BACKPORT: net: check for driver support in netmem TX

We should not enable netmem TX for drivers that don't declare support.

Check for driver netmem TX support during devmem TX binding and fail if
the driver does not have the functionality.

Check for driver support in validate_xmit_skb as well.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Pranjal Shrivastava <praan@google.com>

Author: pran005

net/core/dev.c

@@ -3765,6 +3765,9 @@ static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device
 
 	skb = validate_xmit_xfrm(skb, features, again);
 
+	if (!skb_frags_readable(skb) && !dev->netmem_tx)
+		goto out_kfree_skb;
+
 	return skb;
 
 out_kfree_skb:

net/core/netdev-genl.c

@@ -879,6 +879,13 @@ int netdev_nl_bind_tx_doit(struct sk_buff *skb, struct genl_info *info)
 		goto err_unlock;
 	}
 
+	if (!netdev->netmem_tx) {
+		err = -EOPNOTSUPP;
+		NL_SET_ERR_MSG(info->extack,
+			       "Driver does not support netmem TX");
+		goto err_unlock;
+	}
+
 	binding = net_devmem_bind_dmabuf(netdev, DMA_TO_DEVICE, dmabuf_fd,
 					 info->extack);
 	if (IS_ERR(binding)) {