[asan] Make ASan compatible with linker dead stripping on Linux.

[eugenis]

Summary

See this issue for more context:
https://github.com/google/sanitizers/issues/260

Instead of putting all global variable descriptions in one large
chunk, we emit one descriptor global for each original global. This
is more linker-gc friendly, as now individual globals can be
discarded along with their descriptors when unused.

The address of the descriptor section is obtained in the module
constructor and passed to the global registration function as an
argument. Similarly to MachO, we use a per-DSO registration flag. We
use the new SHT_ASSOCIATED section flag to link the global and its
descriptor together for linker GC.

[pcc]

The InstrumentGlobals function is now basically four functions in one at this point. I'm wondering whether this code could be made more readable if we split each target object format into its own function.

lib/Transforms/Instrumentation/AddressSanitizer.cpp

1838

@rnk Can you remind me why this is not an issue for COFF?

test/Instrumentation/AddressSanitizer/global_metadata.ll

25

Don't you want to check the content of this global? And what about the global metadata for @.str?

[rnk]

In D28498#642272, @pcc wrote:

The InstrumentGlobals function is now basically four functions in one at this point. I'm wondering whether this code could be made more readable if we split each target object format into its own function.

I think I agree. We can extract any common code into helpers.

lib/Transforms/Instrumentation/AddressSanitizer.cpp

1838

In COFF, comdats are not really groups with a string. It's more like, you have N separate comdat sections, one is the "leader", everyone points at the leader by referring to its symbol, and the leader may have static linkage. Basically, because the comdat key is a real symbol with linkage from an object file and not some globally unique string, this isn't a problem.

Also, I thought the point of the LLD change was so that we wouldn't have to do this? I thought the plan was to change our object emission to create a section group without a comdat key.

1844

!ELFUniqueModuleId.empty() maybe?

[eugenis]

Split implementation in one function per target binary format / global instrumentation kind.

[eugenis]

Since the fate of this change is yet unclear, I've moved the valuable refactoring to https://reviews.llvm.org/D28589.

[eugenis]

use SHT_ASSOCIATED

[pcc]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

228 ↗

(On Diff #84753)

Should this be testing a new flag on GlobalObject, rather than the section name?

Also, do we need to teach globaldce about this?

[mehdi_amini]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

228 ↗

(On Diff #84753)

MachO version is using llvm.compiler.used to avoid having to teach GlobalDCE and other part of LLVM about a special section semantics.

[eugenis]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

228 ↗

(On Diff #84753)

Yeah, we could that, at a minimum.

But this code is definitely wrong: we don't want the new flag to be added on .S -> .o transform, unless explicitly given in the assembly.

[eugenis]

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

228 ↗

(On Diff #84753)

I'm not sure why I said that. This code has nothing to do with .S -> .o path.

llvm.compiler.used added.

I'm not convinced we need a whole IR thing (the global flag) for this tiny feature. Maybe e wait until there is more than one user?

[eugenis]

Added a new metadata, !associated, to mark asan globals.

[pcc]

test/Instrumentation/AddressSanitizer/global_metadata.ll

25

Looks like this hasn't been addressed.

[eugenis]

+ globaldce support for !associated

[eugenis]

Moved the new metadata implementation to https://reviews.llvm.org/D29104

[eugenis]

I've re-uploaded this as
https://reviews.llvm.org/D29104
https://reviews.llvm.org/D30120
https://reviews.llvm.org/D30121

include/llvm/Transforms/Utils/ModuleUtils.h

 /// dead.
 ///
 /// After this routine finishes, the only remaining `Function`s in \p
 /// DeadComdatFunctions are those where every member of the comdat is listed
 /// and thus removing them is safe (provided *all* are removed).
 void filterDeadComdatFunctions(
     Module &M, SmallVectorImpl<Function *> &DeadComdatFunctions);
 
