
on some struct pid which retains the pid_namespace pointer of the pid
namespace it was allocated in.

So provide ns_of_pid which allows us to find the pid namespace a pid was
allocated in.

Using this we have the needed infrastructure to do pid namespace related
work at anytime we have a struct pid, removing the chance of accidentally
having a NULL pointer dereference when accessing current->nsproxy.

Signed-off-by: Eric W. Biederman <[e-mail]>
Signed-off-by: Sukadev Bhattiprolu <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Roland McGrath <[e-mail]>
Cc: Bastian Blank <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Nadia Derbey <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/pid.h

···
 extern void free_pid(struct pid *pid);
 
 /*
+ * ns_of_pid() returns the pid namespace in which the specified pid was
+ * allocated.
+ *
+ * NOTE:
+ * 	ns_of_pid() is expected to be called for a process (task) that has
+ * 	an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
+ * 	is expected to be non-NULL. If @pid is NULL, caller should handle
+ * 	the resulting NULL pid-ns.
+ */
+static inline struct pid_namespace *ns_of_pid(struct pid *pid)
+{
+	struct pid_namespace *ns = NULL;
+	if (pid)
+		ns = pid->numbers[pid->level].ns;
+	return ns;
+}
+
+/*
  * the helpers to get the pid's id seen from different namespaces
  *
  * pid_nr()    : global id, i.e. the id seen from the init namespace;