Currently, the CAN FD skb validation logic is based on the MTU: the
interface is deemed FD capable if and only if its MTU is greater or
equal to CANFD_MTU.

This logic is showing its limit with the introduction of CAN XL. For
example, consider the two scenarios below:

  1. An interface configured with CAN FD on and CAN XL on

  2. An interface configured with CAN FD off and CAN XL on

In those two scenarios, the interfaces would have the same MTU:

  CANXL_MTU

making it impossible to differentiate which one has CAN FD turned on
and which one has it off.

Because of the limitation, the only non-UAPI-breaking workaround is to
do the check at the device level using the can_priv->ctrlmode flags.
Unfortunately, the virtual interfaces (vcan, vxcan), which do not have
a can_priv, are left behind.

Add a check on the CAN_CTRLMODE_FD flag in can_dev_dropped_skb() and
drop FD frames whenever the feature is turned off.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
Changelog:

RFC -> v1:

  - add an netdev_info_once() message.

  - this was sent as a standalone patch for discussion, it is now
    integrated in the CAN XL series.

  Link: https://lore.kernel.org/linux-can/20250907080504.598419-2-mailhol@kernel.org/
---
 include/linux/can/dev.h | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 0fe8f80f223e..d59b283c981a 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -103,12 +103,20 @@ static inline bool can_dev_dropped_skb(struct net_device *dev, struct sk_buff *s
 	if (priv->ctrlmode & CAN_CTRLMODE_LISTENONLY) {
 		netdev_info_once(dev,
 				 "interface in listen only mode, dropping skb\n");
-		kfree_skb(skb);
-		dev->stats.tx_dropped++;
-		return true;
+		goto invalid_skb;
+	}
+
+	if (!(priv->ctrlmode & CAN_CTRLMODE_FD) && can_is_canfd_skb(skb)) {
+		netdev_info_once(dev, "CAN FD is disabled, dropping skb\n");
+		goto invalid_skb;
 	}
 
 	return can_dropped_invalid_skb(dev, skb);
+
+invalid_skb:
+	kfree_skb(skb);
+	dev->stats.tx_dropped++;
+	return true;
 }
 
 void can_setup(struct net_device *dev);

-- 
2.51.0