+/// \brief Produce a unique identifier for this module by taking the MD5 sum of
+/// the names of the module's strong external symbols.
+///
+/// This identifier is normally guaranteed to be unique, or the program would
+/// fail to link due to multiply defined symbols.
+///
+/// If the module has no strong external symbols (such a module may still have a
+/// semantic effect if it performs global initialization), we cannot produce a
+/// unique identifier for this module, so we return the empty string.
+std::string getUniqueModuleId(Module *M);
+
 } // End llvm namespace
 
 #endif //  LLVM_TRANSFORMS_UTILS_MODULEUTILS_H

lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp

 #include "llvm/Bitcode/BitcodeWriter.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/ModuleUtils.h"
 using namespace llvm;
 
 namespace {
 
-// Produce a unique identifier for this module by taking the MD5 sum of the
-// names of the module's strong external symbols. This identifier is
-// normally guaranteed to be unique, or the program would fail to link due to
-// multiply defined symbols.
-//
-// If the module has no strong external symbols (such a module may still have a
-// semantic effect if it performs global initialization), we cannot produce a
-// unique identifier for this module, so we return the empty string, which
-// causes the entire module to be written as a regular LTO module.
-std::string getModuleId(Module *M) {
-  MD5 Md5;
-  bool ExportsSymbols = false;
-  auto AddGlobal = [&](GlobalValue &GV) {
-    if (GV.isDeclaration() || GV.getName().startswith("llvm.") ||
-        !GV.hasExternalLinkage())
-      return;
-    ExportsSymbols = true;
-    Md5.update(GV.getName());
-    Md5.update(ArrayRef<uint8_t>{0});
-  };
-
-  for (auto &F : *M)
-    AddGlobal(F);
-  for (auto &GV : M->globals())
-    AddGlobal(GV);
-  for (auto &GA : M->aliases())
-    AddGlobal(GA);
-  for (auto &IF : M->ifuncs())
-    AddGlobal(IF);
-
-  if (!ExportsSymbols)
-    return "";
-
-  MD5::MD5Result R;
-  Md5.final(R);
-
-  SmallString<32> Str;
-  MD5::stringifyResult(R, Str);
-  return ("$" + Str).str();
-}
-
 // Promote each local-linkage entity defined by ExportM and used by ImportM by
 // changing visibility and appending the given ModuleId.
 void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId) {
   auto PromoteInternal = [&](GlobalValue &ExportGV) {
     if (!ExportGV.hasLocalLinkage())
       return;
 
     GlobalValue *ImportGV = ImportM.getNamedValue(ExportGV.getName());
for (Module::alias_iterator I = M->alias_begin(), E = M->alias_end();
     GA->eraseFromParent();
   }
 }
 
 // If it's possible to split M into regular and thin LTO parts, do so and write
 // a multi-module bitcode file with the two parts to OS. Otherwise, write only a
 // regular LTO bitcode file to OS.
 void splitAndWriteThinLTOBitcode(raw_ostream &OS, Module &M) {
-  std::string ModuleId = getModuleId(&M);
+  std::string ModuleId = getUniqueModuleId(&M);
   if (ModuleId.empty()) {
     // We couldn't generate a module ID for this module, just write it out as a
     // regular LTO module.
     WriteBitcodeToFile(&M, OS);
     return;
   }
 
   promoteTypeIds(M, ModuleId);

lib/Transforms/Instrumentation/AddressSanitizer.cpp

 static const char *const kAsanReportErrorTemplate = "__asan_report_";
 static const char *const kAsanRegisterGlobalsName = "__asan_register_globals";
 static const char *const kAsanUnregisterGlobalsName =
     "__asan_unregister_globals";
 static const char *const kAsanRegisterImageGlobalsName =
   "__asan_register_image_globals";
 static const char *const kAsanUnregisterImageGlobalsName =
   "__asan_unregister_image_globals";
+static const char *const kAsanRegisterElfGlobalsName =
+  "__asan_register_elf_globals";
+static const char *const kAsanUnregisterElfGlobalsName =
+  "__asan_unregister_elf_globals";
 static const char *const kAsanPoisonGlobalsName = "__asan_before_dynamic_init";
 static const char *const kAsanUnpoisonGlobalsName = "__asan_after_dynamic_init";
 static const char *const kAsanInitName = "__asan_init";
 static const char *const kAsanVersionCheckName =
     "__asan_version_mismatch_check_v8";
 static const char *const kAsanPtrCmp = "__sanitizer_ptr_cmp";
 static const char *const kAsanPtrSub = "__sanitizer_ptr_sub";
 static const char *const kAsanHandleNoReturnName = "__asan_handle_no_return";
public:
   bool runOnModule(Module &M) override;
   static char ID;  // Pass identification, replacement for typeid
   StringRef getPassName() const override { return "AddressSanitizerModule"; }
 
