dwarf_loader: Support DW_TAG_label outside DW_TAG_lexblock

This happens with asm CUs, noticed when building the Linux kernel with
clang 15, where we have, for instance:

  Contents of the .debug_info section:

    Compilation Unit @ offset 0x0:
     Length:        0x1df (32-bit)
     Version:       5
     Unit Type:     DW_UT_compile (1)
     Abbrev Offset: 0x0
     Pointer Size:  8
   <0><c>: Abbrev Number: 1 (DW_TAG_compile_unit)
      <d>   DW_AT_stmt_list   : 0x0
      <11>   DW_AT_ranges      : 0xc
      <15>   DW_AT_name        : arch/x86/kernel/verify_cpu.S
      <32>   DW_AT_comp_dir    : /home/nathan/cbl/src/linux
      <4d>   DW_AT_producer    : ClangBuiltLinux clang version 16.0.0 (https://github.com/llvm/llvm-project 7e22179d38c438fedb0d9bb0cff1585843bd7082)
      <c2>   DW_AT_language    : 32769    (MIPS assembler)
   <1><c4>: Abbrev Number: 2 (DW_TAG_label)
      <c5>   DW_AT_name        : startup_64
      <d0>   DW_AT_decl_file   : 0x0
      <d4>   DW_AT_decl_line   : 0x364
      <d8>   DW_AT_low_pc      : 0xffffffff81000000
   <1><e0>: Abbrev Number: 2 (DW_TAG_label)
      <e1>   DW_AT_name        : secondary_startup_64
      <f6>   DW_AT_decl_file   : 0x0
      <fa>   DW_AT_decl_line   : 0x399
      <fe>   DW_AT_low_pc      : 0xffffffff81000060
   <1><106>: Abbrev Number: 2 (DW_TAG_label)
      <107>   DW_AT_name        : secondary_startup_64_no_verify
      <126>   DW_AT_decl_file   : 0x0
      <12a>   DW_AT_decl_line   : 0x39f
      <12e>   DW_AT_low_pc      : 0xffffffff81000065
   <1><136>: Abbrev Number: 2 (DW_TAG_label)
      <137>   DW_AT_name        : verify_cpu
      <142>   DW_AT_decl_file   : 0x0
      <146>   DW_AT_decl_line   : 0x430
      <14a>   DW_AT_low_pc      : 0xffffffff81000150
  <SNIP>

Reported-by: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/dwarves/YzWSzXKcm6rSWOC5@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/dwarf_loader.c b/dwarf_loader.c
index 631bbd43..28a912ec 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1485,7 +1485,12 @@ static struct tag *die__create_new_label(Dwarf_Die *die,
 	if (label == NULL)
 		return NULL;
 
-	lexblock__add_label(lexblock, label);
+	if (lexblock != NULL) {
+		// asm CUs have labels and they will be in the cu top level tag list
+		// See die__process_unit()
+		lexblock__add_label(lexblock, label);
+	}
+
 	return &label->ip.tag;
 }
 
@@ -2037,6 +2042,12 @@ static struct tag *__die__process_tag(Dwarf_Die *die, struct cu *cu,
 		 */
 		tag = &unsupported_tag;
 		break;
+	case DW_TAG_label:
+		if (conf->ignore_labels)
+			tag = &unsupported_tag; // callers will assume conf->ignore_labels is true
+		else // We can have labels in asm CUs, no lexblock
+			tag = die__create_new_label(die, NULL, cu, conf);
+		break;
 	}
 
 	if (tag != NULL)
@@ -2055,7 +2066,8 @@ static int die__process_unit(Dwarf_Die *die, struct cu *cu, struct conf_load *co
 		if (tag == &unsupported_tag) {
 			// XXX special case DW_TAG_dwarf_procedure, appears when looking at a recent ~/bin/perf
 			// Investigate later how to properly support this...
-			if (dwarf_tag(die) != DW_TAG_dwarf_procedure)
+			if (dwarf_tag(die) != DW_TAG_dwarf_procedure &&
+			    dwarf_tag(die) != DW_TAG_label) // conf->ignore_labels == true, see die__process_tag()
 				tag__print_not_supported(dwarf_tag(die));
 			continue;
 		}