Revert removal of cred_free_thread(): It is used to ensure that a thread's

credentials are not improperly borrowed when the thread is not current in
the kernel.

Requested by:	jhb, alfred

sys/kern/kern_fork.c

@@ -875,6 +875,9 @@ fork_exit(callout, arg, frame)
 		kthread_exit(0);
 	}
 	PROC_UNLOCK(p);
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 	mtx_assert(&Giant, MA_NOTOWNED);
 }
 

sys/kern/kern_prot.c

@@ -1765,6 +1765,19 @@ crdup(struct ucred *cr)
 	return (newcr);
 }
 
+#ifdef DIAGNOSTIC
+void
+cred_free_thread(struct thread *td)
+{
+	struct ucred *cred;
+
+	cred = td->td_ucred;
+	td->td_ucred = NULL;
+	if (cred != NULL)
+		crfree(cred);
+}
+#endif
+
 /*
  * Fill in a struct xucred based on a struct ucred.
  */

sys/kern/subr_trap.c

@@ -236,5 +236,8 @@ ast(struct trapframe *framep)
 	}
 
 	userret(td, framep, sticks);
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 	mtx_assert(&Giant, MA_NOTOWNED);
 }