 private:
   void initializeCallbacks(Module &M);
 
   bool InstrumentGlobals(IRBuilder<> &IRB, Module &M);
+  void InstrumentGlobalsCOFF(
+      IRBuilder<> &IRB, Module &M,
+      const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+      const SmallVectorImpl<Constant *> &MetadataInitializers);
+  void
+  InstrumentGlobalsELF(IRBuilder<> &IRB, Module &M,
+                       const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+                       const SmallVectorImpl<Constant *> &MetadataInitializers,
+                       const std::string &UniqueModuleId);
+  void InstrumentGlobalsMachO(
+      IRBuilder<> &IRB, Module &M,
+      const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+      const SmallVectorImpl<Constant *> &MetadataInitializers);
+  void InstrumentGlobalsWithMetadataArray(
+      IRBuilder<> &IRB, Module &M,
+      const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+      const SmallVectorImpl<Constant *> &MetadataInitializers);
+
+  GlobalVariable *CreateMetadataGlobal(Module &M, Constant *Initializer,
+                                       StringRef OriginalName);
+  void SetComdatForGlobalMetadata(GlobalVariable *G, GlobalVariable *Metadata,
+                                  StringRef InternalSuffix);
+  IRBuilder<> CreateAsanModuleDtor(Module &M);
+
   bool ShouldInstrumentGlobal(GlobalVariable *G);
   bool ShouldUseMachOGlobalsSection() const;
   StringRef getGlobalMetadataSection() const;
   void poisonOneInitializer(Function &GlobalInit, GlobalValue *ModuleName);
   void createInitializerPoisonCalls(Module &M, GlobalValue *ModuleName);
   size_t MinRedzoneSizeForGlobal() const {
     return RedzoneSizeForScale(Mapping.Scale);
   }
 
   GlobalsMetadata GlobalsMD;
   bool CompileKernel;
   bool Recover;
   Type *IntptrTy;
   LLVMContext *C;
   Triple TargetTriple;
   ShadowMapping Mapping;
   Function *AsanPoisonGlobals;
   Function *AsanUnpoisonGlobals;
   Function *AsanRegisterGlobals;
   Function *AsanUnregisterGlobals;
   Function *AsanRegisterImageGlobals;
   Function *AsanUnregisterImageGlobals;
+  Function *AsanRegisterElfGlobals;
+  Function *AsanUnregisterElfGlobals;
 };
 
 // Stack poisoning does not play well with exception handling.
 // When an exception is thrown, we essentially bypass the code
 // that unpoisones the stack. This is why the run-time library has
 // to intercept __cxa_throw (as well as longjmp, etc) and unpoison the entire
 // stack in the interceptor. This however does not work inside the
 // actual function which catches the exception. Most likely because the
void AddressSanitizerModule::initializeCallbacks(Module &M) {
   AsanRegisterGlobals->setLinkage(Function::ExternalLinkage);
   AsanUnregisterGlobals = checkSanitizerInterfaceFunction(
       M.getOrInsertFunction(kAsanUnregisterGlobalsName, IRB.getVoidTy(),
                             IntptrTy, IntptrTy, nullptr));
   AsanUnregisterGlobals->setLinkage(Function::ExternalLinkage);
 
