diff --git a/src/io/ioutil/ioutil.go b/src/io/ioutil/ioutil.go
index e617be5dae..b1cb841468 100644
--- a/src/io/ioutil/ioutil.go
+++ b/src/io/ioutil/ioutil.go
@@ -74,8 +74,8 @@ func ReadFile(filename string) ([]byte, error) {
 }
 
 // WriteFile writes data to a file named by filename.
-// If the file does not exist, WriteFile creates it with permissions perm;
-// otherwise WriteFile truncates it before writing.
+// If the file does not exist, WriteFile creates it with permissions perm
+// (before umask); otherwise WriteFile truncates it before writing.
 func WriteFile(filename string, data []byte, perm os.FileMode) error {
 	f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
 	if err != nil {