Avoid a gnu readelf warning

[• espindola]

Summary

This avoids a readelf warning, matches bfd ld and should be harmless.

[emaste]

ELF/SyntheticSections.cpp

1233

Why .got.plt and not .plt?

[grimar]

ELF/SyntheticSections.cpp

1233

Do we really want to care about readelf warnings about something that is not in ELF spec (I assume it is not) ? While it is not a error... may be we can just ignore it ?

[ruiu]

ELF/SyntheticSections.cpp

1233

I think I feel the same as George. Is this an issue?

Can you always set or always not set Info?

[emaste]

ELF/SyntheticSections.cpp

1233

Ah, I believe I observed (older) ld.bfd storing .plt's section in .rela.plt Info, but testing now with ld.bfd 2.28 I get behaviour as expected from this patch:

  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
...
  [ 9] .rela.plt         RELA             00000000004003b8  000003b8
       0000000000000060  0000000000000018  AI       5    21     8
...
  [21] .got.plt          PROGBITS         00000000006009f0  000009f0
       0000000000000038  0000000000000008  WA       0     0     8

[ruiu]

It looks a bit of cargo-cult, and I think I'm not very happy about this change. If it is not too annoying, I wouldn't do this at least right now.

[emaste]

FWIW our standard readelf in FreeBSD comes from the ELF Tool Chain project and doesn't have the warning. It's only when the GNU readelf package is installed that we'd encounter this.

Overall I'm indifferent to this change going in.

[dim]

Let's indeed drop this, since upstream readelf has been fixed.

[• espindola]

Fixed in upstream gnu readelf.

ELF/SyntheticSections.cpp

 void RelocationBaseSection::finalizeContents() {
   // If all relocations are R_*_RELATIVE they don't refer to any
   // dynamic symbol and we don't need a dynamic symbol table. If that
   // is the case, just use 0 as the link.
   Link = InX::DynSymTab ? InX::DynSymTab->getParent()->SectionIndex : 0;
 
   // Set required output section properties.
   getParent()->Link = Link;
+
+  // In non relocatable files relocations use virtual addresses, not section
+  // offsets. Given that, there doesn't seem to be any strong reason to set
+  // sh_info for relocation sections. We do it for .rela.plt only because gnu
+  // readelf produces a warning if we don't.
+  if (this == InX::RelaPlt) {
+    Info = InX::GotPlt->getParent()->SectionIndex;
+    getParent()->Info = Info;
+  }
 }
 
 template <class ELFT>
 static void encodeDynamicReloc(typename ELFT::Rela *P,
                                const DynamicReloc &Rel) {
   if (Config->IsRela)
     P->r_addend = Rel.getAddend();
   P->r_offset = Rel.getOffset();

test/ELF/dynamic-reloc.s

 // CHECK-NEXT: Type: SHT_RELA
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address: [[RELAADDR:.*]]
 // CHECK-NEXT: Offset:
 // CHECK-NEXT: Size: [[RELASIZE:.*]]
 // CHECK-NEXT: Link: 1
-// CHECK-NEXT: Info: 0
+// CHECK-NEXT: Info: [[RELALINK:.*]]
 // CHECK-NEXT: AddressAlignment: 8
 // CHECK-NEXT: EntrySize: 24
 
 // CHECK:      Name: .text
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_EXECINSTR
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address: 0x201000
 
+// CHECK:      Index: [[RELALINK]]
+// CHECK-NEXT: Name: .got.plt
+
 // CHECK:      Relocations [
 // CHECK-NEXT:   Section ({{.*}}) .rela.plt {
 // CHECK-NEXT:     Relocation {
 // CHECK-NEXT:       Offset: 0x202018
 // CHECK-NEXT:       Type: R_X86_64_JUMP_SLOT
 // CHECK-NEXT:       Symbol: bar
 // CHECK-NEXT:       Addend: 0x0
 // CHECK-NEXT:     }