KeyboardSettings: Add checkbox to enable Caps Lock mapping to Ctrl

This patch adds an additional control to KeyboardSettings allowing
the user to map Caps Lock to Ctrl. Previously, this was only possible
by writing to /sys/kernel/variables/caps_lock_to_ctrl.

Writing to /sys/kernel/variables/caps_lock_to_ctrl requires root
privileges, but KeyboardSettings will not attempt to elevate
the privilege of the user if they are not root. Instead, the
checkbox is rendered as un-editable.

Userland/Applications/KeyboardSettings/Keyboard.gml

@@ -124,4 +124,18 @@
             name: "num_lock_checkbox"
         }
     }
+
+    @GUI::GroupBox {
+        title: "Caps Lock"
+        fixed_height: 60
+        layout: @GUI::HorizontalBoxLayout {
+            margins: [16, 8, 8]
+            spacing: 16
+        }
+
+        @GUI::CheckBox {
+            text: "Use Caps Lock as an additional Ctrl"
+            name: "caps_lock_remapped_to_ctrl_checkbox"
+        }
+    }
 }

Userland/Applications/KeyboardSettings/KeyboardSettingsWidget.cpp

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2020, Hüseyin Aslıtürk <asliturk@hotmail.com>
  * Copyright (c) 2021-2023, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2022, Sam Cohen <sbcohen2000@gmail.com>
  * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -255,6 +256,19 @@ KeyboardSettingsWidget::KeyboardSettingsWidget()
     m_num_lock_checkbox->on_checked = [&](auto) {
         set_modified(true);
     };
+
+    m_caps_lock_checkbox = find_descendant_of_type_named<GUI::CheckBox>("caps_lock_remapped_to_ctrl_checkbox");
+    auto caps_lock_is_remapped = read_caps_lock_to_ctrl_sys_variable();
+    if (caps_lock_is_remapped.is_error()) {
+        auto error_message = DeprecatedString::formatted("Could not determine if Caps Lock is remapped to Ctrl: {}", caps_lock_is_remapped.error());
+        GUI::MessageBox::show_error(window(), error_message);
+    } else {
+        m_caps_lock_checkbox->set_checked(caps_lock_is_remapped.value());
+    }
+    m_caps_lock_checkbox->set_enabled(getuid() == 0);
+    m_caps_lock_checkbox->on_checked = [&](auto) {
+        set_modified(true);
+    };
 }
 
 KeyboardSettingsWidget::~KeyboardSettingsWidget()
@@ -282,6 +296,7 @@ void KeyboardSettingsWidget::apply_settings()
     }
     m_initial_active_keymap = m_keymaps_list_model.active_keymap();
     Config::write_bool("KeyboardSettings"sv, "StartupEnable"sv, "NumLock"sv, m_num_lock_checkbox->is_checked());
+    write_caps_lock_to_ctrl_sys_variable(m_caps_lock_checkbox->is_checked());
 }
 
 void KeyboardSettingsWidget::set_keymaps(Vector<DeprecatedString> const& keymaps, DeprecatedString const& active_keymap)
@@ -289,3 +304,20 @@ void KeyboardSettingsWidget::set_keymaps(Vector<DeprecatedString> const& keymaps
     auto keymaps_string = DeprecatedString::join(',', keymaps);
     GUI::Process::spawn_or_show_error(window(), "/bin/keymap"sv, Array { "-s", keymaps_string.characters(), "-m", active_keymap.characters() });
 }
+
+void KeyboardSettingsWidget::write_caps_lock_to_ctrl_sys_variable(bool caps_lock_to_ctrl)
+{
+    if (getuid() != 0)
+        return;
+
+    auto write_command = DeprecatedString::formatted("caps_lock_to_ctrl={}", caps_lock_to_ctrl ? "1" : "0");
+    GUI::Process::spawn_or_show_error(window(), "/bin/sysctl"sv, Array { "-w", write_command.characters() });
+}
+
+ErrorOr<bool> KeyboardSettingsWidget::read_caps_lock_to_ctrl_sys_variable()
+{
+    auto file = TRY(Core::File::open("/sys/kernel/variables/caps_lock_to_ctrl"sv, Core::File::OpenMode::Read));
+    auto buffer = TRY(file->read_until_eof());
+    StringView contents_string((char const*)buffer.data(), min(1, buffer.size()));
+    return contents_string == "1";
+}

Userland/Applications/KeyboardSettings/KeyboardSettingsWidget.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/Error.h>
 #include <AK/Vector.h>
 #include <LibGUI/Button.h>
 #include <LibGUI/CheckBox.h>
@@ -28,6 +29,9 @@ private:
 
     void set_keymaps(Vector<DeprecatedString> const& keymaps, DeprecatedString const& active_keymap);
 
+    void write_caps_lock_to_ctrl_sys_variable(bool);
+    ErrorOr<bool> read_caps_lock_to_ctrl_sys_variable();
+
     Vector<DeprecatedString> m_initial_keymap_list;
 
     DeprecatedString m_initial_active_keymap;
@@ -35,6 +39,7 @@ private:
     RefPtr<GUI::ListView> m_selected_keymaps_listview;
     RefPtr<GUI::Label> m_active_keymap_label;
     RefPtr<GUI::CheckBox> m_num_lock_checkbox;
+    RefPtr<GUI::CheckBox> m_caps_lock_checkbox;
     RefPtr<GUI::Button> m_activate_keymap_button;
     RefPtr<GUI::Button> m_add_keymap_button;
     RefPtr<GUI::Button> m_remove_keymap_button;

Userland/Applications/KeyboardSettings/main.cpp

@@ -27,7 +27,9 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     TRY(Core::System::pledge("stdio rpath recvfd sendfd proc exec"));
     TRY(Core::System::unveil("/res", "r"));
     TRY(Core::System::unveil("/bin/keymap", "x"));
+    TRY(Core::System::unveil("/bin/sysctl", "x"));
     TRY(Core::System::unveil("/sys/kernel/keymap", "r"));
+    TRY(Core::System::unveil("/sys/kernel/variables/caps_lock_to_ctrl", "r"));
     TRY(Core::System::unveil("/etc/Keyboard.ini", "r"));
     TRY(Core::System::unveil(nullptr, nullptr));