   // Declare the functions that find globals in a shared object and then invoke
   // the (un)register function on them.
-  AsanRegisterImageGlobals = checkSanitizerInterfaceFunction(
-      M.getOrInsertFunction(kAsanRegisterImageGlobalsName,
-      IRB.getVoidTy(), IntptrTy, nullptr));
+  AsanRegisterImageGlobals =
+      checkSanitizerInterfaceFunction(M.getOrInsertFunction(
+          kAsanRegisterImageGlobalsName, IRB.getVoidTy(), IntptrTy, nullptr));
   AsanRegisterImageGlobals->setLinkage(Function::ExternalLinkage);
 
-  AsanUnregisterImageGlobals = checkSanitizerInterfaceFunction(
-      M.getOrInsertFunction(kAsanUnregisterImageGlobalsName,
-      IRB.getVoidTy(), IntptrTy, nullptr));
+  AsanUnregisterImageGlobals =
+      checkSanitizerInterfaceFunction(M.getOrInsertFunction(
+          kAsanUnregisterImageGlobalsName, IRB.getVoidTy(), IntptrTy, nullptr));
   AsanUnregisterImageGlobals->setLinkage(Function::ExternalLinkage);
+
+  AsanRegisterElfGlobals = checkSanitizerInterfaceFunction(
+      M.getOrInsertFunction(kAsanRegisterElfGlobalsName, IRB.getVoidTy(),
+                            IntptrTy, IntptrTy, IntptrTy, nullptr));
+  AsanRegisterElfGlobals->setLinkage(Function::ExternalLinkage);
+
+  AsanUnregisterElfGlobals = checkSanitizerInterfaceFunction(
+      M.getOrInsertFunction(kAsanUnregisterElfGlobalsName, IRB.getVoidTy(),
+                            IntptrTy, IntptrTy, IntptrTy, nullptr));
+  AsanUnregisterElfGlobals->setLinkage(Function::ExternalLinkage);
+}
+
+// Put the metadata and the instrumented global in the same group. This ensures
+// that the metadata is discarded if the instrumented global is discarded.
+void AddressSanitizerModule::SetComdatForGlobalMetadata(
+    GlobalVariable *G, GlobalVariable *Metadata, StringRef InternalSuffix) {
+  Module &M = *G->getParent();
+  Comdat *C = G->getComdat();
+  if (!C) {
+    if (!G->hasName()) {
+      // If G is unnamed, it must be internal. Give it an artificial name
+      // so we can put it in a comdat.
+      assert(G->hasLocalLinkage());
+      G->setName(Twine(kAsanGenPrefix) + "_anon_global");
+    }
+    if (!InternalSuffix.empty() && G->hasLocalLinkage()) {
+      std::string Name = G->getName();
+      Name += InternalSuffix;
+      C = M.getOrInsertComdat(Name);
+    } else {
+      C = M.getOrInsertComdat(G->getName());
+    }
+    // Make this IMAGE_COMDAT_SELECT_NODUPLICATES on COFF.
+    if (TargetTriple.isOSBinFormatCOFF())
+      C->setSelectionKind(Comdat::NoDuplicates);
+    G->setComdat(C);
+  }
+
+  assert(G->hasComdat());
+  Metadata->setComdat(G->getComdat());
+}
+
+// Create a separate metadata global and put it in the appropriate ASan
+// global registration section.
+GlobalVariable *
+AddressSanitizerModule::CreateMetadataGlobal(Module &M, Constant *Initializer,
+                                             StringRef OriginalName) {
+  auto &DL = M.getDataLayout();
+  GlobalVariable *Metadata =
+      new GlobalVariable(M, Initializer->getType(), false,
+                         GlobalVariable::InternalLinkage, Initializer,
+                         Twine("__asan_global_") +
+                             GlobalValue::getRealLinkageName(OriginalName));
+  Metadata->setSection(getGlobalMetadataSection());
+  unsigned SizeOfGlobalStruct = DL.getTypeAllocSize(Initializer->getType());
+  assert(isPowerOf2_32(SizeOfGlobalStruct) &&
+         "global metadata will not be padded appropriately");
+
+  // The MSVC linker always inserts padding when linking incrementally. We
+  // cope with that by aligning each struct to its size, which must be a power
+  // of two.
+  Metadata->setAlignment(SizeOfGlobalStruct);
+  return Metadata;
+}
+
+IRBuilder<> AddressSanitizerModule::CreateAsanModuleDtor(Module &M) {
