[ELF] - Fix segfault when processing .eh_frame.

[grimar]

Summary

Its a PR34648 which was a segfault that happened because
we stored pointers to elements in DenseMap.
When DenseMap grows such pointers are invalidated.
Solution implemented is to keep elements by pointer
and not by value.

[ruiu]

LGTM

This is a pretty bad bug, I was a bit surprised that it doesn't crash except the situation described in https://bugs.llvm.org/show_bug.cgi?id=34648. It could have been silently creating corrupted outputs, I guess. It seems scary.

[grimar]

In D38034#875487, @ruiu wrote:

LGTM

This is a pretty bad bug, I was a bit surprised that it doesn't crash except the situation described in https://bugs.llvm.org/show_bug.cgi?id=34648. It could have been silently creating corrupted outputs, I guess. It seems scary.

Should we backport it to LLD 5.0.x branch somehow ? 5.0.1 I guess.

ELF/SyntheticSections.h

private:
   template <class RelTy>
   bool isFdeLive(EhSectionPiece &Piece, ArrayRef<RelTy> Rels);
 
   uint64_t getFdePc(uint8_t *Buf, size_t Off, uint8_t Enc);
 
   std::vector<CieRecord *> Cies;
 
   // CIE records are uniquified by their contents and personality functions.
-  llvm::DenseMap<std::pair<ArrayRef<uint8_t>, SymbolBody *>, CieRecord> CieMap;
+  llvm::DenseMap<std::pair<ArrayRef<uint8_t>, SymbolBody *>, CieRecord *>
+      CieMap;
 };
 
 class GotSection : public SyntheticSection {
 public:
   GotSection();
   size_t getSize() const override { return Size; }
   void finalizeContents() override;
   bool empty() const override;

ELF/SyntheticSections.cpp

CieRecord *EhFrameSection<ELFT>::addCie(EhSectionPiece &Piece,
 
   SymbolBody *Personality = nullptr;
   unsigned FirstRelI = Piece.FirstRelocation;
   if (FirstRelI != (unsigned)-1)
     Personality =
         &Sec->template getFile<ELFT>()->getRelocTargetSym(Rels[FirstRelI]);
 
   // Search for an existing CIE by CIE contents/relocation target pair.
-  CieRecord *Cie = &CieMap[{Piece.data(), Personality}];
+  CieRecord *&Cie = CieMap[{Piece.data(), Personality}];
 
   // If not found, create a new one.
-  if (Cie->Piece == nullptr) {
+  if (!Cie) {
+    Cie = make<CieRecord>();
     Cie->Piece = &Piece;
     Cies.push_back(Cie);
   }
   return Cie;
 }
 
 // There is one FDE per function. Returns true if a given FDE
 // points to a live function.