TracerPid is pid_max for parent namespace tracer

For the Stadia debugger flow to work, the game process needs to be able
to detect when it is being traced by a process in its parent PID
namespace, which TracerPid in /proc/[pid]/status won't show. This change
will make TracerPid return pid_max instead of 0 for this case. This will
enable checks that look to see if they are being traced to return true,
even though they can't see the actual process.

Change-Id: I828da9bd59eb3b283ad06587c7dae442169d1064

fs/proc/array.c

@@ -91,6 +91,7 @@
 #include <linux/string_helpers.h>
 #include <linux/user_namespace.h>
 #include <linux/fs_struct.h>
+#include <linux/pid.h>
 
 #include <asm/processor.h>
 #include "internal.h"
@@ -164,8 +165,15 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		task_tgid_nr_ns(rcu_dereference(p->real_parent), ns) : 0;
 
 	tracer = ptrace_parent(p);
-	if (tracer)
+	if (tracer) {
 		tpid = task_pid_nr_ns(tracer, ns);
+		if (tpid == 0) {
+			// Set the pid to a valid number, but not an actual valid pid
+			// so that anything checking if it is being traced will see
+			// that it is traced, even though it can't see the process.
+			tpid = pid_max;
+		}
+	}
 
 	tgid = task_tgid_nr_ns(p, ns);
 	ngid = task_numa_group_id(p);