vt: add fallback to plain map for modifier-aware key types

When a key is pressed with modifiers (Shift, Ctrl, Alt, etc.) and the
modifier-specific keymap has no binding (K_HOLE) or doesn't exist, fall
back to the plain keymap if the plain entry is a modifier-aware type
(KT_CUR or KT_CSI).

This allows arrow keys and CSI navigation keys to automatically handle
all modifier combinations with just a single plain map entry. The key
handlers (k_cur and k_csi) read the modifier state at runtime and encode
it into the output sequence.

For example, with just:
    keycode 103 = Up
    keycode 104 = Csi_Home

All these combinations now work automatically:
    Up         -> ESC [ A
    Shift+Up   -> ESC [ 1 ; 2 A
    Ctrl+Up    -> ESC [ 1 ; 5 A
    Home       -> ESC [ 1 ~
    Shift+Home -> ESC [ 1 ; 2 ~
    Ctrl+Home  -> ESC [ 1 ; 5 ~

Previously, each modifier combination required an explicit keymap entry,
which was tedious and consumed keymap slots.

Explicit modifier bindings still take precedence - the fallback only
triggers when the modifier-specific entry is empty.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://patch.msgid.link/20260203045457.1049793-4-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 44fd67eb723a..dfdea0842149 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -1498,6 +1498,21 @@ static void kbd_keycode(unsigned int keycode, int down, bool hw_raw)
 	param.ledstate = kbd->ledflagstate;
 	key_map = key_maps[shift_final];
 
+	/*
+	 * Fall back to the plain map if modifiers are active, the modifier-
+	 * specific map is missing or has no entry, and the plain map has a
+	 * modifier-aware key type (KT_CUR or KT_CSI). These handlers encode
+	 * the modifier state into the emitted escape sequence.
+	 */
+	if (shift_final && keycode < NR_KEYS &&
+	    (!key_map || key_map[keycode] == K_HOLE) && key_maps[0]) {
+		unsigned short plain = key_maps[0][keycode];
+		unsigned char type = KTYP(plain);
+
+		if (type >= 0xf0 && (type - 0xf0 == KT_CUR || type - 0xf0 == KT_CSI))
+			key_map = key_maps[0];
+	}
+
 	rc = atomic_notifier_call_chain(&keyboard_notifier_list,
 					KBD_KEYCODE, &param);
 	if (rc == NOTIFY_STOP || !key_map) {