bpftool: Support dumping metadata

Dump metadata in the 'bpftool prog' list if it's present.
For some formatting some BTF code is put directly in the
metadata dumping. Sanity checks on the map and the kind of the btf_type
to make sure we are actually dumping what we are expecting.

A helper jsonw_reset is added to json writer so we can reuse the same
json writer without having extraneous commas.

Sample output:

$ bpftool prog
6: cgroup_skb name prog tag bcf7977d3b93787c gpl
[...]
btf_id 4
metadata:
a = "foo"
b = 1

$ bpftool prog --json --pretty
[{
"id": 6,
[...]
"btf_id": 4,
"metadata": {
"a": "foo",
"b": 1
}
}
]

Signed-off-by: YiFei Zhu <[e-mail]>
Signed-off-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Cc: YiFei Zhu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/json_writer.c

···
 	self->pretty = on;
 }
 
+void jsonw_reset(json_writer_t *self)
+{
+	assert(self->depth == 0);
+	self->sep = '\0';
+}
+
 /* Basic blocks */
 static void jsonw_begin(json_writer_t *self, int c)
 {

tools/bpf/bpftool/json_writer.h

···
 /* Cause output to have pretty whitespace */
 void jsonw_pretty(json_writer_t *self, bool on);
 
+/* Reset separator to create new JSON */
+void jsonw_reset(json_writer_t *self);
+
 /* Add property name */
 void jsonw_name(json_writer_t *self, const char *name);
 

tools/bpf/bpftool/prog.c

···
 #include "main.h"
 #include "xlated_dumper.h"
 
+#define BPF_METADATA_PREFIX "bpf_metadata_"
+#define BPF_METADATA_PREFIX_LEN (sizeof(BPF_METADATA_PREFIX) - 1)
+
 const char * const prog_type_name[] = {
 	[BPF_PROG_TYPE_UNSPEC]			= "unspec",
 	[BPF_PROG_TYPE_SOCKET_FILTER]		= "socket_filter",
···
 	}
 }
 
