Re: [RFC PATCH v5 2/2] can: reject CAN FD content when disabled on CAN XL interfaces

[Oliver Hartkopp <socketcan@hartkopp.net>]

On 10.09.25 07:13, Vincent Mailhol wrote:
> On 10/09/2025 at 01:36, Oliver Hartkopp wrote:
>> Hi Vincent,
>>
>> On 09.09.25 11:24, Oliver Hartkopp wrote:
>>
>>
>>> -static unsigned int raw_check_txframe(struct raw_sock *ro, struct sk_buff
>>> *skb, int mtu)
>>> +static unsigned int raw_check_txframe(struct raw_sock *ro, struct sk_buff *skb,
>>> +                      struct net_device *dev)
>>>    {
>>>        /* Classical CAN -> no checks for flags and device capabilities */
>>>        if (can_is_can_skb(skb))
>>>            return CAN_MTU;
>>>    -    /* CAN FD -> needs to be enabled and a CAN FD or CAN XL device */
>>> -    if (ro->fd_frames && can_is_canfd_skb_mtu_len(skb) &&
>>> -        (mtu == CANFD_MTU || can_is_canxl_dev_mtu(mtu)))
>>> -        return CANFD_MTU;
>>> +    /* CAN FD -> needs to be enabled in a CAN FD or CAN XL device */
>>> +    if (ro->fd_frames && can_is_canfd_skb_mtu_len(skb)) {
>>> +        /* real/virtual CAN FD interface */
>>> +        if (dev->mtu == CANFD_MTU)
>>> +            return CANFD_MTU;
>>> +        if (can_is_canxl_dev_mtu(dev->mtu) &&
>>> +            can_dev_ctrlmode_fd_on(dev))
>>> +            return CANFD_MTU;
>>> +    }
>>
>> I've simplified the above code and rewrote the commit message in v6
>>
>>>        /* CAN XL -> needs to be enabled and a CAN XL device */
>>>        if (ro->xl_frames && can_is_canxl_skb(skb) &&
>>> -        can_is_canxl_dev_mtu(mtu))
>>> +        can_is_canxl_dev_mtu(dev->mtu))
>>>            return CANXL_MTU;
>>
>> We might also discuss if we create a can_dev_ctrlmode_xl_on(dev) function to
>> check if the CAN XL interface has CAN_CTRLMODE_XL enabled.
> 
> I checked ISO 11898-1:2024 again and the relevant wording I can found is in
> §6.6.21.4.1:
> 
>    When error signalling is disabled, the node shall transmit and
>    receive only XLFF frames. It shall not transmit EF, OF and RF, nor
>    DFs in CBFF, CEFF, FBFF and FEFF.
> 
> TLDR; Classical CAN is deactivated when error signaling is off.
> 
> So, I think that we also need the same logic for the Classical CAN. The nuance
> is that instead of using CAN_CTRLMODE_CC (which does not exist), we can check
> CAN_CTRLMODE_XL_ERR_SIGNAL. Note that CAN_CTRLMODE_XL_TMS implies that error
> signalling is off, so no need for extra checks on TMS. This is what I have in
> mind at the moment.
> 
>    static inline bool can_dev_cc_on(struct net_device *dev)
>    {
>    	struct can_priv *priv = safe_candev_priv(dev);
> 
>    	/* Classical CAN frames are always allowed on virtual interfaces */
>    	if (!priv)
>    		return true;
> 
>    	/* When error signalling is off only CAN XL frames are allowed */
>    	return !(priv->ctrlmode & CAN_CTRLMODE_XL) ||
>    		(priv->ctrlmode & CAN_CTRLMODE_XL_ERR_SIGNAL);
>    }
> 
>    static inline bool can_dev_fd_on(struct net_device *dev)
>    {
>    	struct can_priv *priv = safe_candev_priv(dev);
> 
> 	/* CAN FD is allowed on virtual interfaces if it fits the MTU */
>    	if (!priv)
>    		return dev->mtu == CANFD_MTU;
> 
>    	return can_dev_cc_on(dev) && (priv->ctrlmode & CAN_CTRLMODE_FD);
>    }
> 
>    static inline bool can_dev_xl_on(struct net_device *dev)
>    {
>    	struct can_priv *priv = safe_candev_priv(dev);
> 
> 	/* CAN XL is allowed on virtual interfaces if it fits the MTU */
>    	if (!priv)
>    		return dev->mtu == CANXL_MTU;

		return can_is_canxl_dev_mtu(mtu);

The MTU of CAN XL interfaces might vary.

> 
>    	return priv->ctrlmode & CAN_CTRLMODE_XL;
>    }
> 
> (the above is not yet tested, so beware of silly errors!)
> 
>> Currently my patches are based on Linus' rc5 tree where CAN_CTRLMODE_XL is not
>> defined. But I can rebase it on your b4/canxl-netlink branch if you like it.
> 
> I still want to finish the XL preparation series before moving to the actual thing.
> 
> You can do what is convinient for you:
> 
>    1/ rebase on b4/canxl-netlink branch but I take no claims if your code breaks
>       after one of my force push.

I will do this with my next attempt.

>    2/ add the flags you need in netlink.h. I will then chery pick your patches in
>       my series when ready and take care of any conflict for you!
> 
> I am going to send the v2 of my XL preparation series. Hopefully we can finalyse
> it soon so that we can focus on one sigle topic :)

ACK.

Best regards,
Oliver