Don't push relocation sections onto InputSection<ELFT>::Discarded.RelocSections

[silvas]

Summary

LLVM3.3 (and earlier) would fail to include a relocation section in
the group that the section it was relocating is in. Object files
affected by this issue have been encountered in the wild when using LLD.

This would result in a siutation like:

Section {
  Index: 5
  Name: .text._Z3fooIiEvv (6)
  Type: SHT_PROGBITS (0x1)
  Flags [ (0x206)
    SHF_ALLOC (0x2)
    SHF_EXECINSTR (0x4)
    SHF_GROUP (0x200)
  ]
  Address: 0x0
  Offset: 0x48
  Size: 5
  Link: 0
  Info: 0
  AddressAlignment: 1
  EntrySize: 0
}
Section {
  Index: 6
  Name: .rela.text._Z3fooIiEvv (1)
  Type: SHT_RELA (0x4)
  Flags [ (0x0)
  ]
  Address: 0x0
  Offset: 0x3F0
  Size: 24
  Link: 8
  Info: 5
  AddressAlignment: 8
  EntrySize: 24
}

In LLD, during symbol resolution, we discard the section containing the
weak symbol, but this amounts to replacing it with
InputSection<ELFT>::Discarded.
When we later saw the corresponding relocation section, we would then
end up pusing onto InputSection<ELFT>::Discarded.RelocSections, which is
bogus.

[ruiu]

LGTM

ELF/InputFiles.cpp

217–218 ↗

(On Diff #46954)

It is better to add a brief comment saying that relocations and relocated sections must be in the same section group, so this is strictly speaking bogus file, but we want to handle that because LLVM 3.3 and earlier creates such file.

lld/trunk/ELF/InputFiles.cpp

case SHT_NULL:
       break;
     case SHT_RELA:
     case SHT_REL: {
       uint32_t RelocatedSectionIndex = Sec.sh_info;
       if (RelocatedSectionIndex >= Size)
         fatal("Invalid relocated section index");
       InputSectionBase<ELFT> *RelocatedSection =
           Sections[RelocatedSectionIndex];
+      // Strictly speaking, a relocation section must be included in the
+      // group of the section it relocates. However, LLVM 3.3 and earlier
+      // would fail to do so, so we gracefully handle that case.
+      if (RelocatedSection == &InputSection<ELFT>::Discarded)
+        continue;
       if (!RelocatedSection)
         fatal("Unsupported relocation reference");
       if (auto *S = dyn_cast<InputSection<ELFT>>(RelocatedSection)) {
         S->RelocSections.push_back(&Sec);
       } else if (auto *S = dyn_cast<EHInputSection<ELFT>>(RelocatedSection)) {
         if (S->RelocSection)
           fatal("Multiple relocation sections to .eh_frame are not supported");
         S->RelocSection = &Sec;

lld/trunk/test/ELF/llvm33-rela-outside-group.s

+// Input file generated with:
+// llvm33/llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %S/Inputs/llvm33-rela-outside-group.o
+//
+// RUN: ld.lld -shared %S/Inputs/llvm33-rela-outside-group.o %S/Inputs/llvm33-rela-outside-group.o
+
+  .global bar
+  .weak _Z3fooIiEvv
+
+  .section  .text._Z3fooIiEvv,"axG",@progbits,_Z3fooIiEvv,comdat
+_Z3fooIiEvv:
+  callq bar