core: Check that we're adding DW_TAG_member sorted by byte offset

Rust reorder struct fields and pahole assumes them to be in order, as is
the case for languages like C and C++, etc. So when adding a
DW_TAG_member, add it in order.

Using: https://github.com/Rust-for-Linux/pahole-rust-cases/blob/main/inverted.o

Before:

  $ pahole --show_private_classes ../pahole-rust-cases/inverted.o
  struct S {

  	/* XXX 4 bytes hole, try to pack */

  	bool                       a __attribute__((__aligned__(1))); /*     4     1 */

  	/* XXX 65531 bytes hole, try to pack */
  	/* Bitfield combined with previous fields */

  	u32                        b __attribute__((__aligned__(4))); /*     0     4 */

  	/* size: 8, cachelines: 1, members: 2 */
  	/* sum members: 5, holes: 2, sum holes: 65535 */
  	/* padding: 4 */
  	/* forced alignments: 2, forced holes: 2, sum forced holes: 65535 */
  	/* last cacheline: 8 bytes */

  	/* BRAIN FART ALERT! 8 bytes != 5 (member bytes) + 0 (member bits) + 65535 (byte holes) + 0 (bit holes), diff = -524288 bits */
  } __attribute__((__aligned__(4)));
  $

After:

  $ readelf -wi ../pahole-rust-cases/inverted.o | grep DW_TAG_compile_unit -A9
   <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
      <c>   DW_AT_producer    : (indirect string, offset: 0x0): clang LLVM (rustc version 1.60.0 (7737e0b5c 2022-04-04))
      <10>   DW_AT_language    : 28	(Rust)
      <12>   DW_AT_name        : (indirect string, offset: 0x39): inverted.rs/@/inverted.c4dda47b-cgu.0
      <16>   DW_AT_stmt_list   : 0x0
      <1a>   DW_AT_comp_dir    : (indirect string, offset: 0x5f): /root/pahole-rust
      <1e>   DW_AT_GNU_pubnames: 1
      <1e>   DW_AT_low_pc      : 0x0
      <26>   DW_AT_high_pc     : 0x62
   <1><2a>: Abbrev Number: 2 (DW_TAG_namespace)
  $ pahole --show_private_classes ../pahole-rust-cases/inverted.o
  struct S {
  	u32                        b __attribute__((__aligned__(4))); /*     0     4 */
  	bool                       a __attribute__((__aligned__(1))); /*     4     1 */

  	/* size: 8, cachelines: 1, members: 2 */
  	/* padding: 3 */
  	/* forced alignments: 2 */
  	/* last cacheline: 8 bytes */
  } __attribute__((__aligned__(4)));
  $

  $ cp ../pahole-rust-cases/inverted.o .
  $ pahole --btf_encode inverted.o
  $ readelf -SW inverted.o  | grep -i BTF
    [26] .BTF              PROGBITS        0000000000000000 000922 00006c 00      0   0  1
  $
  $ pahole -F btf inverted.o
  struct S {
  	u32                        b;                    /*     0     4 */
  	bool                       a;                    /*     4     1 */

  	/* size: 8, cachelines: 1, members: 2 */
  	/* padding: 3 */
  	/* last cacheline: 8 bytes */
  };
  $

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Eric Curtin <ecurtin@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Cc: Neal Gompa <neal@gompa.dev>
Cc: Yonghong Song <yhs@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

dwarf_loader.c

@@ -2835,9 +2835,51 @@ static int class_member__cache_byte_size(struct tag *tag, struct cu *cu,
 	return 0;
 }
 
+static bool cu__language_reorders_offsets(const struct cu *cu)
+{
+	return cu->language == DW_LANG_Rust;
+}
+
+static int type__sort_by_offset(struct tag *tag, struct cu *cu, void *cookie __maybe_unused)
+{
+	if (!tag__is_type(tag))
+		return 0;
+
+	struct type *type = tag__type(tag);
+	struct class_member *current_member;
+
+	// There may be more than DW_TAG_members entries in the type tags, so do a simple
+	// bubble sort for now, so that the other non tags stay where they are.
+restart:
+	type__for_each_data_member(type, current_member) {
+		if (list_is_last(&current_member->tag.node, &type->namespace.tags))
+		       break;
+
+		struct class_member *next_member = list_entry(current_member->tag.node.next, typeof(*current_member), tag.node);
+
+		if (current_member->byte_offset < next_member->byte_offset)
+			continue;
+
+		list_del(&current_member->tag.node);
+		list_add(&current_member->tag.node, &next_member->tag.node);
+		goto restart;
+	}
+
+	return 0;
+}
+
+static void cu__sort_types_by_offset(struct cu *cu, struct conf_load *conf)
+{
+	cu__for_all_tags(cu, type__sort_by_offset, conf);
+}
+
 static int cu__finalize(struct cu *cu, struct conf_load *conf, void *thr_data)
 {
 	cu__for_all_tags(cu, class_member__cache_byte_size, conf);
+
+	if (cu__language_reorders_offsets(cu))
+		cu__sort_types_by_offset(cu, conf);
+
 	if (conf && conf->steal) {
 		return conf->steal(cu, conf, thr_data);
 	}