diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4d78abaf..e9d70353 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,11 @@
 # Changelog
 All notable changes to this project will be documented in this file.
 
+## Unreleased - ???
+- Add os/umask
+- Add os/perm-int
+- Add os/perm-str
+
 ## 1.8.1 - 2020-03-31
 - Fix bugs for big endian systems
 - Fix 1.8.0 regression on BSDs
diff --git a/src/core/os.c b/src/core/os.c
index 7a8162c3..fb1e2c31 100644
--- a/src/core/os.c
+++ b/src/core/os.c
@@ -797,16 +797,6 @@ static Janet os_symlink(int32_t argc, Janet *argv) {
 #endif
 }
 
-static Janet os_umask(int32_t argc, Janet *argv) {
-    janet_fixarity(argc, 1);
-#ifdef JANET_WINDOWS
-    janet_panicf("os/umask not supported on Windows");
-#else
-    int32_t mask = janet_getinteger(argv, 0);
-    return janet_wrap_integer(umask(mask));
-#endif
-}
-
 static Janet os_mkdir(int32_t argc, Janet *argv) {
     janet_fixarity(argc, 1);
     const char *path = janet_getcstring(argv, 0);
@@ -1152,6 +1142,17 @@ static Janet os_chmod(int32_t argc, Janet *argv) {
     return janet_wrap_nil();
 }
 
+static Janet os_umask(int32_t argc, Janet *argv) {
+    janet_fixarity(argc, 1);
+    int mask = (int) os_getmode(argv, 0);
+#ifdef JANET_WINDOWS
+    int res = _umask(mask);
+#else
+    int res = umask(mask);
+#endif
+    return janet_wrap_integer(janet_perm_to_unix(res));
+}
+
 static Janet os_dir(int32_t argc, Janet *argv) {
     janet_arity(argc, 1, 2);
     const char *dir = janet_getcstring(argv, 0);