[PATCH] D24750: [GC] Don't crash while processing Discarded sections

[Davide Italiano via llvm-commits]

davide added a comment.

The reason why gcc doesn't crash (and presumably also MSVC) is that this code is hitting UB.
`getFile() == nullptr`, but gcc decides to not crash, instead it returns a bogus pointer for the object pointed by `getFile()->getObj()`.

  diff --git a/ELF/MarkLive.cpp b/ELF/MarkLive.cpp
  index 4965377..0c6d510 100644
  --- a/ELF/MarkLive.cpp
  +++ b/ELF/MarkLive.cpp
  @@ -81,6 +81,14 @@ static ResolvedReloc<ELFT> resolveReloc(InputSectionBase<ELFT> &Sec,
   template <class ELFT>
   static void forEachSuccessor(InputSection<ELFT> &Sec,
                                std::function<void(ResolvedReloc<ELFT>)> Fn) {
  +
  +  if (&Sec == &InputSection<ELFT>::Discarded) {
  +    if (Sec.getFile() == nullptr) {
  +      ELFFile<ELFT> &Obj = Sec.getFile()->getObj();
  +      llvm::errs() << "Still alive: " << &Obj << "\n";
  +    }
  +  }
  +
     ELFFile<ELFT> &Obj = Sec.getFile()->getObj();
     for (const typename ELFT::Shdr *RelSec : Sec.RelocSections) {
       if (RelSec->sh_type == SHT_RELA) {



  $ /home/davide/work/llvm/build-gcc/./bin/ld.lld -shared /home/davide/work/llvm/build-gcc/tools/lld/test/ELF/Output/comdat.s.tmp.o /home/davide/work/llvm/build-gcc/tools/lld/test/ELF/Output/comdat.s.tmp.o /home/davide/work/llvm/build-gcc/tools/lld/test/ELF/Output/comdat.s.tmp2.o -o /home/davide/work/llvm/build-gcc/tools/lld/test/ELF/Output/comdat.s.tmp --gc-sections
  Still alive: 0x48


https://reviews.llvm.org/D24750