diff --git a/samples/ftrace/ftrace-direct-modify.c b/samples/ftrace/ftrace-direct-modify.c
index 5b9a09957c6e..89e6bf27cd9f 100644
--- a/samples/ftrace/ftrace-direct-modify.c
+++ b/samples/ftrace/ftrace-direct-modify.c
@@ -3,6 +3,9 @@
 #include <linux/kthread.h>
 #include <linux/ftrace.h>
 
+extern void my_direct_func1(void);
+extern void my_direct_func2(void);
+
 void my_direct_func1(void)
 {
 	trace_printk("my direct func1\n");
diff --git a/samples/ftrace/ftrace-direct-too.c b/samples/ftrace/ftrace-direct-too.c
index 3f0079c9bd6f..11b99325f3db 100644
--- a/samples/ftrace/ftrace-direct-too.c
+++ b/samples/ftrace/ftrace-direct-too.c
@@ -4,6 +4,9 @@
 #include <linux/mm.h> /* for handle_mm_fault() */
 #include <linux/ftrace.h>
 
+extern void my_direct_func(struct vm_area_struct *vma,
+			   unsigned long address, unsigned int flags);
+
 void my_direct_func(struct vm_area_struct *vma,
 			unsigned long address, unsigned int flags)
 {
diff --git a/samples/ftrace/ftrace-direct.c b/samples/ftrace/ftrace-direct.c
index a2729d1ef17f..642c50b5f716 100644
--- a/samples/ftrace/ftrace-direct.c
+++ b/samples/ftrace/ftrace-direct.c
@@ -4,6 +4,8 @@
 #include <linux/sched.h> /* for wake_up_process() */
 #include <linux/ftrace.h>
 
+extern void my_direct_func(struct task_struct *p);
+
 void my_direct_func(struct task_struct *p)
 {
 	trace_printk("waking up %s-%d\n", p->comm, p->pid);