[ELF] - Fail the link if something happens on DWARF parsing stage of -gdb-index building

[grimar]

Summary

This is relative to PR33173,

Previously if something wrong happened on DWARF parsers side during parsing
object for building gdb index (like was in PR: unsupported relocation) then LLD continued
and finished the link. DWARF parsers sure showed error message on their side, but that is all.

Patch changes behavior to fail the link in this case and show more detailed message.

[ruiu]

lld/trunk/ELF/SyntheticSections.cpp

1817

What does this ErrorPolicy::Continue mean? If there's something like Abort, you want to abort, as if something is broken in a DWARF debug info, it is very likely that the following bytes are also garbage.

[grimar]

lld/trunk/ELF/SyntheticSections.cpp

1817

There are ErrorPolicy::Continue and ErrorPolicy::Halt currently imlemented.
First one allows parsers to continue parsing object, second terminates parsing.

Continue used here intentionally because allows to report as many errors as possible and not Abort instantly.
(that is consistent with how compilers (and linkers) behave usually).

it is very likely that the following bytes are also garbage.

Broken objects is only a one side, there are many examples where Continue behavior may be more desirable:

• For example currently we have case when one of X86_64 relocations is not supported (PR33173), we probably want to report all such cases and not fail after very first error.
• Another example - we currently have AVR target in LLD, and DWARF parsers do not support its relocations at all. With Continue we will list ErrorLimit amount of places where it faces unimplemented relocations, what can be useful for working on AVR and other possible new targets.
• If something wrong with compressed section, DWARF parsers would report a error currently ("failed to decompress .... "), it makes sence to continue parsing other sections to find more possible issues I think.

lld/trunk/ELF/SyntheticSections.cpp

 GdbIndexChunk GdbIndexSection::readDwarf(InputSection *Sec) {
   Expected<std::unique_ptr<object::ObjectFile>> Obj =
       object::ObjectFile::createObjectFile(Sec->File->MB);
   if (!Obj) {
     error(toString(Sec->File) + ": error creating DWARF context");
     return {};
   }
 
-  DWARFContextInMemory Dwarf(*Obj.get());
+  DWARFContextInMemory Dwarf(*Obj.get(), nullptr, [&](Error E) {
+    error(toString(Sec->File) + ": error parsing DWARF data:\n>>> " +
+          toString(std::move(E)));
+    return ErrorPolicy::Continue;
+  });
 
   GdbIndexChunk Ret;
   Ret.CompilationUnits = readCuList(Dwarf, Sec);
   Ret.AddressArea = readAddressArea(Dwarf, Sec);
   Ret.NamesAndTypes = readPubNamesAndTypes(Dwarf, Config->IsLE);
   return Ret;
 }
 

lld/trunk/test/ELF/invalid/invalid-debug-relocations.test

+# REQUIRES: x86
+# RUN: yaml2obj %s -o %t.o
+# RUN: not ld.lld -gdb-index %t.o -o %t.exe 2>&1 | FileCheck %s
+
+# CHECK:      error: {{.*}}.o: error parsing DWARF data:
+# CHECK-NEXT: >>> failed to compute relocation: Unknown, Invalid data was encountered while parsing the file
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_386
+Sections:
+  - Type:            SHT_PROGBITS
+    Name:            .text
+    Flags:           [ ]
+    AddressAlign:    0x04
+    Content:         "0000"
+  - Type:            SHT_PROGBITS
+    Name:            .debug_info
+    Flags:           [ ]
+    AddressAlign:    0x04
+    Content:         "0000"
+  - Type:            SHT_REL
+    Name:            .rel.debug_info
+    Link:            .symtab
+    Info:            .debug_info
+    Relocations:
+      - Offset:          0
+        Symbol:          _start
+        Type:            0xFF
+      - Offset:          4
+        Symbol:          _start
+        Type:            0xFF
+Symbols:
+  Global:
+    - Name:     _start
+      Type:     STT_FUNC
+      Section:  .text
+      Value:    0x0