+  Function *AsanDtorFunction =
+      Function::Create(FunctionType::get(Type::getVoidTy(*C), false),
+                       GlobalValue::InternalLinkage, kAsanModuleDtorName, &M);
+  BasicBlock *AsanDtorBB = BasicBlock::Create(*C, "", AsanDtorFunction);
+  appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority);
+
+  return IRBuilder<>(ReturnInst::Create(*C, AsanDtorBB));
+}
+
+void AddressSanitizerModule::InstrumentGlobalsCOFF(
+    IRBuilder<> &IRB, Module &M,
+    const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+    const SmallVectorImpl<Constant *> &MetadataInitializers) {
+  assert(ExtendedGlobals.size() == MetadataInitializers.size());
+
+  for (size_t i = 0; i < ExtendedGlobals.size(); i++) {
+    GlobalVariable *G = ExtendedGlobals[i];
+    GlobalVariable *Metadata =
+        CreateMetadataGlobal(M, MetadataInitializers[i], G->getName());
+    SetComdatForGlobalMetadata(G, Metadata, "");
+  }
+}
+
+void AddressSanitizerModule::InstrumentGlobalsELF(
+    IRBuilder<> &IRB, Module &M,
+    const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+    const SmallVectorImpl<Constant *> &MetadataInitializers,
+    const std::string &UniqueModuleId) {
+  assert(ExtendedGlobals.size() == MetadataInitializers.size());
+
+  for (size_t i = 0; i < ExtendedGlobals.size(); i++) {
+    GlobalVariable *G = ExtendedGlobals[i];
+    GlobalVariable *Metadata =
+        CreateMetadataGlobal(M, MetadataInitializers[i], G->getName());
+    SetComdatForGlobalMetadata(G, Metadata, UniqueModuleId);
+  }
+
+  // RegisteredFlag serves two purposes. First, we can pass it to dladdr()
+  // to look up the loaded image that contains it. Second, we can store in it
+  // whether registration has already occurred, to prevent duplicate
+  // registration.
+  //
+  // common linkage ensures that there is only one global per shared library.
+  GlobalVariable *RegisteredFlag = new GlobalVariable(
+      M, IntptrTy, false, GlobalVariable::CommonLinkage,
+      ConstantInt::get(IntptrTy, 0), kAsanGlobalsRegisteredFlagName);
+  RegisteredFlag->setVisibility(GlobalVariable::HiddenVisibility);
+
+  // Create start and stop symbols.
+  GlobalVariable *StartELFMetadata = new GlobalVariable(
+      M, IntptrTy, false, GlobalVariable::ExternalWeakLinkage, nullptr,
+      "__start_" + getGlobalMetadataSection());
+  StartELFMetadata->setVisibility(GlobalVariable::HiddenVisibility);
+  GlobalVariable *StopELFMetadata = new GlobalVariable(
+      M, IntptrTy, false, GlobalVariable::ExternalWeakLinkage, nullptr,
+      "__stop_" + getGlobalMetadataSection());
+  StopELFMetadata->setVisibility(GlobalVariable::HiddenVisibility);
+
+  // Create a call to register the globals with the runtime.
+  IRB.CreateCall(AsanRegisterElfGlobals,
+                 {IRB.CreatePointerCast(RegisteredFlag, IntptrTy),
+                  IRB.CreatePointerCast(StartELFMetadata, IntptrTy),
+                  IRB.CreatePointerCast(StopELFMetadata, IntptrTy)});
+
+  // We also need to unregister globals at the end, e.g., when a shared library
+  // gets closed.
+  IRBuilder<> IRB_Dtor = CreateAsanModuleDtor(M);
+  IRB_Dtor.CreateCall(AsanUnregisterElfGlobals,
+                      {IRB.CreatePointerCast(RegisteredFlag, IntptrTy),
+                       IRB.CreatePointerCast(StartELFMetadata, IntptrTy),
+                       IRB.CreatePointerCast(StopELFMetadata, IntptrTy)});
+}
+
+void AddressSanitizerModule::InstrumentGlobalsMachO(
+    IRBuilder<> &IRB, Module &M,
+    const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+    const SmallVectorImpl<Constant *> &MetadataInitializers) {
+  assert(ExtendedGlobals.size() == MetadataInitializers.size());
+
+  // On recent Mach-O platforms, use a structure which binds the liveness of
