chore: add test for setgid bit inheritance in mkdir -p

Author: 0xferrous

tests/by-util/test_mkdir.rs

@@ -908,6 +908,32 @@ fn test_mkdir_parent_mode_with_explicit_mode() {
     );
 }
 
+/// Test that nested directories inherit the setgid bit with mkdir -p.
+#[test]
+#[cfg(not(windows))]
+fn test_mkdir_parent_inherits_setgid() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.mkdir("parent");
+    at.set_mode("parent", 0o2755);
+
+    ucmd.arg("-p")
+        .arg("parent/child/grandchild")
+        .succeeds()
+        .no_stderr()
+        .no_stdout();
+
+    // All descendants should inherit the setgid bit (0o2000)
+    let parent_mode = at.metadata("parent").permissions().mode() as mode_t;
+    assert_eq!(parent_mode & 0o2000, 0o2000);
+
+    let child_mode = at.metadata("parent/child").permissions().mode() as mode_t;
+    assert_eq!(child_mode & 0o2000, 0o2000);
+
+    let grandchild_mode = at.metadata("parent/child/grandchild").permissions().mode() as mode_t;
+    assert_eq!(grandchild_mode & 0o2000, 0o2000);
+}
+
 #[test]
 fn test_mkdir_concurrent_creation() {
     // Test concurrent mkdir -p operations: 10 iterations, 8 threads, 40 levels nesting