chmod: fix EROFS showing as Permission denied

src/uu/chmod/locales/en-US.ftl

@@ -10,6 +10,7 @@ chmod-error-no-such-file = cannot access {$file}: No such file or directory
 chmod-error-preserve-root = it is dangerous to operate recursively on {$file}
   chmod: use --no-preserve-root to override this failsafe
 chmod-error-permission-denied = cannot access {$file}: Permission denied
+chmod-error-read-only-file-system = changing permissions of {$file}: Read-only file system
 chmod-error-new-permissions = {$file}: new permissions are {$actual}, not {$expected}
 chmod-error-missing-operand = missing operand
 

src/uu/chmod/locales/fr-FR.ftl

@@ -22,6 +22,7 @@ chmod-error-no-such-file = impossible d'accéder à {$file} : Aucun fichier ou r
 chmod-error-preserve-root = il est dangereux d'opérer récursivement sur {$file}
   chmod: utiliser --no-preserve-root pour outrepasser cette protection
 chmod-error-permission-denied = impossible d'accéder à {$file} : Permission refusée
+chmod-error-read-only-file-system = modification des permissions de {$file} : Système de fichiers en lecture seule
 chmod-error-new-permissions = {$file} : les nouvelles permissions sont {$actual}, pas {$expected}
 chmod-error-missing-operand = opérande manquant
 

src/uu/chmod/src/chmod.rs

@@ -35,6 +35,8 @@ enum ChmodError {
     PreserveRoot(PathBuf),
     #[error("{}", translate!("chmod-error-permission-denied", "file" => _0.quote()))]
     PermissionDenied(PathBuf),
+    #[error("{}", translate!("chmod-error-read-only-file-system", "file" => _0.quote()))]
+    ReadOnlyFileSystem(PathBuf),
     #[error("{}", translate!("chmod-error-new-permissions", "file" => _0.maybe_quote(), "actual" => _1.clone(), "expected" => _2.clone()))]
     NewPermissions(PathBuf, String, String),
 }
@@ -609,14 +611,20 @@ impl Chmoder {
         let (new_mode, _) = self.calculate_new_mode(current_mode, file_path.is_dir())?;
 
         // Use safe traversal to change the mode
-        if let Err(_e) = dir_fd.chmod_at(entry_name, new_mode, symlink_behavior) {
+        if let Err(e) = dir_fd.chmod_at(entry_name, new_mode, symlink_behavior) {
             if self.verbose {
                 println!(
                     "failed to change mode of {} to {new_mode:o}",
                     file_path.quote(),
                 );
             }
-            return Err(ChmodError::PermissionDenied(file_path.into()).into());
+            let err = if e.kind() == std::io::ErrorKind::ReadOnlyFilesystem {
+                ChmodError::ReadOnlyFileSystem(file_path.into())
+            } else {
+                ChmodError::PermissionDenied(file_path.into())
+            };
+
+            return Err(err.into());
         }
 
         // Report the change using the helper method

tests/by-util/test_chmod.rs

@@ -403,6 +403,60 @@ fn test_permission_denied() {
         .stderr_is("chmod: cannot access 'd/no-x/y': Permission denied\n");
 }
 
+#[test]
+#[cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd"))]
+fn test_chmod_readonly_filesystem() {
+    let mut scene = TestScenario::new(util_name!());
+
+    // Test must be run as root (or with `sudo -E`)
+    if scene.cmd("whoami").run().stdout_str() != "root\n" {
+        return;
+    }
+
+    // Prepare the mount
+    let mountpoint = "readonly_mount";
+    scene.fixtures.mkdir(mountpoint);
+    let mountpoint_path = scene.fixtures.plus_as_string(mountpoint);
+
+    scene
+        .mount_temp_fs(&mountpoint_path)
+        .expect("mounting tmpfs failed");
+
+    // Create a file and set permissions so chmod will attempt to change them
+    scene.fixtures.touch(format!("{mountpoint}/file.txt"));
+    scene
+        .cmd("chmod")
+        .arg("400")
+        .arg(format!("{mountpoint_path}/file.txt"))
+        .run();
+
+    // Remount as read-only
+    scene
+        .cmd("mount")
+        .arg("-o")
+        .arg("remount,ro")
+        .arg(&mountpoint_path)
+        .run();
+
+    // Should say "Read-only file system" not "Permission denied"
+    scene
+        .ucmd()
+        .arg("ugo+w")
+        .arg(format!("{mountpoint_path}/file.txt"))
+        .fails()
+        .stderr_contains("Read-only file system");
+
+    // Remount as read-write so umount can clean up
+    scene
+        .cmd("mount")
+        .arg("-o")
+        .arg("remount,rw")
+        .arg(&mountpoint_path)
+        .run();
+
+    scene.umount_temp_fs();
+}
+
 #[test]
 #[allow(clippy::unreadable_literal)]
 fn test_chmod_recursive_correct_exit_code() {