+  // the global variable to the metadata struct. Keep the list of "Liveness" GV
+  // created to be added to llvm.compiler.used
+  StructType *LivenessTy = StructType::get(IntptrTy, IntptrTy, nullptr);
+  SmallVector<GlobalValue *, 16> LivenessGlobals(ExtendedGlobals.size());
+
+  for (size_t i = 0; i < ExtendedGlobals.size(); i++) {
+    Constant *Initializer = MetadataInitializers[i];
+    GlobalVariable *G = ExtendedGlobals[i];
+    GlobalVariable *Metadata =
+        CreateMetadataGlobal(M, Initializer, G->getName());
+
+    // On recent Mach-O platforms, we emit the global metadata in a way that
+    // allows the linker to properly strip dead globals.
+    auto LivenessBinder = ConstantStruct::get(
+        LivenessTy, Initializer->getAggregateElement(0u),
+        ConstantExpr::getPointerCast(Metadata, IntptrTy), nullptr);
+    GlobalVariable *Liveness = new GlobalVariable(
+        M, LivenessTy, false, GlobalVariable::InternalLinkage, LivenessBinder,
+        Twine("__asan_binder_") + G->getName());
+    Liveness->setSection("__DATA,__asan_liveness,regular,live_support");
+    LivenessGlobals[i] = Liveness;
+  }
+
+  // Update llvm.compiler.used, adding the new liveness globals. This is
+  // needed so that during LTO these variables stay alive. The alternative
+  // would be to have the linker handling the LTO symbols, but libLTO
+  // current API does not expose access to the section for each symbol.
+  if (!LivenessGlobals.empty())
+    appendToCompilerUsed(M, LivenessGlobals);
+
+  // RegisteredFlag serves two purposes. First, we can pass it to dladdr()
+  // to look up the loaded image that contains it. Second, we can store in it
+  // whether registration has already occurred, to prevent duplicate
+  // registration.
+  //
+  // common linkage ensures that there is only one global per shared library.
+  GlobalVariable *RegisteredFlag = new GlobalVariable(
+      M, IntptrTy, false, GlobalVariable::CommonLinkage,
+      ConstantInt::get(IntptrTy, 0), kAsanGlobalsRegisteredFlagName);
+  RegisteredFlag->setVisibility(GlobalVariable::HiddenVisibility);
+
+  IRB.CreateCall(AsanRegisterImageGlobals,
+                 {IRB.CreatePointerCast(RegisteredFlag, IntptrTy)});
+
+  // We also need to unregister globals at the end, e.g., when a shared library
+  // gets closed.
+  IRBuilder<> IRB_Dtor = CreateAsanModuleDtor(M);
+  IRB_Dtor.CreateCall(AsanUnregisterImageGlobals,
+                      {IRB.CreatePointerCast(RegisteredFlag, IntptrTy)});
+}
+
+void AddressSanitizerModule::InstrumentGlobalsWithMetadataArray(
+    IRBuilder<> &IRB, Module &M,
+    const SmallVectorImpl<GlobalVariable *> &ExtendedGlobals,
+    const SmallVectorImpl<Constant *> &MetadataInitializers) {
+  assert(ExtendedGlobals.size() == MetadataInitializers.size());
+  unsigned N = ExtendedGlobals.size();
+  assert(N > 0);
+
+  // On platforms that don't have a custom metadata section, we emit an array
+  // of global metadata structures.
+  ArrayType *ArrayOfGlobalStructTy =
+      ArrayType::get(MetadataInitializers[0]->getType(), N);
+  auto AllGlobals = new GlobalVariable(
+      M, ArrayOfGlobalStructTy, false, GlobalVariable::InternalLinkage,
+      ConstantArray::get(ArrayOfGlobalStructTy, MetadataInitializers), "");
+
+  IRB.CreateCall(AsanRegisterGlobals,
+                 {IRB.CreatePointerCast(AllGlobals, IntptrTy),
+                  ConstantInt::get(IntptrTy, N)});
+
+  // We also need to unregister globals at the end, e.g., when a shared library
+  // gets closed.
+  IRBuilder<> IRB_Dtor = CreateAsanModuleDtor(M);
+  IRB_Dtor.CreateCall(AsanUnregisterGlobals,
+                      {IRB.CreatePointerCast(AllGlobals, IntptrTy),
+                       ConstantInt::get(IntptrTy, N)});
 }
 
