[PATCH bpf] libbpf: Fix bpf_xdp_query() in old kernels

[Yonghong Song <yhs@fb.com>]

Commit 04d58f1b26a4("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 04d58f1b26a4.
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: 04d58f1b26a4 ("libbpf: add API to get XDP/XSK supported features")
Cc: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 tools/lib/bpf/netlink.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index 1653e7a8b0a1..4c1b3502f88d 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -468,8 +468,11 @@ 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)
+			return 0;
 		return libbpf_err(err);
+	}
 
 	memset(&req, 0, sizeof(req));
 	req.nh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
-- 
2.30.2