[WebAssembly] Don't generate invalid modules when function signatures mismatch

Previously we could emit a warning and generate a potentially invalid
wasm module (due to call sites and functions having conflicting
signatures). Now, rather than create invalid binaries we handle such
cases by creating stub functions containing unreachable, effectively
turning these into runtime errors rather than validation failures.

Differential Revision: https://reviews.llvm.org/D57909

llvm-svn: 354528

5 files changed, 267 insertions, 73 deletions

diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
index 40e88f2ac2a..d3286af258b 100644
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -534,17 +534,6 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   for (auto *Arg : Args.filtered(OPT_undefined))
     handleUndefined(Arg->getValue());
 
-  // Handle the `--export <sym>` options
-  // This works like --undefined but also exports the symbol if its found
-  for (auto *Arg : Args.filtered(OPT_export)) {
-    Symbol *Sym = handleUndefined(Arg->getValue());
-    if (Sym && Sym->isDefined())
-      Sym->ForceExport = true;
-    else if (!Config->AllowUndefined)
-      error(Twine("symbol exported via --export not found: ") +
-            Arg->getValue());
-  }
-
   Symbol *EntrySym = nullptr;
   if (!Config->Relocatable) {
     if (!Config->Shared && !Config->Entry.empty()) {
@@ -555,26 +544,47 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
         error("entry symbol not defined (pass --no-entry to supress): " +
               Config->Entry);
     }
-
-    // Make sure we have resolved all symbols.
-    if (!Config->AllowUndefined)
-      Symtab->reportRemainingUndefines();
   }
 
   if (errorCount())
     return;
 
+  // Handle the `--export <sym>` options
+  // This works like --undefined but also exports the symbol if its found
+  for (auto *Arg : Args.filtered(OPT_export))
+    handleUndefined(Arg->getValue());
+
   // Do link-time optimization if given files are LLVM bitcode files.
   // This compiles bitcode files into real object files.
   Symtab->addCombinedLTOObject();
   if (errorCount())
     return;
 
-  // Add synthetic dummies for weak undefined functions.  Must happen
-  // after LTO otherwise functions may not yet have signatures.
-  if (!Config->Relocatable)
+  // Resolve any variant symbols that were created due to signature
+  // mismatchs.
+  Symtab->handleSymbolVariants();
+  if (errorCount())
+    return;
+
+  for (auto *Arg : Args.filtered(OPT_export)) {
+    Symbol *Sym = Symtab->find(Arg->getValue());
+    if (Sym && Sym->isDefined())
+      Sym->ForceExport = true;
+    else if (!Config->AllowUndefined)
+      error(Twine("symbol exported via --export not found: ") +
+            Arg->getValue());
+  }
+
+  if (!Config->Relocatable) {
+    // Add synthetic dummies for weak undefined functions.  Must happen
+    // after LTO otherwise functions may not yet have signatures.
     Symtab->handleWeakUndefines();
 
+    // Make sure we have resolved all symbols.
+    if (!Config->AllowUndefined)
+      Symtab->reportRemainingUndefines();
+  }
+
   if (EntrySym)
     EntrySym->setHidden(false);
 
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
index 35ac2c5a6da..b196d5017c9 100644
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -26,6 +26,38 @@ using namespace lld::wasm;
 
 SymbolTable *lld::wasm::Symtab;
 
+static char encodeValType(ValType Type) {
+  switch (Type) {
+  case ValType::I32:
+    return 'i';
+  case ValType::I64:
+    return 'j';
+  case ValType::F32:
+    return 'f';
+  case ValType::F64:
+    return 'd';
+  case ValType::V128:
+    return 'V';
+  case ValType::EXCEPT_REF:
+    return 'e';
+  }
+  llvm_unreachable("invalid wasm type");
+}
+
+static std::string encodeSignature(const WasmSignature &Sig) {
+  std::string S = ":";
+  for (ValType Type : Sig.Returns)
+    S += encodeValType(Type);
+  S += ':';
+  for (ValType Type : Sig.Params)
+    S += encodeValType(Type);
+  return S;
+}
+
+static StringRef getVariantName(StringRef BaseName, const WasmSignature &Sig) {
+  return Saver.save(BaseName + encodeSignature(Sig));
+}
+
 void SymbolTable::addFile(InputFile *File) {
   log("Processing: " + toString(File));
   if (Config->Trace)
@@ -81,6 +113,11 @@ Symbol *SymbolTable::find(StringRef Name) {
   return SymVector[It->second];
 }
 
