[ELF] Fix copy relocation when two symbols share the same address.

[MaskRay]

Summary

In glibc libc.so.6, the multiple versions of _sys_errlist share the same address. When _sys_errlist is copy relocated, we would replace SharedSymbol with Defined in the first iteration of the following loop:

for (SharedSymbol *Sym : getSymbolsAt<ELFT>(SS))

Then in the second iteration, we think the symbol (which has been changed to Defined) is still SharedSymbol and screw up.

[MaskRay]

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[MaskRay]

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[MaskRay]

Landed in 2389a240d02bd8440995e2ad630958e1971ebec7

ELF/Relocations.cpp

 #include "OutputSections.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "SyntheticSections.h"
 #include "Target.h"
 #include "Thunks.h"
 #include "lld/Common/Memory.h"
 #include "lld/Common/Strings.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::object;
 using namespace llvm::support::endian;
template <class ELFT> static bool isReadOnly(SharedSymbol &SS) {
   return false;
 }
 
 // Returns symbols at the same offset as a given symbol, including SS itself.
 //
 // If two or more symbols are at the same offset, and at least one of
 // them are copied by a copy relocation, all of them need to be copied.
 // Otherwise, they would refer different places at runtime.
+// Use a set so that if two or more symbols share the same address, we only copy
+// one but with the largest size.
 template <class ELFT>
-static std::vector<SharedSymbol *> getSymbolsAt(SharedSymbol &SS) {
+static SmallSet<SharedSymbol *, 4> getSymbolsAt(SharedSymbol &SS) {
   typedef typename ELFT::Sym Elf_Sym;
 
   SharedFile<ELFT> &File = SS.getFile<ELFT>();
 
-  std::vector<SharedSymbol *> Ret;
+  SmallSet<SharedSymbol *, 4> Ret;
   for (const Elf_Sym &S : File.getGlobalELFSyms()) {
     if (S.st_shndx == SHN_UNDEF || S.st_shndx == SHN_ABS ||
         S.st_value != SS.Value)
       continue;
     StringRef Name = check(S.getName(File.getStringTable()));
     Symbol *Sym = Symtab->find(Name);
-    if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym))
-      Ret.push_back(Alias);
+    if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym)) {
+      SharedSymbol* SSym = *Ret.insert(Alias).first;
+      SSym->Size = std::max(SSym->Size, Alias->Size);
+    }
   }
   return Ret;
 }
 
 // When a symbol is copy relocated or we create a canonical plt entry, it is
 // effectively a defined symbol. In the case of copy relocation the symbol is
 // in .bss and in the case of a canonical plt entry it is in .plt. This function
 // replaces the existing symbol with a Defined pointing to the appropriate