bpf: Open code obj_get_info_by_fd in bpf preload.

Open code obj_get_info_by_fd in bpf preload.
It's the last part of libbpf that preload/iterators were using.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220131220528.98088-7-alexei.starovoitov@gmail.com

kernel/bpf/preload/iterators/iterators.c

@@ -16,6 +16,22 @@
 int to_kernel = -1;
 int from_kernel = 0;
 
+static int __bpf_obj_get_info_by_fd(int bpf_fd, void *info, __u32 *info_len)
+{
+	union bpf_attr attr;
+	int err;
+
+	memset(&attr, 0, sizeof(attr));
+	attr.info.bpf_fd = bpf_fd;
+	attr.info.info_len = *info_len;
+	attr.info.info = (long) info;
+
+	err = skel_sys_bpf(BPF_OBJ_GET_INFO_BY_FD, &attr, sizeof(attr));
+	if (!err)
+		*info_len = attr.info.info_len;
+	return err;
+}
+
 static int send_link_to_kernel(int link_fd, const char *link_name)
 {
 	struct bpf_preload_info obj = {};
@@ -23,7 +39,7 @@ static int send_link_to_kernel(int link_fd, const char *link_name)
 	__u32 info_len = sizeof(info);
 	int err;
 
-	err = bpf_obj_get_info_by_fd(link_fd, &info, &info_len);
+	err = __bpf_obj_get_info_by_fd(link_fd, &info, &info_len);
 	if (err)
 		return err;
 	obj.link_id = info.id;