linux-kernel - Re: [PATCH 2/3] tty/vt: Prevent 8th bit corruption with soft cursor

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Message-ID: <6e52d938-3ac2-4779-b70d-9cc58c16c266@kernel.org>
Date: Sun, 31 Aug 2025 08:25:46 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Zsolt Kajtar <soci@....rulez.org>, linux-serial@...r.kernel.org,
 linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org
Subject: Re: [PATCH 2/3] tty/vt: Prevent 8th bit corruption with soft cursor

On 29. 08. 25, 21:49, Zsolt Kajtar wrote:
> The attributes of the soft cursor are configurable and one would rightly
> expect that only the attributes are to be affected. But that's not
> guaranteed for a font with 512 glyphs as the 8th bit is in the attribute
> byte. This patch makes sure that really only the attribute bits are
> changed by the cursor and not the glyph's appearance.
> 
> Signed-off-by: Zsolt Kajtar <soci@....rulez.org>
> ---
>   drivers/tty/vt/vt.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index c6c931047..d54f4d24e 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -823,6 +823,7 @@ static void add_softcursor(struct vc_data *vc)
>   {
>   	int i = scr_readw((u16 *) vc->vc_pos);

It's terrible how the current code uses weird types (int here).

>   	u32 type = vc->vc_cursor_type;
> +	int mask = vc->vc_hi_font_mask | 0xff;

But since you are introducing a new one, use u16, not int. Esp. when it 
is a mask.
>   	if (!(type & CUR_SW))
>   		return;
> @@ -836,6 +837,7 @@ static void add_softcursor(struct vc_data *vc)
>   		i ^= CUR_BG;
>   	if ((type & CUR_INVERT_FG_BG) && (i & CUR_FG) == ((i & CUR_BG) >> 4))
>   		i ^= CUR_FG;
> +	i = (i & ~mask) | (softcursor_original & mask);
>   	scr_writew(i, (u16 *)vc->vc_pos);
>   	if (con_should_update(vc))
>   		con_putc(vc, i, vc->state.y, vc->state.x);

thanks,
-- 
js
suse labs