net: dsa: refuse cross-chip mirroring operations

In case of a tc mirred action from one switch to another, the behavior
is not correct. We simply tell the source switch driver to program a
mirroring entry towards mirror->to_local_port = to_dp->index, but it is
not even guaranteed that the to_dp belongs to the same switch as dp.

For proper cross-chip support, we would need to go through the
cross-chip notifier layer in switch.c, program the entry on cascade
ports, and introduce new, explicit API for cross-chip mirroring, given
that intermediary switches should have introspection into the DSA tags
passed through the cascade port (and not just program a port mirror on
the entire cascade port). None of that exists today.

Reject what is not implemented so that user space is not misled into
thinking it works.

Fixes: f50f212 ("net: dsa: Add plumbing for port mirroring")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241008094320.3340980-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/user.c

@@ -1392,16 +1392,21 @@ dsa_user_add_cls_matchall_mirred(struct net_device *dev,
 	if (!dsa_user_dev_check(act->dev))
 		return -EOPNOTSUPP;
 
+	to_dp = dsa_user_to_port(act->dev);
+
+	if (dp->ds != to_dp->ds) {
+		NL_SET_ERR_MSG_MOD(extack,
+				   "Cross-chip mirroring not implemented");
+		return -EOPNOTSUPP;
+	}
+
 	mall_tc_entry = kzalloc(sizeof(*mall_tc_entry), GFP_KERNEL);
 	if (!mall_tc_entry)
 		return -ENOMEM;
 
 	mall_tc_entry->cookie = cls->cookie;
 	mall_tc_entry->type = DSA_PORT_MALL_MIRROR;
 	mirror = &mall_tc_entry->mirror;
-
-	to_dp = dsa_user_to_port(act->dev);
-
 	mirror->to_local_port = to_dp->index;
 	mirror->ingress = ingress;