+void SymbolTable::replace(StringRef Name, Symbol* Sym) {
+  auto It = SymMap.find(CachedHashStringRef(Name));
+  SymVector[It->second] = Sym;
+}
+
 std::pair<Symbol *, bool> SymbolTable::insertName(StringRef Name) {
   bool Trace = false;
   auto P = SymMap.insert({CachedHashStringRef(Name), (int)SymVector.size()});
@@ -123,29 +160,19 @@ static void reportTypeError(const Symbol *Existing, const InputFile *File,
 }
 
 // Check the type of new symbol matches that of the symbol is replacing.
-// For functions this can also involve verifying that the signatures match.
-static void checkFunctionType(Symbol *Existing, const InputFile *File,
-                              const WasmSignature *NewSig) {
-  auto ExistingFunction = dyn_cast<FunctionSymbol>(Existing);
-  if (!ExistingFunction) {
-    reportTypeError(Existing, File, WASM_SYMBOL_TYPE_FUNCTION);
-    return;
-  }
-
+// Returns true if the function types match, false is there is a singature
+// mismatch.
+bool signatureMatches(FunctionSymbol *Existing, const WasmSignature *NewSig) {
   if (!NewSig)
-    return;
+    return true;
 
-  const WasmSignature *OldSig = ExistingFunction->Signature;
+  const WasmSignature *OldSig = Existing->Signature;
   if (!OldSig) {
-    ExistingFunction->Signature = NewSig;
-    return;
+    Existing->Signature = NewSig;
+    return true;
   }
 
-  if (*NewSig != *OldSig)
-    warn("function signature mismatch: " + Existing->getName() +
-         "\n>>> defined as " + toString(*OldSig) + " in " +
-         toString(Existing->getFile()) + "\n>>> defined as " +
-         toString(*NewSig) + " in " + toString(File));
+  return *NewSig == *OldSig;
 }
 
 static void checkGlobalType(const Symbol *Existing, const InputFile *File,
@@ -255,26 +282,45 @@ Symbol *SymbolTable::addDefinedFunction(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name, File);
 
+  auto Replace = [&](Symbol* Sym) {
+    // If the new defined function doesn't have signture (i.e. bitcode
+    // functions) but the old symbol does, then preserve the old signature
+    const WasmSignature *OldSig = S->getSignature();
+    auto* NewSym = replaceSymbol<DefinedFunction>(Sym, Name, Flags, File, Function);
+    if (!NewSym->Signature)
+      NewSym->Signature = OldSig;
+  };
+
   if (WasInserted || S->isLazy()) {
-    replaceSymbol<DefinedFunction>(S, Name, Flags, File, Function);
+    Replace(S);
     return S;
   }
 
-  if (Function)
-    checkFunctionType(S, File, &Function->Signature);
+  auto ExistingFunction = dyn_cast<FunctionSymbol>(S);
+  if (!ExistingFunction) {
+    reportTypeError(S, File, WASM_SYMBOL_TYPE_FUNCTION);
+    return S;
+  }
 
-  if (shouldReplace(S, File, Flags)) {
-    // If the new defined function doesn't have signture (i.e. bitcode
-    // functions) but the old symbol does then preserve the old signature
-    const WasmSignature *OldSig = nullptr;
-    if (auto* F = dyn_cast<FunctionSymbol>(S))
-      OldSig = F->Signature;
-    if (auto *L = dyn_cast<LazySymbol>(S))
-      OldSig = L->Signature;
-    auto NewSym = replaceSymbol<DefinedFunction>(S, Name, Flags, File, Function);
-    if (!NewSym->Signature)
-      NewSym->Signature = OldSig;
+  if (Function && !signatureMatches(ExistingFunction, &Function->Signature)) {
+    Symbol* Variant;
+    if (getFunctionVariant(S, &Function->Signature, File, &Variant))
+      // New variant, always replace
+      Replace(Variant);
+    else if (shouldReplace(S, File, Flags))
+      // Variant already exists, replace it after checking shouldReplace
+      Replace(Variant);
+
+    // This variant we found take the place in the symbol table as the primary
+    // variant.
+    replace(Name, Variant);
+    return Variant;
   }
+
+  // Existing function with matching signature.
+  if (shouldReplace(S, File, Flags))
+    Replace(S);
+
   return S;
 }
 
