libbpf: Add support for bpf_link-based cgroup attachment

Add bpf_program__attach_cgroup(), which uses BPF_LINK_CREATE subcommand to
create an FD-based kernel bpf_link. Also add low-level bpf_link_create() API.

If expected_attach_type is not specified explicitly with
bpf_program__set_expected_attach_type(), libbpf will try to determine proper
attach type from BPF program's section definition.

Also add support for bpf_link's underlying BPF program replacement:
  - unconditional through high-level bpf_link__update_program() API;
  - cmpxchg-like with specifying expected current BPF program through
    low-level bpf_link_update() API.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200330030001.2312810-4-andriin@fb.com

src/bpf.h

@@ -168,6 +168,25 @@ LIBBPF_API int bpf_prog_detach(int attachable_fd, enum bpf_attach_type type);
 LIBBPF_API int bpf_prog_detach2(int prog_fd, int attachable_fd,
 				enum bpf_attach_type type);
 
+struct bpf_link_create_opts {
+	size_t sz; /* size of this struct for forward/backward compatibility */
+};
+#define bpf_link_create_opts__last_field sz
+
+LIBBPF_API int bpf_link_create(int prog_fd, int target_fd,
+			       enum bpf_attach_type attach_type,
+			       const struct bpf_link_create_opts *opts);
+
+struct bpf_link_update_opts {
+	size_t sz; /* size of this struct for forward/backward compatibility */
+	__u32 flags;	   /* extra flags */
+	__u32 old_prog_fd; /* expected old program FD */
+};
+#define bpf_link_update_opts__last_field old_prog_fd
+
+LIBBPF_API int bpf_link_update(int link_fd, int new_prog_fd,
+			       const struct bpf_link_update_opts *opts);
+
 struct bpf_prog_test_run_attr {
 	int prog_fd;
 	int repeat;