x86/resctrl: Only show tasks' pid in current pid namespace

When writing a task id to the "tasks" file in an rdtgroup,
rdtgroup_tasks_write() treats the pid as a number in the current pid
namespace. But when reading the "tasks" file, rdtgroup_tasks_show() shows
the list of global pids from the init namespace, which is confusing and
incorrect.

To be more robust, let the "tasks" file only show pids in the current pid
namespace.

Fixes: e02737d5 ("x86/intel_rdt: Add tasks files")
Signed-off-by: Shawn Wang <shawnwang@linux.alibaba.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Acked-by: Fenghua Yu <fenghua.yu@intel.com>
Tested-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/all/20230116071246.97717-1-shawnwang@linux.alibaba.com/

arch/x86/kernel/cpu/resctrl/rdtgroup.c

@@ -726,11 +726,15 @@ static ssize_t rdtgroup_tasks_write(struct kernfs_open_file *of,
 static void show_rdt_tasks(struct rdtgroup *r, struct seq_file *s)
 {
 	struct task_struct *p, *t;
+	pid_t pid;
 
 	rcu_read_lock();
 	for_each_process_thread(p, t) {
-		if (is_closid_match(t, r) || is_rmid_match(t, r))
-			seq_printf(s, "%d\n", t->pid);
+		if (is_closid_match(t, r) || is_rmid_match(t, r)) {
+			pid = task_pid_vnr(t);
+			if (pid)
+				seq_printf(s, "%d\n", pid);
+		}
 	}
 	rcu_read_unlock();
 }