bpftool: Add support for querying cgroup_iter link

Support dumping info of a cgroup_iter link. This includes
showing the cgroup's id and the order for walking the cgroup
hierarchy. Example output is as follows:

> bpftool link show
1: iter  prog 2  target_name bpf_map
2: iter  prog 3  target_name bpf_prog
3: iter  prog 12  target_name cgroup  cgroup_id 72  order self_only

> bpftool -p link show
[{
        "id": 1,
        "type": "iter",
        "prog_id": 2,
        "target_name": "bpf_map"
    },{
        "id": 2,
        "type": "iter",
        "prog_id": 3,
        "target_name": "bpf_prog"
    },{
        "id": 3,
        "type": "iter",
        "prog_id": 12,
        "target_name": "cgroup",
        "cgroup_id": 72,
        "order": "self_only"
    }
]
Signed-off-by: Hao Luo <haoluo@google.com>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220829231828.1016835-1-haoluo@google.comSigned-off-by: Martin KaFai Lau <martin.lau@linux.dev>

tools/bpf/bpftool/link.c

@@ -83,6 +83,29 @@ static bool is_iter_map_target(const char *target_name)
 	       strcmp(target_name, "bpf_sk_storage_map") == 0;
 }
 
+static bool is_iter_cgroup_target(const char *target_name)
+{
+	return strcmp(target_name, "cgroup") == 0;
+}
+
+static const char *cgroup_order_string(__u32 order)
+{
+	switch (order) {
+	case BPF_CGROUP_ITER_ORDER_UNSPEC:
+		return "order_unspec";
+	case BPF_CGROUP_ITER_SELF_ONLY:
+		return "self_only";
+	case BPF_CGROUP_ITER_DESCENDANTS_PRE:
+		return "descendants_pre";
+	case BPF_CGROUP_ITER_DESCENDANTS_POST:
+		return "descendants_post";
+	case BPF_CGROUP_ITER_ANCESTORS_UP:
+		return "ancestors_up";
+	default: /* won't happen */
+		return "unknown";
+	}
+}
+
 static void show_iter_json(struct bpf_link_info *info, json_writer_t *wtr)
 {
 	const char *target_name = u64_to_ptr(info->iter.target_name);
@@ -91,6 +114,12 @@ static void show_iter_json(struct bpf_link_info *info, json_writer_t *wtr)
 
 	if (is_iter_map_target(target_name))
 		jsonw_uint_field(wtr, "map_id", info->iter.map.map_id);
+
+	if (is_iter_cgroup_target(target_name)) {
+		jsonw_lluint_field(wtr, "cgroup_id", info->iter.cgroup.cgroup_id);
+		jsonw_string_field(wtr, "order",
+				   cgroup_order_string(info->iter.cgroup.order));
+	}
 }
 
 static int get_prog_info(int prog_id, struct bpf_prog_info *info)
@@ -208,6 +237,12 @@ static void show_iter_plain(struct bpf_link_info *info)
 
 	if (is_iter_map_target(target_name))
 		printf("map_id %u  ", info->iter.map.map_id);
+
+	if (is_iter_cgroup_target(target_name)) {
+		printf("cgroup_id %llu  ", info->iter.cgroup.cgroup_id);
+		printf("order %s  ",
+		       cgroup_order_string(info->iter.cgroup.order));
+	}
 }
 
 static int show_link_close_plain(int fd, struct bpf_link_info *info)