[llvm-readelf] - Do not omit a zero symbol value when printing relocations.

[grimar]

Summary

Previously we only printed a symbol value when it has a non-empty name
or non-zero value.

This patch changes the behavior. Now we only omit a symbols value when
a relocation does not reference a symbol (i.e. symbol index == 0).

I think it is what GNU readelf does, looking on its output.
And such behavior looks much more reasonable to me.

[grimar]

Note: reloc-zero-name-or-value.test was added in D59823. Seems it only documented the existent behavior.

[jhenderson]

LGTM, if it is matching GNU.

llvm/test/tools/llvm-readobj/ELF/reloc-zero-name-or-value.test

-# Show that the value field is omitted if a symbol has no name or value, but is
-# printed if one is present. Test for both static and dynamic relocation
-# printing.
+## Show that the value field is omitted when a relocation does not reference a symbol.
+## In other cases, particularly when a symbol has a zero value or when it has an empty
+## name, we print it. Test for both static and dynamic relocation printing.
 
 # RUN: yaml2obj %s -o %t
 # RUN: llvm-readelf --relocations --dyn-relocations %t | FileCheck %s
 
-# CHECK:      Relocation section '.rela.text' at offset {{.*}} contains 4 entries:
+# CHECK:      Relocation section '.rela.text' at offset {{.*}} contains 5 entries:
 # CHECK-NEXT:     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
 # CHECK-NEXT: 0000000000000000  0000000000000000 R_X86_64_NONE                           1
 # CHECK-NEXT: 0000000000000000  0000000100000000 R_X86_64_NONE          0000000000000000 sym + 1
 # CHECK-NEXT: 0000000000000000  0000000200000000 R_X86_64_NONE          0000000000000123 456
-# CHECK-NEXT: 0000000000000000  0000000300000000 R_X86_64_NONE                           678
+# CHECK-NEXT: 0000000000000000  0000000300000000 R_X86_64_NONE          0000000000000000 678
+# CHECK-NEXT: 0000000000000000  0000000400000000 R_X86_64_NONE          0000000000000000 2
 
-# CHECK:      Relocation section '.rela.dyn' at offset {{.*}} contains 4 entries:
+# CHECK:      Relocation section '.rela.dyn' at offset {{.*}} contains 5 entries:
 # CHECK-NEXT:     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
 # CHECK-NEXT: 0000000000000000  0000000000000000 R_X86_64_NONE                           1
 # CHECK-NEXT: 0000000000000000  0000000100000000 R_X86_64_NONE          0000000000000000 sym + 1
 # CHECK-NEXT: 0000000000000000  0000000200000000 R_X86_64_NONE          0000000000000123 456
-# CHECK-NEXT: 0000000000000000  0000000300000000 R_X86_64_NONE                           678
+# CHECK-NEXT: 0000000000000000  0000000300000000 R_X86_64_NONE          0000000000000000 678
+# CHECK-NEXT: 0000000000000000  0000000400000000 R_X86_64_NONE          0000000000000000 2
 
