[ELF2] Don't crash on symbolless relocations when writing shared libraries

[hfinkel]

Summary

This is, in a sense, a related fix to D13564. For certain special relocations, such as R_PPC64_TOC, there is no actual corresponding symbol in the object file. In these cases Obj.getRelocationSymbol will return nullptr.

This allows us to produce shared libraries from PPC64 object files. I've yet to verify that the resulting behavior is completely correct, but in the name of incremental progress, at least we won't segfault.

[ruiu]

LGTM

[hfinkel]

This patch is no longer necessary after r250095. The test case change will be merged into D13565 prior to commit.

ELF/OutputSections.cpp

for (const DynamicReloc<ELFT> &Rel : Relocs) {
     uint32_t Type = RI.getType(IsMips64EL);
 
     bool CanBePreempted = canBePreempted(Body);
     uintX_t Addend = 0;
     if (!CanBePreempted) {
       if (IsRela) {
         if (Body)
           Addend += getSymVA<ELFT>(cast<ELFSymbolBody<ELFT>>(*Body));
-        else
-          Addend += getLocalSymVA(
-              Obj.getRelocationSymbol(&RI, File.getSymbolTable()), File);
+        else if (const auto *Sym =
+                 Obj.getRelocationSymbol(&RI, File.getSymbolTable()))
+          Addend += getLocalSymVA(Sym, File);
       }
       P->setSymbolAndType(0, Target->getRelativeReloc(), IsMips64EL);
     }
 
     if (Body && Target->relocNeedsGot(Type, *Body)) {
       P->r_offset = Out<ELFT>::Got->getEntryAddr(*Body);
       if (CanBePreempted)
         P->setSymbolAndType(Body->getDynamicSymbolTableIndex(),

test/elf2/Inputs/shared-ppc64.s

 .section        ".opd","aw"
 .global bar
 bar:
-# FIXME: This should be: .quad   .Lbar,.TOC.@tocbase,0
-# But we can't handle .TOC.@tocbase for shared libraries yet
+.quad   .Lbar,.TOC.@tocbase,0
 .quad   .Lbar,0,0
 
 .text
 .Lbar:
         blr