find: reject the deprecated -perm +MODE octal form to match GNU

src/find/matchers/perm.rs

@@ -53,6 +53,13 @@ mod parsing {
     }
 
     pub fn parse_mode(pattern: &str, for_dir: bool) -> Result<u32, Box<dyn Error>> {
+        // GNU rejects the old `-perm +MODE` octal form; a leading + needs a symbolic mode.
+        if let Some(rest) = pattern.strip_prefix('+') {
+            if rest.contains(|c: char| c.is_ascii_digit()) {
+                return Err(From::from(format!("invalid mode '+{rest}'")));
+            }
+        }
+
         let mode = if pattern.contains(|c: char| c.is_ascii_digit()) {
             parse_numeric(0, pattern, for_dir)?
         } else {

tests/test_find.rs

@@ -790,6 +790,21 @@ fn find_perm() {
     ucmd().args(&["-perm", "u=g"]).succeeds();
 }
 
+#[cfg(unix)]
+#[test]
+fn find_perm_plus_octal_is_rejected() {
+    // GNU find rejects the deprecated `-perm +MODE` octal form; only symbolic
+    // modes may follow a leading `+`.
+    ucmd()
+        .args(&["-perm", "+644"])
+        .fails()
+        .stderr_contains("invalid mode '+644'");
+    ucmd()
+        .args(&["-perm", "+0"])
+        .fails()
+        .stderr_contains("invalid mode '+0'");
+}
+
 #[cfg(unix)]
 #[test]
 fn find_inum() {