@@ -287,15 +333,19 @@ Symbol *SymbolTable::addDefinedData(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name, File);
 
-  if (WasInserted || S->isLazy()) {
+  auto Replace = [&]() {
     replaceSymbol<DefinedData>(S, Name, Flags, File, Segment, Address, Size);
+  };
+
+  if (WasInserted || S->isLazy()) {
+    Replace();
     return S;
   }
 
   checkDataType(S, File);
 
   if (shouldReplace(S, File, Flags))
-    replaceSymbol<DefinedData>(S, Name, Flags, File, Segment, Address, Size);
+    Replace();
   return S;
 }
 
@@ -307,15 +357,19 @@ Symbol *SymbolTable::addDefinedGlobal(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name, File);
 
-  if (WasInserted || S->isLazy()) {
+  auto Replace = [&]() {
     replaceSymbol<DefinedGlobal>(S, Name, Flags, File, Global);
+  };
+
+  if (WasInserted || S->isLazy()) {
+    Replace();
     return S;
   }
 
   checkGlobalType(S, File, &Global->getType());
 
   if (shouldReplace(S, File, Flags))
-    replaceSymbol<DefinedGlobal>(S, Name, Flags, File, Global);
+    Replace();
   return S;
 }
 
@@ -327,15 +381,19 @@ Symbol *SymbolTable::addDefinedEvent(StringRef Name, uint32_t Flags,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name, File);
 
-  if (WasInserted || S->isLazy()) {
+  auto Replace = [&]() {
     replaceSymbol<DefinedEvent>(S, Name, Flags, File, Event);
+  };
+
+  if (WasInserted || S->isLazy()) {
+    Replace();
     return S;
   }
 
   checkEventType(S, File, &Event->getType(), &Event->Signature);
 
   if (shouldReplace(S, File, Flags))
-    replaceSymbol<DefinedEvent>(S, Name, Flags, File, Event);
+    Replace();
   return S;
 }
 
@@ -350,13 +408,25 @@ Symbol *SymbolTable::addUndefinedFunction(StringRef Name, StringRef ImportName,
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name, File);
 
-  if (WasInserted)
+  auto Replace = [&]() {
     replaceSymbol<UndefinedFunction>(S, Name, ImportName, ImportModule, Flags,
                                      File, Sig);
+  };
+
+  if (WasInserted)
+    Replace();
   else if (auto *Lazy = dyn_cast<LazySymbol>(S))
     Lazy->fetch();
-  else
-    checkFunctionType(S, File, Sig);
+  else {
+    auto ExistingFunction = dyn_cast<FunctionSymbol>(S);
+    if (!ExistingFunction) {
+      reportTypeError(S, File, WASM_SYMBOL_TYPE_FUNCTION);
+      return S;
+    }
+    if (!signatureMatches(ExistingFunction, Sig))
+      if (getFunctionVariant(S, Sig, File, &S))
+        Replace();
+  }
 
   return S;
 }
@@ -438,6 +508,44 @@ bool SymbolTable::addComdat(StringRef Name) {
   return Comdats.insert(CachedHashStringRef(Name)).second;
 }
 
+// The new signature doesn't match.  Create a variant to the symbol with the
+// signature encoded in the name and return that instead.  These symbols are
+// then unified later in handleSymbolVariants.
+bool SymbolTable::getFunctionVariant(Symbol* Sym, const WasmSignature *Sig,
+                                     const InputFile *File, Symbol **Out) {
+  StringRef NewName = getVariantName(Sym->getName(), *Sig);
+  LLVM_DEBUG(dbgs() << "getFunctionVariant: " << Sym->getName() << " -> " << NewName
+                    << " " << toString(*Sig) << "\n");
+  Symbol *Variant = nullptr;
+
+  // Linear search through symbol variants.  Should never be more than two
+  // or three entries here.
+  auto &Variants = SymVariants[CachedHashStringRef(Sym->getName())];
+  if (Variants.size() == 0)
+    Variants.push_back(Sym);
+
+  for (Symbol* V : Variants) {
+    if (*V->getSignature() == *Sig) {
+      Variant = V;
+      break;
+    }
+  }
+
+  bool WasAdded = !Variant;
+  if (WasAdded) {
+    // Create a new variant;
+    LLVM_DEBUG(dbgs() << "added new variant\n");
+    Variant = reinterpret_cast<Symbol *>(make<SymbolUnion>());
+    Variants.push_back(Variant);
+  } else {
+    LLVM_DEBUG(dbgs() << "variant already exists: " << toString(*Variant) << "\n");
+    assert(*Variant->getSignature() == *Sig);
+  }
+
+  *Out = Variant;
+  return WasAdded;
+}
+
 // Set a flag for --trace-symbol so that we can print out a log message
 // if a new symbol with the same name is inserted into the symbol table.
 void SymbolTable::trace(StringRef Name) {
@@ -451,14 +559,16 @@ static const uint8_t UnreachableFn[] = {
 
