[ELF] - Teach LLD to use information from .debug_str for error reporting.

[grimar]

Summary

Recently we teached LLD to report line numbers for duplicate variables
definitions, though currently LLD is unable to do that for case when
strings are not built in .debug_info, but stored in .debug_str instead.
That is because out LLDDwarfObj does not handle .debug_str yet.
Patch fixes that.

[ruiu]

LGTM but please update the commit message before committing.

[grimar]

It was reverted in r317378 because of https://bugs.llvm.org/show_bug.cgi?id=35199.

Something looks just broken in libDebugInfo though not sure this patch is OK, so reopening it for now.
I'll be on vacations next week, and will be able to investigate the issue after 13 nov (if nobody fix it earlier).

[grimar]

Reason of crash was that DWARF parsers performs an attempt to parse address ranges
from .dwo files. Previously when no .debug_str was provided by LLD, it failed to do that
and things just worked. After this patch started to provide ".debug_str", code of DWARF
parsers attempts to call DWARFObject::getFileName() to open .dwo and method was not
implemented in LLD.

I don't know why DWARF parser might want to collect address ranges from .dwo files and
wrote a mail "Collecting address ranges in DWARFUnit::collectAddressRanges." to llvm-dev with question
about that. It seems excessive logic, at least it is not covered by any testcases I found.

In this patch I suggest to implement stub returning empty string for now. That way nothing
changes for LLD, we will return empty file name and DWARF parser will fail to parse .dwo file
for collecting address ranges. But at the same time we will not crash and so can land this patch.

Added testcase which crashed with llvm_unreachable at
https://github.com/llvm-mirror/llvm/blob/master/lib/DebugInfo/DWARF/DWARFUnit.cpp#L335
in previous iteration.

[ruiu]

LGTM

ELF/GdbIndex.h

 
 template <class ELFT> class LLDDwarfObj final : public llvm::DWARFObject {
   LLDDWARFSection InfoSection;
   LLDDWARFSection RangeSection;
   LLDDWARFSection LineSection;
   StringRef AbbrevSection;
   StringRef GnuPubNamesSection;
   StringRef GnuPubTypesSection;
+  StringRef StrSection;
 
   template <class RelTy>
   llvm::Optional<llvm::RelocAddrEntry> findAux(const InputSectionBase &Sec,
                                                uint64_t Pos,
                                                ArrayRef<RelTy> Rels) const;
 
 public:
   explicit LLDDwarfObj(ObjFile<ELFT> *Obj);
   const llvm::DWARFSection &getInfoSection() const override {
     return InfoSection;
   }
   const llvm::DWARFSection &getRangeSection() const override {
     return RangeSection;
   }
   const llvm::DWARFSection &getLineSection() const override {
     return LineSection;
   }
   StringRef getCUIndexSection() const override { return ""; }
   StringRef getAbbrevSection() const override { return AbbrevSection; }
-  StringRef getStringSection() const override { return ""; }
+  StringRef getStringSection() const override { return StrSection; }
   StringRef getGnuPubNamesSection() const override {
     return GnuPubNamesSection;
   }
   StringRef getGnuPubTypesSection() const override {
     return GnuPubTypesSection;
   }
   bool isLittleEndian() const override {
     return ELFT::TargetEndianness == llvm::support::little;

ELF/GdbIndex.cpp

if (LLDDWARFSection *M = StringSwitch<LLDDWARFSection *>(Sec->Name)
       continue;
     }
     if (Sec->Name == ".debug_abbrev")
       AbbrevSection = toStringRef(Sec->Data);
     else if (Sec->Name == ".debug_gnu_pubnames")
       GnuPubNamesSection = toStringRef(Sec->Data);
     else if (Sec->Name == ".debug_gnu_pubtypes")
       GnuPubTypesSection = toStringRef(Sec->Data);
+    else if (Sec->Name == ".debug_str")
+      StrSection = toStringRef(Sec->Data);
   }
 }
 
