libbpf: Fix bpf_xdp_query() in old kernels

Commit 04d58f1b("libbpf: add API to get XDP/XSK supported features")
added feature_flags to struct bpf_xdp_query_opts. If a user uses
bpf_xdp_query_opts with feature_flags member, the bpf_xdp_query()
will check whether 'netdev' family exists or not in the kernel.
If it does not exist, the bpf_xdp_query() will return -ENOENT.

But 'netdev' family does not exist in old kernels as it is
introduced in the same patch set as Commit 04d58f1b.
So old kernel with newer libbpf won't work properly with
bpf_xdp_query() api call.

To fix this issue, if the return value of
libbpf_netlink_resolve_genl_family_id() is -ENOENT, bpf_xdp_query()
will just return 0, skipping the rest of xdp feature query.
This preserves backward compatibility.

Fixes: 04d58f1b ("libbpf: add API to get XDP/XSK supported features")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230227224943.1153459-1-yhs@fb.com

tools/lib/bpf/netlink.c

@@ -468,8 +468,13 @@ int bpf_xdp_query(int ifindex, int xdp_flags, struct bpf_xdp_query_opts *opts)
 		return 0;
 
 	err = libbpf_netlink_resolve_genl_family_id("netdev", sizeof("netdev"), &id);
-	if (err < 0)
+	if (err < 0) {
+		if (err == -ENOENT) {
+			opts->feature_flags = 0;
+			goto skip_feature_flags;
+		}
 		return libbpf_err(err);
+	}
 
 	memset(&req, 0, sizeof(req));
 	req.nh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
@@ -489,6 +494,7 @@ int bpf_xdp_query(int ifindex, int xdp_flags, struct bpf_xdp_query_opts *opts)
 
 	opts->feature_flags = md.flags;
 
+skip_feature_flags:
 	return 0;
 }