chmod: always call chmod(2) even when mode bits are unchanged

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

@@ -11,6 +11,7 @@ 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-new-permissions = {$file}: new permissions are {$actual}, not {$expected}
+chmod-error-changing-permissions = changing permissions of {$file}: {$err}
 chmod-error-missing-operand = missing operand
 
 # Help messages

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

@@ -23,6 +23,7 @@ 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-new-permissions = {$file} : les nouvelles permissions sont {$actual}, pas {$expected}
+chmod-error-changing-permissions = changement des permissions de {$file} : {$err}
 chmod-error-missing-operand = opérande manquant
 
 # Messages verbeux/de statut

src/uu/chmod/src/chmod.rs

@@ -13,7 +13,9 @@ use std::os::unix::fs::{MetadataExt, PermissionsExt};
 use std::path::{Path, PathBuf};
 use thiserror::Error;
 use uucore::display::Quotable;
-use uucore::error::{ExitCode, UError, UResult, USimpleError, UUsageError, set_exit_code};
+use uucore::error::{
+    ExitCode, UError, UResult, USimpleError, UUsageError, set_exit_code, strip_errno,
+};
 use uucore::fs::{FileInformation, display_permissions_unix};
 use uucore::mode;
 use uucore::perms::{TraverseSymlinks, configure_symlink_and_recursion};
@@ -38,6 +40,8 @@ enum ChmodError {
     PermissionDenied(PathBuf),
     #[error("{}", translate!("chmod-error-new-permissions", "file" => _0.maybe_quote(), "actual" => _1.clone(), "expected" => _2.clone()))]
     NewPermissions(PathBuf, String, String),
+    #[error("{}", translate!("chmod-error-changing-permissions", "file" => _0.quote(), "err" => strip_errno(_1)))]
+    ChangingPermissions(PathBuf, std::io::Error),
 }
 
 impl UError for ChmodError {}
@@ -772,13 +776,12 @@ impl Chmoder {
     }
 
     fn change_file(&self, fperm: u32, mode: u32, file: &Path) -> Result<(), i32> {
-        if fperm == mode {
-            // Use the helper method for consistent reporting
-            self.report_permission_change(file, fperm, mode);
-            Ok(())
-        } else if let Err(err) = fs::set_permissions(file, fs::Permissions::from_mode(mode)) {
+        // Always issue the chmod(2) call, even when the bits are unchanged: the
+        // syscall can still fail (e.g. lacking permission on the file) and that
+        // failure must be reported, matching GNU.
+        if let Err(err) = fs::set_permissions(file, fs::Permissions::from_mode(mode)) {
             if !self.quiet {
-                show_error!("{err}");
+                show_error!("{}", ChmodError::ChangingPermissions(file.into(), err));
             }
             if self.verbose {
                 println!(

tests/by-util/test_chmod.rs

@@ -5,7 +5,7 @@
 // spell-checker:ignore (words) dirfd subdirs openat FDCWD rwxr
 
 use std::fs::{OpenOptions, Permissions, metadata, set_permissions};
-use std::os::unix::fs::{OpenOptionsExt, PermissionsExt};
+use std::os::unix::fs::{MetadataExt, OpenOptionsExt, PermissionsExt};
 use uutests::at_and_ucmd;
 use uutests::util::{AtPath, TestScenario, UCommand};
 
@@ -1409,6 +1409,31 @@ fn test_chmod_recursive_uses_dirfd_for_subdirs() {
     );
 }
 
+#[test]
+fn test_chmod_operator_only_still_calls_syscall() {
+    use uucore::process::geteuid;
+
+    // An operator with no permission letters ('+', '-', '=') leaves the mode
+    // bits unchanged, yet chmod must still issue the chmod(2) call so that a
+    // lack of permission is reported instead of silently succeeding. As a
+    // non-root user, '/' (owned by root) is a file we cannot chmod.
+    if geteuid() == 0 {
+        return;
+    }
+    if metadata("/").map_or(0, |m| m.uid()) != 0 {
+        return; // '/' is not root-owned in this environment
+    }
+
+    for op in ["+", "-", "="] {
+        new_ucmd!()
+            .arg(op)
+            .arg("/")
+            .fails()
+            .code_is(1)
+            .stderr_contains("changing permissions of '/'");
+    }
+}
+
 #[test]
 fn test_chmod_colored_output() {
     // Test colored help message