 // Find if there is a relocation at Pos in Sec.  The code is a bit
 // more complicated than usual because we need to pass a section index
 // to llvm since it has no idea about InputSection.
 template <class ELFT>
 template <class RelTy>

test/ELF/conflict-debug-variable2.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+
+# RUN: llvm-dwarfdump -v %t.o | FileCheck -check-prefix=INPUT %s
+# INPUT:     .debug_info contents:
+# INPUT:       DW_TAG_variable
+# INPUT-NEXT:    DW_AT_name [DW_FORM_strp]       ( .debug_str[0x00000027] = "foo")
+# INPUT-NEXT:    DW_AT_type [DW_FORM_ref4]       (cu + 0x0033 => {0x00000033} "int")
+# INPUT-NEXT:    DW_AT_external [DW_FORM_flag_present]   (true)
+# INPUT-NEXT:    DW_AT_decl_file [DW_FORM_data1] ("/home/path/test.c")
+# INPUT-NEXT:    DW_AT_decl_line [DW_FORM_data1] (1)
+# INPUT-NEXT:    DW_AT_location [DW_FORM_exprloc]        (DW_OP_addr 0x0)
+# INPUT:       DW_TAG_variable
+# INPUT-NEXT:    DW_AT_name [DW_FORM_strp]       ( .debug_str[0x0000002f] = "bar")
+# INPUT-NEXT:    DW_AT_type [DW_FORM_ref4]       (cu + 0x0033 => {0x00000033} "int")
+# INPUT-NEXT:    DW_AT_external [DW_FORM_flag_present]   (true)
+# INPUT-NEXT:    DW_AT_decl_file [DW_FORM_data1] ("/home/path/test.c")
+# INPUT-NEXT:    DW_AT_decl_line [DW_FORM_data1] (2)
+# INPUT-NEXT:    DW_AT_location [DW_FORM_exprloc]        (DW_OP_addr 0x0)
+
+## Check we use information from .debug_info in messages.
+# RUN: not ld.lld %t.o %t.o -o %t 2>&1 | FileCheck %s
+# CHECK:      duplicate symbol: bar
+# CHECK-NEXT: >>> defined at test.c:2
+# CHECK-NEXT: >>>            {{.*}}:(bar)
+# CHECK-NEXT: >>> defined at test.c:2
+# CHECK-NEXT: >>>            {{.*}}:(.data+0x0)
+# CHECK:      duplicate symbol: foo
+# CHECK-NEXT: >>> defined at test.c:1
+# CHECK-NEXT: >>>            {{.*}}:(foo)
+# CHECK-NEXT: >>> defined at test.c:1
+# CHECK-NEXT: >>>            {{.*}}:(.bss+0x0)
+
+# Used reduced output from following code and clang
+# version 6.0.0 (trunk 316661) to produce this input file:
+# Source (test.c):
+#  int foo = 0;
+#  int bar = 1;
+# Invocation: g++ -g -S test.c
+
+.text
+.file  "test.c"
+.file  1 "test.c"
+
+.type  foo,@object
+.bss
+.globl  foo
+.p2align  2
+foo:
+ .long  0
+ .size  foo, 4
+
+.type  bar,@object
+.data
+.globl  bar
+.p2align  2
+bar:
+ .long  1
+ .size  bar, 4
+
+.section  .debug_str,"MS",@progbits,1
+.Linfo_string0:
+  .asciz  "clang version 6.0.0"
+.Linfo_string1:
+  .asciz  "test.c"
+.Linfo_string2:
+  .asciz  "/home/path/"
+.Linfo_string3:
+  .asciz  "foo"
+.Linfo_string4:
+  .asciz  "int"
+.Linfo_string5:
+  .asciz  "bar"
+  
+.section  .debug_abbrev,"",@progbits
+  .byte  1                       # Abbreviation Code
+  .byte  17                      # DW_TAG_compile_unit
+  .byte  1                       # DW_CHILDREN_yes
+  .byte  37                      # DW_AT_producer
+  .byte  14                      # DW_FORM_strp
+  .byte  19                      # DW_AT_language
+  .byte  5                       # DW_FORM_data2
+  .byte  3                       # DW_AT_name
+  .byte  14                      # DW_FORM_strp
+  .byte  16                      # DW_AT_stmt_list
+  .byte  23                      # DW_FORM_sec_offset
+  .byte  27                      # DW_AT_comp_dir