 // Replace the given symbol body with an unreachable function.
 // This is used by handleWeakUndefines in order to generate a callable
-// equivalent of an undefined function.
+// equivalent of an undefined function and also handleSymbolVariants for
+// undefined functions that don't match the signature of the definition.
 InputFunction *SymbolTable::replaceWithUnreachable(Symbol *Sym,
                                                    const WasmSignature &Sig,
                                                    StringRef DebugName) {
   auto *Func = make<SyntheticFunction>(Sig, Sym->getName(), DebugName);
   Func->setBody(UnreachableFn);
   SyntheticFunctions.emplace_back(Func);
-  replaceSymbol<DefinedFunction>(Sym, Sym->getName(), Sym->getFlags(), nullptr, Func);
+  replaceSymbol<DefinedFunction>(Sym, Sym->getName(), Sym->getFlags(), nullptr,
+                                 Func);
   return Func;
 }
 
@@ -471,21 +581,15 @@ void SymbolTable::handleWeakUndefines() {
     if (!Sym->isUndefWeak())
       continue;
 
-    const WasmSignature *Sig = nullptr;
-
-    if (auto *FuncSym = dyn_cast<FunctionSymbol>(Sym)) {
+    const WasmSignature *Sig = Sym->getSignature();
+    if (!Sig) {
       // It is possible for undefined functions not to have a signature (eg. if
       // added via "--undefined"), but weak undefined ones do have a signature.
-      assert(FuncSym->Signature);
-      Sig = FuncSym->Signature;
-    } else if (auto *LazySym = dyn_cast<LazySymbol>(Sym)) {
-      // Lazy symbols may not be functions and therefore can have a null
-      // signature.
-      Sig = LazySym->Signature;
-    }
-
-    if (!Sig)
+      // Lazy symbols may not be functions and therefore Sig can still be null
+      // in some circumstantce.
+      assert(!isa<FunctionSymbol>(Sym));
       continue;
+    }
 
