ethtool: don't allow disabling queues with umem installed

We already check the RSS indirection table does not use queues which
would be disabled by channel reconfiguration. Make sure user does not
try to disable queues which have a UMEM and zero-copy AF_XDP socket
installed.

Signed-off-by: Jakub Kicinski <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

include/net/xdp_sock.h

···
 struct xdp_umem_fq_reuse *xsk_reuseq_swap(struct xdp_umem *umem,
 					  struct xdp_umem_fq_reuse *newq);
 void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq);
+struct xdp_umem *xdp_get_umem_from_qid(struct net_device *dev, u16 queue_id);
 
 static inline char *xdp_umem_get_data(struct xdp_umem *umem, u64 addr)
 {
···
 }
 static inline void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq)
 {
+}
+
+static inline struct xdp_umem *xdp_get_umem_from_qid(struct net_device *dev,
+						     u16 queue_id)
+{
+	return NULL;
 }
 
 static inline char *xdp_umem_get_data(struct xdp_umem *umem, u64 addr)

net/core/ethtool.c

···
 #include <linux/rtnetlink.h>
 #include <linux/sched/signal.h>
 #include <linux/net.h>
+#include <net/xdp_sock.h>
 
 /*
  * Some useful ethtool_ops methods that're device independent.
···
 						   void __user *useraddr)
 {
 	struct ethtool_channels channels, curr = { .cmd = ETHTOOL_GCHANNELS };
+	u16 from_channel, to_channel;
 	u32 max_rx_in_use = 0;
+	unsigned int i;
 
 	if (!dev->ethtool_ops->set_channels || !dev->ethtool_ops->get_channels)
 		return -EOPNOTSUPP;
···
 	    !ethtool_get_max_rxfh_channel(dev, &max_rx_in_use) &&
 	    (channels.combined_count + channels.rx_count) <= max_rx_in_use)
 	    return -EINVAL;
+
+	/* Disabling channels, query zero-copy AF_XDP sockets */
+	from_channel = channels.combined_count +
+		min(channels.rx_count, channels.tx_count);
+	to_channel = curr.combined_count + max(curr.rx_count, curr.tx_count);
+	for (i = from_channel; i < to_channel; i++)
+		if (xdp_get_umem_from_qid(dev, i))
+			return -EINVAL;
 
 	return dev->ethtool_ops->set_channels(dev, &channels);
 }

net/xdp/xdp_umem.c

···
 		dev->_tx[queue_id].umem = umem;
 }
 
-static struct xdp_umem *xdp_get_umem_from_qid(struct net_device *dev,
-					      u16 queue_id)
+struct xdp_umem *xdp_get_umem_from_qid(struct net_device *dev,
+				       u16 queue_id)
 {
 	if (queue_id < dev->real_num_rx_queues)
 		return dev->_rx[queue_id].umem;