bpftool: Dump more info about DATASEC members

Dump succinct information for each member of DATASEC: its kinds and name. This
is extremely helpful to see at a quick glance what is inside each DATASEC of
a given BTF. Without this, one has to jump around BTF data to just find out
the name of a VAR or FUNC. DATASEC's var_secinfo member is special in that
regard because it doesn't itself contain the name of the member, delegating
that to the referenced VAR and FUNC kinds. Other kinds, like
STRUCT/UNION/FUNC/ENUM, encode member names directly and thus are clearly
identifiable in BTF dump.

The new output looks like this:

[35] DATASEC '.bss' size=0 vlen=6
type_id=8 offset=0 size=4 (VAR 'input_bss1')
type_id=13 offset=0 size=4 (VAR 'input_bss_weak')
type_id=16 offset=0 size=4 (VAR 'output_bss1')
type_id=17 offset=0 size=4 (VAR 'output_data1')
type_id=18 offset=0 size=4 (VAR 'output_rodata1')
type_id=20 offset=0 size=8 (VAR 'output_sink1')
[36] DATASEC '.data' size=0 vlen=2
type_id=9 offset=0 size=4 (VAR 'input_data1')
type_id=14 offset=0 size=4 (VAR 'input_data_weak')
[37] DATASEC '.kconfig' size=0 vlen=2
type_id=25 offset=0 size=4 (VAR 'LINUX_KERNEL_VERSION')
type_id=28 offset=0 size=1 (VAR 'CONFIG_BPF_SYSCALL')
[38] DATASEC '.ksyms' size=0 vlen=1
type_id=30 offset=0 size=1 (VAR 'bpf_link_fops')
[39] DATASEC '.rodata' size=0 vlen=2
type_id=12 offset=0 size=4 (VAR 'input_rodata1')
type_id=15 offset=0 size=4 (VAR 'input_rodata_weak')
[40] DATASEC 'license' size=0 vlen=1
type_id=24 offset=0 size=4 (VAR 'LICENSE')

Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/btf.c

···
 	return btf__name_by_offset(btf, off) ? : "(invalid)";
 }
 
+static int btf_kind_safe(int kind)
+{
+	return kind <= BTF_KIND_MAX ? kind : BTF_KIND_UNKN;
+}
+
 static int dump_btf_type(const struct btf *btf, __u32 id,
 			 const struct btf_type *t)
 {
 	json_writer_t *w = json_wtr;
-	int kind, safe_kind;
-
-	kind = BTF_INFO_KIND(t->info);
-	safe_kind = kind <= BTF_KIND_MAX ? kind : BTF_KIND_UNKN;
+	int kind = btf_kind(t);
 
 	if (json_output) {
 		jsonw_start_object(w);
 		jsonw_uint_field(w, "id", id);
-		jsonw_string_field(w, "kind", btf_kind_str[safe_kind]);
+		jsonw_string_field(w, "kind", btf_kind_str[btf_kind_safe(kind)]);
 		jsonw_string_field(w, "name", btf_str(btf, t->name_off));
 	} else {
-		printf("[%u] %s '%s'", id, btf_kind_str[safe_kind],
+		printf("[%u] %s '%s'", id, btf_kind_str[btf_kind_safe(kind)],
 		       btf_str(btf, t->name_off));
 	}
 
-	switch (BTF_INFO_KIND(t->info)) {
+	switch (kind) {
 	case BTF_KIND_INT: {
 		__u32 v = *(__u32 *)(t + 1);
 		const char *enc;
···
 		break;
 	}
 	case BTF_KIND_DATASEC: {
-		const struct btf_var_secinfo *v = (const void *)(t+1);
+		const struct btf_var_secinfo *v = (const void *)(t + 1);
+		const struct btf_type *vt;
 		__u16 vlen = BTF_INFO_VLEN(t->info);
 		int i;
 
···
 			} else {
 				printf("\n\ttype_id=%u offset=%u size=%u",
 				       v->type, v->offset, v->size);
+
+				if (v->type <= btf__get_nr_types(btf)) {
+					vt = btf__type_by_id(btf, v->type);
+					printf(" (%s '%s')",
+					       btf_kind_str[btf_kind_safe(btf_kind(vt))],
+					       btf_str(btf, vt->name_off));
+				}
 			}
 		}
 		if (json_output)