can: netlink: can_changelink(): allow disabling of automatic restart

Since the commit c1f3f97 ("can: netlink: can_changelink(): fix NULL
pointer deref of struct can_priv::do_set_mode"), the automatic restart
delay can only be set for devices that implement the restart handler struct
can_priv::do_set_mode. As it makes no sense to configure a automatic
restart for devices that doesn't support it.

However, since systemd commit 13ce5d4632e3 ("network/can: properly handle
CAN.RestartSec=0") [1], systemd-networkd correctly handles a restart delay
of "0" (i.e. the restart is disabled). Which means that a disabled restart
is always configured in the kernel.

On systems with both changes active this causes that CAN interfaces that
don't implement a restart handler cannot be brought up by systemd-networkd.

Solve this problem by allowing a delay of "0" to be configured, even if the
device does not implement a restart handler.

[1] systemd/systemd@13ce5d4

Cc: stable@vger.kernel.org
Cc: Andrei Lalaev <andrey.lalaev@gmail.com>
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Closes: https://lore.kernel.org/all/20251020-certain-arrogant-vole-of-sunshine-141841-mkl@pengutronix.de
Fixes: c1f3f97 ("can: netlink: can_changelink(): fix NULL pointer deref of struct can_priv::do_set_mode")
Link: https://patch.msgid.link/20251020-netlink-fix-restart-v1-1-3f53c7f8520b@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: marckleinebudde

drivers/net/can/dev/netlink.c

@@ -452,7 +452,9 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
 	}
 
 	if (data[IFLA_CAN_RESTART_MS]) {
-		if (!priv->do_set_mode) {
+		unsigned int restart_ms = nla_get_u32(data[IFLA_CAN_RESTART_MS]);
+
+		if (restart_ms != 0 && !priv->do_set_mode) {
 			NL_SET_ERR_MSG(extack,
 				       "Device doesn't support restart from Bus Off");
 			return -EOPNOTSUPP;
@@ -461,7 +463,7 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
 		/* Do not allow changing restart delay while running */
 		if (dev->flags & IFF_UP)
 			return -EBUSY;
-		priv->restart_ms = nla_get_u32(data[IFLA_CAN_RESTART_MS]);
+		priv->restart_ms = restart_ms;
 	}
 
 	if (data[IFLA_CAN_RESTART]) {