xdp: Provide extack messages when prog attachment failed

In order to provide more meaningful messages to user when the process of
loading xdp program onto network interface failed, let's add extack
messages within dev_change_xdp_fd.

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/dev.c

@@ -7983,20 +7983,26 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 	query = flags & XDP_FLAGS_HW_MODE ? XDP_QUERY_PROG_HW : XDP_QUERY_PROG;
 
 	bpf_op = bpf_chk = ops->ndo_bpf;
-	if (!bpf_op && (flags & (XDP_FLAGS_DRV_MODE | XDP_FLAGS_HW_MODE)))
+	if (!bpf_op && (flags & (XDP_FLAGS_DRV_MODE | XDP_FLAGS_HW_MODE))) {
+		NL_SET_ERR_MSG(extack, "underlying driver does not support XDP in native mode");
 		return -EOPNOTSUPP;
+	}
 	if (!bpf_op || (flags & XDP_FLAGS_SKB_MODE))
 		bpf_op = generic_xdp_install;
 	if (bpf_op == bpf_chk)
 		bpf_chk = generic_xdp_install;
 
 	if (fd >= 0) {
 		if (__dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG) ||
-		    __dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG_HW))
+		    __dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG_HW)) {
+			NL_SET_ERR_MSG(extack, "native and generic XDP can't be active at the same time");
 			return -EEXIST;
+		}
 		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) &&
-		    __dev_xdp_query(dev, bpf_op, query))
+		    __dev_xdp_query(dev, bpf_op, query)) {
+			NL_SET_ERR_MSG(extack, "XDP program already attached");
 			return -EBUSY;
+		}
 
 		prog = bpf_prog_get_type_dev(fd, BPF_PROG_TYPE_XDP,
 					     bpf_op == ops->ndo_bpf);