[LLD][ELF] - Do not forget to clean synthetic sections pointers before link().

[grimar]

Summary

This is https://bugs.llvm.org/show_bug.cgi?id=38739,

user tries to use LLD as a library and seems faced the old pointer in InX::EhFrame
on non-first call of link(). We should clean the global variables correctly.

[grimar]

Or should we add struct InX::clean() method instead may be?

[ruiu]

I'd memset InX to zero it out instead. Also, don't you need to clear In as well?

[grimar]

In D51409#1217154, @ruiu wrote:

I'd memset InX to zero it out instead.

But we do not have an instance of InX to use memset.

Also, don't you need to clear In as well?

Right. Will you agree to do cleanup for synthetics in createSyntheticSections?
We might set them to nullptr there for those that are not always created. Also its the place where we know and have ELFT.

[grimar]

• Changed implementation.

[grimar]

Ping. What about this? I honestly think it is not that nice to have 'First', but it is probably better than other solutions I can think of.

[ruiu]

ELF/SyntheticSections.h

977

Is it really guaranteed that static data members are adjacent in memory?

[pcc]

ELF/Writer.cpp

263

Won't sizeof(InX) evaluate to 1 here because all of the members are static?

[grimar]

Honestly saying, this is a bit strange feature, though I assumed it should work because we already have memset(&Out::First, 0, sizeof(Out)); line.
So, I am sorry, I am not sure about the correctness of this patch, it looked unusual to me, but I supposed it is ok to use it because of above.

[grimar]

Need to investigate this.

[grimar]

Added David. I guess he might know something about the correctness of approach used. If it is not correct, we already have a bug in LLD then.

[grimar]

Abandoning in favor of D52506.

ELF/SyntheticSections.h

 template <class ELFT> void splitSections();
 void mergeSections();
 
 Defined *addSyntheticLocal(StringRef Name, uint8_t Type, uint64_t Value,
                            uint64_t Size, InputSectionBase &Section);
 
 // Linker generated sections which can be used as inputs.
 struct InX {
+  static uint8_t First;
   static InputSection *ARMAttributes;
   static BssSection *Bss;
   static BssSection *BssRelRo;
   static BuildIdSection *BuildId;
   static EhFrameHeader *EhFrameHdr;
   static EhFrameSection *EhFrame;
   static SyntheticSection *Dynamic;
   static StringTableSection *DynStrTab;
struct InX {
   static RelocationBaseSection *RelaIplt;
   static StringTableSection *ShStrTab;
   static StringTableSection *StrTab;
   static SymbolTableBaseSection *SymTab;
   static SymtabShndxSection* SymTabShndx;
 };
 
 template <class ELFT> struct In {
+  static uint8_t First;
   static VersionDefinitionSection<ELFT> *VerDef;
   static VersionTableSection<ELFT> *VerSym;
   static VersionNeedSection<ELFT> *VerNeed;
 };
 
+template <class ELFT> uint8_t In<ELFT>::First;
 template <class ELFT> VersionDefinitionSection<ELFT> *In<ELFT>::VerDef;
 template <class ELFT> VersionTableSection<ELFT> *In<ELFT>::VerSym;
 template <class ELFT> VersionNeedSection<ELFT> *In<ELFT>::VerNeed;
 } // namespace elf
 } // namespace lld
 
 #endif

ELF/SyntheticSections.cpp

for (Thunk *T : Thunks) {
     T->getThunkTargetSym()->Size = Size;
     Off += Size;
   }
   bool Changed = Off != Size;
   Size = Off;
   return Changed;
 }
 
+uint8_t InX::First;
 InputSection *InX::ARMAttributes;
 BssSection *InX::Bss;
 BssSection *InX::BssRelRo;
 BuildIdSection *InX::BuildId;
 EhFrameHeader *InX::EhFrameHdr;
 EhFrameSection *InX::EhFrame;
 SyntheticSection *InX::Dynamic;
 StringTableSection *InX::DynStrTab;

ELF/Writer.cpp

static OutputSection *findSection(StringRef Name) {
   return nullptr;
 }
 
 // Initialize Out members.
 template <class ELFT> static void createSyntheticSections() {
   // Initialize all pointers with NULL. This is needed because
   // you can call lld::elf::main more than once as a library.
   memset(&Out::First, 0, sizeof(Out));
+  memset(&InX::First, 0, sizeof(InX));
+  memset(&In<ELFT>::First, 0, sizeof(In<ELFT>));
 
   auto Add = [](InputSectionBase *Sec) { InputSections.push_back(Sec); };
 
   InX::DynStrTab = make<StringTableSection>(".dynstr", true);
   InX::Dynamic = make<DynamicSection<ELFT>>();
   if (Config->AndroidPackDynRelocs) {
     InX::RelaDyn = make<AndroidPackedRelocationSection<ELFT>>(
         Config->IsRela ? ".rela.dyn" : ".rel.dyn");