+static void *find_metadata(int prog_fd, struct bpf_map_info *map_info)
+{
+	struct bpf_prog_info prog_info;
+	__u32 prog_info_len;
+	__u32 map_info_len;
+	void *value = NULL;
+	__u32 *map_ids;
+	int nr_maps;
+	int key = 0;
+	int map_fd;
+	int ret;
+	__u32 i;
+
+	memset(&prog_info, 0, sizeof(prog_info));
+	prog_info_len = sizeof(prog_info);
+	ret = bpf_obj_get_info_by_fd(prog_fd, &prog_info, &prog_info_len);
+	if (ret)
+		return NULL;
+
+	if (!prog_info.nr_map_ids)
+		return NULL;
+
+	map_ids = calloc(prog_info.nr_map_ids, sizeof(__u32));
+	if (!map_ids)
+		return NULL;
+
+	nr_maps = prog_info.nr_map_ids;
+	memset(&prog_info, 0, sizeof(prog_info));
+	prog_info.nr_map_ids = nr_maps;
+	prog_info.map_ids = ptr_to_u64(map_ids);
+	prog_info_len = sizeof(prog_info);
+
+	ret = bpf_obj_get_info_by_fd(prog_fd, &prog_info, &prog_info_len);
+	if (ret)
+		goto free_map_ids;
+
+	for (i = 0; i < prog_info.nr_map_ids; i++) {
+		map_fd = bpf_map_get_fd_by_id(map_ids[i]);
+		if (map_fd < 0)
+			goto free_map_ids;
+
+		memset(map_info, 0, sizeof(*map_info));
+		map_info_len = sizeof(*map_info);
+		ret = bpf_obj_get_info_by_fd(map_fd, map_info, &map_info_len);
+		if (ret < 0) {
+			close(map_fd);
+			goto free_map_ids;
+		}
+
+		if (map_info->type != BPF_MAP_TYPE_ARRAY ||
+		    map_info->key_size != sizeof(int) ||
+		    map_info->max_entries != 1 ||
+		    !map_info->btf_value_type_id ||
+		    !strstr(map_info->name, ".rodata")) {
+			close(map_fd);
+			continue;
+		}
+
+		value = malloc(map_info->value_size);
+		if (!value) {
+			close(map_fd);
+			goto free_map_ids;
+		}
+
+		if (bpf_map_lookup_elem(map_fd, &key, value)) {
+			close(map_fd);
+			free(value);
+			value = NULL;
+			goto free_map_ids;
+		}
+
+		close(map_fd);
+		break;
+	}
+
+free_map_ids:
+	free(map_ids);
+	return value;
+}
+
+static bool has_metadata_prefix(const char *s)
+{
+	return strncmp(s, BPF_METADATA_PREFIX, BPF_METADATA_PREFIX_LEN) == 0;
+}
+
+static void show_prog_metadata(int fd, __u32 num_maps)
+{
+	const struct btf_type *t_datasec, *t_var;
+	struct bpf_map_info map_info;
+	struct btf_var_secinfo *vsi;
+	bool printed_header = false;
+	struct btf *btf = NULL;
+	unsigned int i, vlen;
+	void *value = NULL;
+	const char *name;
+	int err;
+
+	if (!num_maps)
+		return;
+
+	memset(&map_info, 0, sizeof(map_info));
+	value = find_metadata(fd, &map_info);
+	if (!value)
+		return;
+
+	err = btf__get_from_id(map_info.btf_id, &btf);
+	if (err || !btf)
+		goto out_free;
+
+	t_datasec = btf__type_by_id(btf, map_info.btf_value_type_id);
+	if (!btf_is_datasec(t_datasec))
+		goto out_free;
+
+	vlen = btf_vlen(t_datasec);
+	vsi = btf_var_secinfos(t_datasec);
+
+	/* We don't proceed to check the kinds of the elements of the DATASEC.
+	 * The verifier enforces them to be BTF_KIND_VAR.
+	 */
+
+	if (json_output) {
+		struct btf_dumper d = {
+			.btf = btf,
+			.jw = json_wtr,
+			.is_plain_text = false,
+		};
+
+		for (i = 0; i < vlen; i++, vsi++) {
+			t_var = btf__type_by_id(btf, vsi->type);
+			name = btf__name_by_offset(btf, t_var->name_off);
+
+			if (!has_metadata_prefix(name))
+				continue;
+
+			if (!printed_header) {
+				jsonw_name(json_wtr, "metadata");
+				jsonw_start_object(json_wtr);
+				printed_header = true;
+			}
+
+			jsonw_name(json_wtr, name + BPF_METADATA_PREFIX_LEN);
+			err = btf_dumper_type(&d, t_var->type, value + vsi->offset);
+			if (err) {
+				p_err("btf dump failed: %d", err);
+				break;
+			}
+		}
+		if (printed_header)
+			jsonw_end_object(json_wtr);
+	} else {
+		json_writer_t *btf_wtr = jsonw_new(stdout);
+		struct btf_dumper d = {
+			.btf = btf,
+			.jw = btf_wtr,
+			.is_plain_text = true,
+		};
+
+		if (!btf_wtr) {
+			p_err("jsonw alloc failed");
+			goto out_free;
+		}
+
+		for (i = 0; i < vlen; i++, vsi++) {
+			t_var = btf__type_by_id(btf, vsi->type);
+			name = btf__name_by_offset(btf, t_var->name_off);
+
+			if (!has_metadata_prefix(name))
+				continue;
+
+			if (!printed_header) {
+				printf("\tmetadata:");
+				printed_header = true;
+			}
+
+			printf("\n\t\t%s = ", name + BPF_METADATA_PREFIX_LEN);
+
+			jsonw_reset(btf_wtr);
+			err = btf_dumper_type(&d, t_var->type, value + vsi->offset);
+			if (err) {
+				p_err("btf dump failed: %d", err);
+				break;
+			}
+		}
+		if (printed_header)
+			jsonw_destroy(&btf_wtr);
+	}
+
+out_free:
+	btf__free(btf);
+	free(value);
+}
+
 static void print_prog_header_json(struct bpf_prog_info *info)
 {
 	jsonw_uint_field(json_wtr, "id", info->id);
···
 
 	emit_obj_refs_json(&refs_table, info->id, json_wtr);
 
+	show_prog_metadata(fd, info->nr_map_ids);
+
 	jsonw_end_object(json_wtr);
 }
 
···
 	emit_obj_refs_plain(&refs_table, info->id, "\n\tpids ");
 
 	printf("\n");
+
+	show_prog_metadata(fd, info->nr_map_ids);
 }
 
 static int show_prog(int fd)