bpftool: Print the enum's name instead of value

This patch prints the enum's name if there is one found in
the array of btf_enum.

The commit 9eea9849 ("bpf: fix BTF verification of enums")
has details about an enum could have any power-of-2 size (up to 8 bytes).
This patch also takes this chance to accommodate these non 4 byte
enums.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200318171637.128862-1-kafai@fb.com

tools/bpf/bpftool/btf_dumper.c

@@ -43,9 +43,42 @@ static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
 	return btf_dumper_do_type(d, actual_type_id, bit_offset, data);
 }
 
-static void btf_dumper_enum(const void *data, json_writer_t *jw)
+static int btf_dumper_enum(const struct btf_dumper *d,
+			    const struct btf_type *t,
+			    const void *data)
 {
-	jsonw_printf(jw, "%d", *(int *)data);
+	const struct btf_enum *enums = btf_enum(t);
+	__s64 value;
+	__u16 i;
+
+	switch (t->size) {
+	case 8:
+		value = *(__s64 *)data;
+		break;
+	case 4:
+		value = *(__s32 *)data;
+		break;
+	case 2:
+		value = *(__s16 *)data;
+		break;
+	case 1:
+		value = *(__s8 *)data;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	for (i = 0; i < btf_vlen(t); i++) {
+		if (value == enums[i].val) {
+			jsonw_string(d->jw,
+				     btf__name_by_offset(d->btf,
+							 enums[i].name_off));
+			return 0;
+		}
+	}
+
+	jsonw_int(d->jw, value);
+	return 0;
 }
 
 static int btf_dumper_array(const struct btf_dumper *d, __u32 type_id,
@@ -366,8 +399,7 @@ static int btf_dumper_do_type(const struct btf_dumper *d, __u32 type_id,
 	case BTF_KIND_ARRAY:
 		return btf_dumper_array(d, type_id, data);
 	case BTF_KIND_ENUM:
-		btf_dumper_enum(data, d->jw);
-		return 0;
+		return btf_dumper_enum(d, t, data);
 	case BTF_KIND_PTR:
 		btf_dumper_ptr(data, d->jw, d->is_plain_text);
 		return 0;