ethtool: Fix context creation with no parameters

The 'at least one change' requirement is not applicable for context
creation, skip the check in such case.
This allows a command such as 'ethtool -X eth0 context new' to work.

The command works by mistake when using older versions of userspace
ethtool due to an incompatibility issue where rxfh.input_xfrm is passed
as zero (unset) instead of RXH_XFRM_NO_CHANGE as done with recent
userspace. This patch does not try to solve the incompatibility issue.

Link: https://lore.kernel.org/netdev/05ae8316-d3aa-4356-98c6-55ed4253c8a7@nvidia.com/
Fixes: 84a1d9c ("net: ethtool: extend RXNFC API to support RSS spreading of filter matches")
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Jianbo Liu <jianbol@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20240807173352.3501746-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: gal-pressman

net/ethtool/ioctl.c

@@ -1369,14 +1369,17 @@ static noinline_for_stack int ethtool_set_rxfh(struct net_device *dev,
 		return -EOPNOTSUPP;
 	create = rxfh.rss_context == ETH_RXFH_CONTEXT_ALLOC;
 
-	/* If either indir, hash key or function is valid, proceed further.
-	 * Must request at least one change: indir size, hash key, function
-	 * or input transformation.
-	 */
 	if ((rxfh.indir_size &&
 	     rxfh.indir_size != ETH_RXFH_INDIR_NO_CHANGE &&
 	     rxfh.indir_size != dev_indir_size) ||
-	    (rxfh.key_size && (rxfh.key_size != dev_key_size)) ||
+	    (rxfh.key_size && rxfh.key_size != dev_key_size))
+		return -EINVAL;
+
+	/* Must request at least one change: indir size, hash key, function
+	 * or input transformation.
+	 * There's no need for any of it in case of context creation.
+	 */
+	if (!create &&
 	    (rxfh.indir_size == ETH_RXFH_INDIR_NO_CHANGE &&
 	     rxfh.key_size == 0 && rxfh.hfunc == ETH_RSS_HASH_NO_CHANGE &&
 	     rxfh.input_xfrm == RXH_XFRM_NO_CHANGE))