libbpf: Support attaching struct_ops to a cgroup

Add bpf_map__attach_cgroup_opts() to attach a struct_ops map to a cgroup
through a BPF link.

Also extend struct bpf_prog_query_opts with a type_id field so a
BPF_STRUCT_OPS query on a cgroup can select the struct_ops type to
enumerate.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://patch.msgid.link/20260917200542.3689605-14-ameryhung@gmail.com

src/libbpf.c

@@ -14174,6 +14174,70 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
 	return &link->link;
 }
 
+struct bpf_link *bpf_map__attach_cgroup_opts(const struct bpf_map *map, int cgroup_fd,
+					     const struct bpf_cgroup_opts *opts)
+{
+	LIBBPF_OPTS(bpf_link_create_opts, link_create_opts);
+	struct bpf_link_struct_ops *link;
+	__u32 relative_id, zero = 0;
+	int err, fd, relative_fd;
+
+	if (!OPTS_VALID(opts, bpf_cgroup_opts))
+		return libbpf_err_ptr(-EINVAL);
+
+	if (!bpf_map__is_struct_ops(map)) {
+		pr_warn("map '%s': can't attach non-struct_ops map\n", map->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
+	if (map->fd < 0) {
+		pr_warn("map '%s': can't attach BPF map without FD (was it created?)\n", map->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
+	if (!(map->def.map_flags & BPF_F_LINK)) {
+		pr_warn("map '%s': can't attach to cgroup without BPF_F_LINK\n", map->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
+	relative_id = OPTS_GET(opts, relative_id, 0);
+	relative_fd = OPTS_GET(opts, relative_fd, 0);
+
+	if (relative_fd && relative_id) {
+		pr_warn("map '%s': relative_fd and relative_id cannot be set at the same time\n",
+			map->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
+	link_create_opts.cgroup.expected_revision = OPTS_GET(opts, expected_revision, 0);
+	link_create_opts.cgroup.relative_fd = relative_fd;
+	link_create_opts.cgroup.relative_id = relative_id;
+	link_create_opts.flags = OPTS_GET(opts, flags, 0);
+
+	link = calloc(1, sizeof(*link));
+	if (!link)
+		return libbpf_err_ptr(-ENOMEM);
+
+	err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
+	if (err && err != -EBUSY) {
+		free(link);
+		return libbpf_err_ptr(err);
+	}
+
+	link->link.detach = bpf_link__detach_struct_ops;
+
+	fd = bpf_link_create(map->fd, cgroup_fd, BPF_STRUCT_OPS, &link_create_opts);
+	if (fd < 0) {
+		free(link);
+		return libbpf_err_ptr(fd);
+	}
+
+	link->link.fd = fd;
+	link->map_fd = map->fd;
+
+	return &link->link;
+}
+
 /*
  * Swap the back struct_ops of a link with a new struct_ops map.
  */