+  .byte  14                      # DW_FORM_strp
+  .ascii  "\264B"                # DW_AT_GNU_pubnames
+  .byte  25                      # DW_FORM_flag_present
+  .byte  0                       # EOM(1)
+  .byte  0                       # EOM(2)
+
+  .byte  2                       # Abbreviation Code
+  .byte  52                      # DW_TAG_variable
+  .byte  0                       # DW_CHILDREN_no
+  .byte  3                       # DW_AT_name
+  .byte  14                      # DW_FORM_strp
+  .byte  73                      # DW_AT_type
+  .byte  19                      # DW_FORM_ref4
+  .byte  63                      # DW_AT_external
+  .byte  25                      # DW_FORM_flag_present
+  .byte  58                      # DW_AT_decl_file
+  .byte  11                      # DW_FORM_data1
+  .byte  59                      # DW_AT_decl_line
+  .byte  11                      # DW_FORM_data1
+  .byte  2                       # DW_AT_location
+  .byte  24                      # DW_FORM_exprloc
+  .byte  0                       # EOM(1)
+  .byte  0                       # EOM(2)
+
+  .byte  3                       # Abbreviation Code
+  .byte  36                      # DW_TAG_base_type
+  .byte  0                       # DW_CHILDREN_no
+  .byte  3                       # DW_AT_name
+  .byte  14                      # DW_FORM_strp
+  .byte  62                      # DW_AT_encoding
+  .byte  11                      # DW_FORM_data1
+  .byte  11                      # DW_AT_byte_size
+  .byte  11                      # DW_FORM_data1
+  .byte  0                       # EOM(1)
+  .byte  0                       # EOM(2)
+  .byte  0                       # EOM(3)
+
+.section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long  76                      # Length of Unit
+  .short  4                      # DWARF version number
+  .long  .debug_abbrev           # Offset Into Abbrev. Section
+  .byte  8                       # Address Size (in bytes)
+
+  .byte  1                       # Abbrev [1] 0xb:0x45 DW_TAG_compile_unit
+  .long  .Linfo_string0          # DW_AT_producer
+  .short  12                     # DW_AT_language
+  .long  .Linfo_string1          # DW_AT_name
+  .long  0                       # DW_AT_stmt_list
+  .long  .Linfo_string2          # DW_AT_comp_dir
+
+  .byte  2                       # Abbrev [2] 0x1e:0x15 DW_TAG_variable
+  .long  .Linfo_string3          # DW_AT_name
+  .long  51                      # DW_AT_type
+  .byte  1                       # DW_AT_decl_file
+  .byte  1                       # DW_AT_decl_line
+  .byte  9                       # DW_AT_location
+  .byte  3
+  .quad  foo
+
+  .byte  3                       # Abbrev [3] 0x33:0x7 DW_TAG_base_type
+  .long  .Linfo_string4          # DW_AT_name
+  .byte  5                       # DW_AT_encoding
+  .byte  4                       # DW_AT_byte_size
+
+  .byte  2                       # Abbrev [2] 0x3a:0x15 DW_TAG_variable
+  .long  .Linfo_string5          # DW_AT_name
+  .long  51                      # DW_AT_type
+  .byte  1                       # DW_AT_decl_file
+  .byte  2                       # DW_AT_decl_line
+  .byte  9                       # DW_AT_location
+  .byte  3
+  .quad  bar
+  .byte  0                       # End Of Children Mark