diff --git a/drivers/pci/endpoint/pci-ep-cfs.c b/drivers/pci/endpoint/pci-ep-cfs.c
index e8f65eb51c1a605cf57e4cae945f37a2173767e6..c9a2fb9dd49d97965f368560d85a9a22fbafa533 100644
--- a/drivers/pci/endpoint/pci-ep-cfs.c
+++ b/drivers/pci/endpoint/pci-ep-cfs.c
@@ -103,7 +103,10 @@ static int pci_epc_epf_link(struct config_item *epc_item,
 	struct pci_epf *epf = epf_group->epf;
 
 	func_no = find_first_zero_bit(&epc_group->function_num_map,
-				      sizeof(epc_group->function_num_map));
+				      BITS_PER_LONG);
+	if (func_no >= BITS_PER_LONG)
+		return -EINVAL;
+
 	set_bit(func_no, &epc_group->function_num_map);
 	epf->func_no = func_no;