proc: hide kernel addresses via %pK in /proc/<pid>/stack

This file is readable for the task owner.  Hide kernel addresses from
unprivileged users, leave them function names and offsets.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Kees Cook <kees.cook@canonical.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -340,7 +340,7 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
 	save_stack_trace_tsk(task, &trace);
 
 	for (i = 0; i < trace.nr_entries; i++) {
-		seq_printf(m, "[<%p>] %pS\n",
+		seq_printf(m, "[<%pK>] %pS\n",
 			   (void *)entries[i], (void *)entries[i]);
 	}
 	kfree(entries);