tools: bpftool: fix a bitfield pretty print issue

Commit b12d6ec0 ("bpf: btf: add btf print functionality")
added btf pretty print functionality to bpftool.
There is a problem though in printing a bitfield whose type
has modifiers.

For example, for a type like
  typedef int ___int;
  struct tmp_t {
          int a:3;
          ___int b:3;
  };
Suppose we have a map
  struct bpf_map_def SEC("maps") tmpmap = {
          .type = BPF_MAP_TYPE_HASH,
          .key_size = sizeof(__u32),
          .value_size = sizeof(struct tmp_t),
          .max_entries = 1,
  };
and the hash table is populated with one element with
key 0 and value (.a = 1 and .b = 2).

In BTF, the struct member "b" will have a type "typedef" which
points to an int type. The current implementation does not
pass the bit offset during transition from typedef to int type,
hence incorrectly print the value as
  $ bpftool m d id 79
  [{
          "key": 0,
          "value": {
              "a": 0x1,
              "b": 0x1
          }
      }
  ]

This patch fixed the issue by carrying bit_offset along the type
chain during bit_field print. The correct result can be printed as
  $ bpftool m d id 76
  [{
          "key": 0,
          "value": {
              "a": 0x1,
              "b": 0x2
          }
      }
  ]

The kernel pretty print is implemented correctly and does not
have this issue.

Fixes: b12d6ec0 ("bpf: btf: add btf print functionality")
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/btf_dumper.c

@@ -32,7 +32,7 @@ static void btf_dumper_ptr(const void *data, json_writer_t *jw,
 }
 
 static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
-			       const void *data)
+			       __u8 bit_offset, const void *data)
 {
 	int actual_type_id;
 
@@ -40,7 +40,7 @@ static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
 	if (actual_type_id < 0)
 		return actual_type_id;
 
-	return btf_dumper_do_type(d, actual_type_id, 0, data);
+	return btf_dumper_do_type(d, actual_type_id, bit_offset, data);
 }
 
 static void btf_dumper_enum(const void *data, json_writer_t *jw)
@@ -237,7 +237,7 @@ static int btf_dumper_do_type(const struct btf_dumper *d, __u32 type_id,
 	case BTF_KIND_VOLATILE:
 	case BTF_KIND_CONST:
 	case BTF_KIND_RESTRICT:
-		return btf_dumper_modifier(d, type_id, data);
+		return btf_dumper_modifier(d, type_id, bit_offset, data);
 	default:
 		jsonw_printf(d->jw, "(unsupported-kind");
 		return -EINVAL;