dwarf_loader: Support btf_type_tag attribute

LLVM patches ([1] for clang, [2] and [3] for BPF backend)
added support for btf_type_tag attributes. The following is
an example:

  [$ ~] cat t.c
  #define __tag1 __attribute__((btf_type_tag("tag1")))
  #define __tag2 __attribute__((btf_type_tag("tag2")))
  int __tag1 * __tag1 __tag2 *g __attribute__((section(".data..percpu")));
  [$ ~] clang -O2 -g -c t.c
  [$ ~] llvm-dwarfdump --debug-info t.o
  t.o:    file format elf64-x86-64
  0x0000001e:   DW_TAG_variable
                  DW_AT_name      ("g")
                  DW_AT_type      (0x00000033 "int **")
                  DW_AT_external  (true)
                  DW_AT_decl_file ("/home/yhs/t.c")
                  DW_AT_decl_line (3)
                  DW_AT_location  (DW_OP_addr 0x0)
  0x00000033:   DW_TAG_pointer_type
                  DW_AT_type      (0x0000004b "int *")
  0x00000038:     DW_TAG_LLVM_annotation
                    DW_AT_name    ("btf_type_tag")
                    DW_AT_const_value     ("tag1")
  0x00000041:     DW_TAG_LLVM_annotation
                    DW_AT_name    ("btf_type_tag")
                    DW_AT_const_value     ("tag2")
  0x0000004a:     NULL
  0x0000004b:   DW_TAG_pointer_type
                  DW_AT_type      (0x0000005a "int")
  0x00000050:     DW_TAG_LLVM_annotation
                    DW_AT_name    ("btf_type_tag")
                    DW_AT_const_value     ("tag1")
  0x00000059:     NULL
  0x0000005a:   DW_TAG_base_type
                  DW_AT_name      ("int")
                  DW_AT_encoding  (DW_ATE_signed)
                  DW_AT_byte_size (0x04)
  0x00000061:   NULL

From the above example, you can see that DW_TAG_pointer_type may contain
one or more DW_TAG_LLVM_annotation btf_type_tag tags.  If
DW_TAG_LLVM_annotation tags are present inside DW_TAG_pointer_type, for
BTF encoding, pahole will need to follow [3] to generate a type chain

  var -> ptr -> tag2 -> tag1 -> ptr -> tag1 -> int

This patch implemented dwarf_loader support. If a pointer type contains
DW_TAG_LLVM_annotation tags, a new type btf_type_tag_ptr_type will be
created which will store the pointer tag itself and all
DW_TAG_LLVM_annotation tags.  During recoding stage, the type chain will
be formed properly based on the above example.

An option "--skip_encoding_btf_type_tag" is added to disable
this new functionality.

  [1] https://reviews.llvm.org/D111199
  [2] https://reviews.llvm.org/D113222
  [3] https://reviews.llvm.org/D113496

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Cc: kernel-team@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
3 files changed