diff --git a/samples/ftrace/ftrace-direct-modify.c b/samples/ftrace/ftrace-direct-modify.c
index 690e4a9ff33304f065c00422511b0aa829d3e7df..2877cb053a8293b42e3e7b5a8e7e4d1190b7d436 100644
--- a/samples/ftrace/ftrace-direct-modify.c
+++ b/samples/ftrace/ftrace-direct-modify.c
@@ -4,6 +4,9 @@
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.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-multi-modify.c b/samples/ftrace/ftrace-direct-multi-modify.c
index 91bc42a7adb9167057bbe6c460d0392cf93c3f9a..6f43a39decd02d25be90f2a30aeebd682f6b8521 100644
--- a/samples/ftrace/ftrace-direct-multi-modify.c
+++ b/samples/ftrace/ftrace-direct-multi-modify.c
@@ -4,6 +4,9 @@
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.h>
 
+extern void my_direct_func1(unsigned long ip);
+extern void my_direct_func2(unsigned long ip);
+
 void my_direct_func1(unsigned long ip)
 {
 	trace_printk("my direct func1 ip %lx\n", ip);
diff --git a/samples/ftrace/ftrace-direct-too.c b/samples/ftrace/ftrace-direct-too.c
index 6e0de725bf221f97a60de3d8a3062b2a29dd7b5d..b97e5ed46233ba56bb12a3e66abd24a0107e84b5 100644
--- a/samples/ftrace/ftrace-direct-too.c
+++ b/samples/ftrace/ftrace-direct-too.c
@@ -5,6 +5,9 @@
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.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 a30aa42ec76a8ff92ee0377264eb8166e7d206b2..c918b13edb490d2600dedda173088ee553329a21 100644
--- a/samples/ftrace/ftrace-direct.c
+++ b/samples/ftrace/ftrace-direct.c
@@ -5,6 +5,8 @@
 #include <linux/ftrace.h>
 #include <asm/asm-offsets.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);