     // Add a synthetic dummy for weak undefined functions.  These dummies will
     // be GC'd if not used as the target of any "call" instructions.
@@ -498,3 +602,64 @@ void SymbolTable::handleWeakUndefines() {
     Sym->setHidden(true);
   }
 }
+
+static void reportFunctionSignatureMismatch(StringRef SymName,
+                                            FunctionSymbol *A,
+                                            FunctionSymbol *B, bool Error) {
+  std::string msg = ("function signature mismatch: " + SymName +
+                     "\n>>> defined as " + toString(*A->Signature) + " in " +
+                     toString(A->getFile()) + "\n>>> defined as " +
+                     toString(*B->Signature) + " in " + toString(B->getFile()))
+                        .str();
+  if (Error)
+    error(msg);
+  else
+    warn(msg);
+}
+
+// Remove any variant symbols that were created due to function signature
+// mismatches.
+void SymbolTable::handleSymbolVariants() {
+  for (auto Pair : SymVariants) {
+    // Push the initial symbol onto the list of variants.
+    StringRef SymName = Pair.first.val();
+    std::vector<Symbol *> &Variants = Pair.second;
+
+#ifndef NDEBUG
+    dbgs() << "symbol with (" << Variants.size()
+                      << ") variants: " << SymName << "\n";
+    for (auto *S: Variants) {
+      auto *F = cast<FunctionSymbol>(S);
+      dbgs() << " variant: " + F->getName() << " " << toString(*F->Signature) << "\n";
+    }
+#endif
+
+    // Find the one definition.
+    DefinedFunction *Defined = nullptr;
+    for (auto *Symbol : Variants) {
+      if (auto F = dyn_cast<DefinedFunction>(Symbol)) {
+        Defined = F;
+        break;
+      }
+    }
+
+    // If there are no definitions, and the undefined symbols disagree on
+    // the signature, there is not we can do since we don't know which one
+    // to use as the signature on the import.
+    if (!Defined) {
+      reportFunctionSignatureMismatch(SymName,
+                                      cast<FunctionSymbol>(Variants[0]),
+                                      cast<FunctionSymbol>(Variants[1]), true);
+      return;
+    }
+
+    for (auto *Symbol : Variants) {
+      if (Symbol != Defined) {
+        auto *F = cast<FunctionSymbol>(Symbol);
+        reportFunctionSignatureMismatch(SymName, F, Defined, false);
+        StringRef DebugName = Saver.save("unreachable:" + toString(*F));
+        replaceWithUnreachable(F, *F->Signature, DebugName);
+      }
+    }
+  }
+}
diff --git a/lld/wasm/SymbolTable.h b/lld/wasm/SymbolTable.h
index f03980d7066..3d0529af41d 100644
--- a/lld/wasm/SymbolTable.h
+++ b/lld/wasm/SymbolTable.h
@@ -49,6 +49,8 @@ public:
 
   Symbol *find(StringRef Name);
 
+  void replace(StringRef Name, Symbol* Sym);
+
   void trace(StringRef Name);
 
   Symbol *addDefinedFunction(StringRef Name, uint32_t Flags, InputFile *File,
@@ -79,12 +81,15 @@ public:
   DefinedFunction *addSyntheticFunction(StringRef Name, uint32_t Flags,
                                         InputFunction *Function);
 
+  void handleSymbolVariants();
   void handleWeakUndefines();
 
 private:
   std::pair<Symbol *, bool> insert(StringRef Name, const InputFile *File);
   std::pair<Symbol *, bool> insertName(StringRef Name);
 
+  bool getFunctionVariant(Symbol* Sym, const WasmSignature *Sig,
+                          const InputFile *File, Symbol **Out);
   InputFunction *replaceWithUnreachable(Symbol *Sym, const WasmSignature &Sig,
                                         StringRef DebugName);
 
@@ -94,6 +99,10 @@ private:
   llvm::DenseMap<llvm::CachedHashStringRef, int> SymMap;
   std::vector<Symbol *> SymVector;
 
+  // For certain symbols types, e.g. function symbols, we allow for muliple
+  // variants of the same symbol with different signatures.
+  llvm::DenseMap<llvm::CachedHashStringRef, std::vector<Symbol *>> SymVariants;
+
   llvm::DenseSet<llvm::CachedHashStringRef> Comdats;
 
   // For LTO.
diff --git a/lld/wasm/Symbols.cpp b/lld/wasm/Symbols.cpp
index 60030bef3fa..f8ea47176a3 100644
--- a/lld/wasm/Symbols.cpp
+++ b/lld/wasm/Symbols.cpp
@@ -45,6 +45,14 @@ WasmSymbolType Symbol::getWasmType() const {
   llvm_unreachable("invalid symbol kind");
 }
 
+const WasmSignature *Symbol::getSignature() const {
+  if (auto* F = dyn_cast<FunctionSymbol>(this))
+    return F->Signature;
+  if (auto *L = dyn_cast<LazySymbol>(this))
+    return L->Signature;
+  return nullptr;
+}
+
 InputChunk *Symbol::getChunk() const {
   if (auto *F = dyn_cast<DefinedFunction>(this))
     return F->Function;
diff --git a/lld/wasm/Symbols.h b/lld/wasm/Symbols.h
index fd501ed64cd..3ed9e7abf55 100644
--- a/lld/wasm/Symbols.h
+++ b/lld/wasm/Symbols.h
@@ -105,6 +105,8 @@ public:
   // True if this symbol is specified by --trace-symbol option.
   unsigned Traced : 1;
 
+  const WasmSignature* getSignature() const;
+
 protected:
   Symbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F)
       : IsUsedInRegularObj(false), ForceExport(false), Traced(false),