linux-kernel - Re: [PATCH] Input: xen-kbdfront - drop keys to shrink modalias

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Date:   Thu, 20 Oct 2022 10:31:34 +0200
From:   Mattijs Korpershoek <mkorpershoek@...libre.com>
To:     Jason Andryuk <jandryuk@...il.com>, linux-kernel@...r.kernel.org
Cc:     xen-devel@...ts.xenproject.org, Jason Andryuk <jandryuk@...il.com>,
        Phillip Susi <phill@...susis.net>, stable@...r.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-input@...r.kernel.org
Subject: Re: [PATCH] Input: xen-kbdfront - drop keys to shrink modalias

On Wed, Oct 19, 2022 at 16:14, Jason Andryuk <jandryuk@...il.com> wrote:

> xen kbdfront registers itself as being able to deliver *any* key since
> it doesn't know what keys the backend may produce.
>
> Unfortunately, the generated modalias gets too large and uevent creation
> fails with -ENOMEM.
>
> This can lead to gdm not using the keyboard since there is no seat
> associated [1] and the debian installer crashing [2].
>
> Trim the ranges of key capabilities by removing some BTN_* ranges.
> While doing this, some neighboring undefined ranges are removed to trim
> it further.
>
> This removes:
> BTN_DPAD_UP(0x220)..BTN_DPAD_RIGHT(0x223)
> Empty space 0x224..0x229
>
> Emtpy space 0x2bd..0x2bf
> BTN_TRIGGER_HAPPY(0x2c0)..BTN_TRIGGER_HAPPY40(0x2e7)
> Empty space 0x2e8..0x2ff
>
> The modalias shrinks from 2082 to 1754 bytes.
>
> [1] https://github.com/systemd/systemd/issues/22944
> [2] https://lore.kernel.org/xen-devel/87o8dw52jc.fsf@vps.thesusis.net/T/
>
> Cc: Phillip Susi <phill@...susis.net>
> Cc: stable@...r.kernel.org
> Signed-off-by: Jason Andryuk <jandryuk@...il.com>

Reviewed-by: Mattijs Korpershoek <mkorpershoek@...libre.com>

> ---
>  drivers/input/misc/xen-kbdfront.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/input/misc/xen-kbdfront.c b/drivers/input/misc/xen-kbdfront.c
> index 8d8ebdc2039b..23f37211be78 100644
> --- a/drivers/input/misc/xen-kbdfront.c
> +++ b/drivers/input/misc/xen-kbdfront.c
> @@ -256,7 +256,14 @@ static int xenkbd_probe(struct xenbus_device *dev,
>  		__set_bit(EV_KEY, kbd->evbit);
>  		for (i = KEY_ESC; i < KEY_UNKNOWN; i++)
>  			__set_bit(i, kbd->keybit);
> -		for (i = KEY_OK; i < KEY_MAX; i++)
> +		/* In theory we want to go KEY_OK..KEY_MAX, but that grows the
> +		 * modalias line too long.  KEY_KBD_LCD_MENU5 is the last
> +		 * defined non-button key. There is a gap of buttons from
> +		 * BTN_DPAD_UP..BTN_DPAD_RIGHT and KEY_ALS_TOGGLE is the next
> +		 * defined. */
> +		for (i = KEY_OK; i < BTN_DPAD_UP; i++)
> +			__set_bit(i, kbd->keybit);
> +		for (i = KEY_ALS_TOGGLE; i <= KEY_KBD_LCD_MENU5; i++)
>  			__set_bit(i, kbd->keybit);
>  
>  		ret = input_register_device(kbd);
> -- 
> 2.37.3