-# CHECK:      'RELA' relocation section at offset {{.*}} contains 96 bytes:
+# CHECK:      'RELA' relocation section at offset {{.*}} contains 120 bytes:
 # CHECK-NEXT:     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
 # CHECK-NEXT: 0000000000000000  0000000000000000 R_X86_64_NONE                           1
 # CHECK-NEXT: 0000000000000000  0000000100000000 R_X86_64_NONE          0000000000000000 sym + 1
 # CHECK-NEXT: 0000000000000000  0000000200000000 R_X86_64_NONE          0000000000000123 456
-# CHECK-NEXT: 0000000000000000  0000000300000000 R_X86_64_NONE                           678
+# CHECK-NEXT: 0000000000000000  0000000300000000 R_X86_64_NONE          0000000000000000 678
+# CHECK-NEXT: 0000000000000000  0000000400000000 R_X86_64_NONE          0000000000000000 2
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_EXEC
   Machine: EM_X86_64
 Sections:
Relocations:
       - Offset: 0
         Type: R_X86_64_NONE
         Addend: 0x456
         Symbol: 2
       - Offset: 0
         Addend: 0x678
         Type:   R_X86_64_NONE
         Symbol: 3
+## References a symbol with a zero value and an empty name.
+      - Offset: 0
+        Type: R_X86_64_NONE
+        Addend: 2
+        Symbol: 4
   - Name: .dynamic
     Type: SHT_DYNAMIC
     Flags: [SHF_ALLOC]
     Address: 0x1000
     AddressAlign: 0x1000
     Entries:
       - Tag: DT_RELA
         Value: 0x1100
       - Tag: DT_RELASZ
-        Value: 96
+        Value: 120
       - Tag: DT_RELAENT
         Value: 24
       - Tag: DT_NULL
         Value: 0
   - Name: .rela.dyn
     Type: SHT_RELA
     Flags: [SHF_ALLOC]
     Info: .text
Relocations:
       - Offset: 0
         Type: R_X86_64_NONE
         Addend: 0x456
         Symbol: 2
       - Offset: 0
         Addend: 0x678
         Type:   R_X86_64_NONE
         Symbol: 3
+## References a symbol with a zero value and an empty name.
+      - Offset: 0
+        Type: R_X86_64_NONE
+        Addend: 2
+        Symbol: 4
 Symbols:
   - Name:    sym
     Value:   0
     Section: .text
     Binding: STB_GLOBAL
   - Value:   0x123
     Section: .text
     Binding: STB_GLOBAL
   - Type:  STT_SECTION
     Index: 0
+  - Value:   0x0
+    Section: .text
+    Binding: STB_GLOBAL
 DynamicSymbols:
   - Name:    sym
     Value:   0
     Section: .text
     Binding: STB_GLOBAL
   - Value:   0x123
     Section: .text
     Binding: STB_GLOBAL
   - Type:  STT_SECTION
     Index: 0
+  - Value:   0x0
+    Section: .text
+    Binding: STB_GLOBAL
 ProgramHeaders:
   - Type: PT_LOAD
     VAddr: 0x1000
     Sections:
       - Section: .dynamic
       - Section: .rela.dyn
   - Type: PT_DYNAMIC
     VAddr: 0x1000
     Sections:
       - Section: .dynamic

llvm/tools/llvm-readobj/ELFDumper.cpp

void GNUStyle<ELFT>::printRelocation(const ELFO *Obj, const Elf_Sym *Sym,
 
   Fields[0].Str = to_string(format_hex_no_prefix(R.r_offset, Width));
   Fields[1].Str = to_string(format_hex_no_prefix(R.r_info, Width));
 
   SmallString<32> RelocName;
   Obj->getRelocationTypeName(R.getType(Obj->isMips64EL()), RelocName);
   Fields[2].Str = RelocName.c_str();
 
-  if (Sym && (!SymbolName.empty() || Sym->getValue() != 0))
+  if (Sym)
     Fields[3].Str = to_string(format_hex_no_prefix(Sym->getValue(), Width));
 
   Fields[4].Str = std::string(SymbolName);
   for (const Field &F : Fields)
     printField(F);
 
   std::string Addend;
   if (IsRela) {
return WarnAndReturn(
         "index is greater than or equal to the number of dynamic symbols (" +
             Twine(Symbols.size()) + ")");
 
   const typename ELFT::Sym *Sym = FirstSym + SymIndex;
   Expected<StringRef> ErrOrName = Sym->getName(Dumper->getDynamicStringTable());
   if (!ErrOrName)
     return WarnAndReturn(Sym, toString(ErrOrName.takeError()));
 
-  return {Sym, maybeDemangle(*ErrOrName)};
+  return {Sym == FirstSym ? nullptr : Sym, maybeDemangle(*ErrOrName)};
 }
 } // namespace
 
 template <class ELFT>
 void GNUStyle<ELFT>::printDynamicRelocation(const ELFO *Obj, Elf_Rela R,
                                             bool IsRela) {
   RelSymbol<ELFT> S = getSymbolForReloc(Obj, this->FileName, this->dumper(), R);
   printRelocation(Obj, S.Sym, S.Name, R, IsRela);