Re: [PATCH] vt: fix spurious modifier in CSI/cursor key sequences

[Nicolas Pitre]

Ping.

On Thu, 25 Jun 2026, Nicolas Pitre wrote:

> From: Nicolas Pitre <npitre@xxxxxxxxxxxx>
> 
> csi_modifier_param() builds the xterm modifier parameter from
> shift_state, counting KG_SHIFTL/KG_SHIFTR as Shift, KG_ALTGR as Alt
> and KG_CTRLL/KG_CTRLR as Ctrl in addition to the canonical KG_SHIFT,
> KG_ALT and KG_CTRL.
> 
> That is wrong when those weights are not plain modifiers. Keymaps
> derived from XKB layouts (by kbd's xkbsupport, and by the
> console-setup used in Debian, Ubuntu and others) encode the active
> layout group using KG_SHIFTL/KG_SHIFTR:
> 
> 	group 1: -
> 	group 2: shiftl
> 	group 3: shiftr
> 	group 4: shiftl | shiftr
> 
> So while a non-default layout group is selected, KG_SHIFTL and/or
> KG_SHIFTR are set in shift_state with no Shift key held.
> csi_modifier_param() then adds a spurious Shift to every cursor and
> CSI key: pressing Up while group 2 is active emits ESC[1;2A (Shift+Up)
> instead of ESC[A. KG_ALTGR has the same problem since it is the
> standard third-level selector.
> 
> Normal keymaps bind the physical Shift/Ctrl/Alt keys to KG_SHIFT,
> KG_CTRL and KG_ALT, leaving the left/right and AltGr weights free for
> layout and level selection. Count only those canonical weights, so
> genuine modifiers are still encoded while layout/level selectors are
> not.
> 
> Fixes: 4af70f151671 ("vt: add modifier support to cursor keys")
> Reported-by: Alexey Gladkov <legion@xxxxxxxxxx>
> Closes: https://lore.kernel.org/kbd/aj2gR0Y7sM6i9s2G@xxxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Nicolas Pitre <npitre@xxxxxxxxxxxx>
> ---
>  drivers/tty/vt/keyboard.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index dfdea0842149..763a3f1b7be0 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -765,16 +765,22 @@ static void k_fn(struct vc_data *vc, unsigned char value, char up_flag)
>  /*
>   * Compute xterm-style modifier parameter for CSI sequences.
>   * Returns 1 + (shift ? 1 : 0) + (alt ? 2 : 0) + (ctrl ? 4 : 0)
> + *
> + * Only the canonical modifier weights are counted. The left/right variants
> + * (KG_SHIFTL, KG_SHIFTR, KG_CTRLL, KG_CTRLR) and KG_ALTGR are commonly
> + * repurposed as keymap layout-group or level selectors rather than as plain
> + * modifiers (for instance XKB-derived keymaps select the layout group with
> + * KG_SHIFTL/KG_SHIFTR), so counting them would encode a spurious modifier.
>   */
>  static int csi_modifier_param(void)
>  {
>  	int mod = 1;
>  
> -	if (shift_state & (BIT(KG_SHIFT) | BIT(KG_SHIFTL) | BIT(KG_SHIFTR)))
> +	if (shift_state & BIT(KG_SHIFT))
>  		mod += 1;
> -	if (shift_state & (BIT(KG_ALT) | BIT(KG_ALTGR)))
> +	if (shift_state & BIT(KG_ALT))
>  		mod += 2;
> -	if (shift_state & (BIT(KG_CTRL) | BIT(KG_CTRLL) | BIT(KG_CTRLR)))
> +	if (shift_state & BIT(KG_CTRL))
>  		mod += 4;
>  	return mod;
>  }
> -- 
> 2.54.0
> 
>