 // This function replaces all global variables with new variables that have
 // trailing redzones. It also creates a function that poisons
 // redzones and inserts this function into llvm.global_ctors.
 bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M) {
   GlobalsMD.init(M);
 
   SmallVector<GlobalVariable *, 16> GlobalsToChange;
 
   for (auto &G : M.globals()) {
     if (ShouldInstrumentGlobal(&G)) GlobalsToChange.push_back(&G);
   }
 
   size_t n = GlobalsToChange.size();
   if (n == 0) return false;
 
   auto &DL = M.getDataLayout();
-  bool UseComdatMetadata = TargetTriple.isOSBinFormatCOFF();
-  bool UseMachOGlobalsSection = ShouldUseMachOGlobalsSection();
-  bool UseMetadataArray = !(UseComdatMetadata || UseMachOGlobalsSection);
 
   // A global is described by a structure
   //   size_t beg;
   //   size_t size;
   //   size_t size_with_redzone;
   //   const char *name;
   //   const char *module_name;
   //   size_t has_dynamic_init;
   //   void *source_location;
   //   size_t odr_indicator;
   // We initialize an array of such structures and pass it to a run-time call.
   StructType *GlobalStructTy =
       StructType::get(IntptrTy, IntptrTy, IntptrTy, IntptrTy, IntptrTy,
                       IntptrTy, IntptrTy, IntptrTy, nullptr);
-  unsigned SizeOfGlobalStruct = DL.getTypeAllocSize(GlobalStructTy);
-  assert(isPowerOf2_32(SizeOfGlobalStruct) &&
-         "global metadata will not be padded appropriately");
-  SmallVector<Constant *, 16> Initializers(UseMetadataArray ? n : 0);
-
-  // On recent Mach-O platforms, use a structure which binds the liveness of
-  // the global variable to the metadata struct. Keep the list of "Liveness" GV
-  // created to be added to llvm.compiler.used
-  StructType *LivenessTy  = nullptr;
-  if (UseMachOGlobalsSection)
-    LivenessTy = StructType::get(IntptrTy, IntptrTy, nullptr);
-  SmallVector<GlobalValue *, 16> LivenessGlobals(
-      UseMachOGlobalsSection ? n : 0);
+  SmallVector<GlobalVariable *, 16> NewGlobals(n);
+  SmallVector<Constant *, 16> Initializers(n);
 
   bool HasDynamicallyInitializedGlobals = false;
 
   // We shouldn't merge same module names, as this string serves as unique
   // module ID in runtime.
   GlobalVariable *ModuleName = createPrivateGlobalForString(
       M, M.getModuleIdentifier(), /*AllowMerging*/ false);
 
for (size_t i = 0; i < n; i++) {
     Value *Indices2[2];
     Indices2[0] = IRB.getInt32(0);
     Indices2[1] = IRB.getInt32(0);
 
     G->replaceAllUsesWith(
         ConstantExpr::getGetElementPtr(NewTy, NewGlobal, Indices2, true));
     NewGlobal->takeName(G);
     G->eraseFromParent();
-    G = NewGlobal;
-
-    if (UseComdatMetadata) {
-      // Get or create a COMDAT for G so that we can use it with our metadata.
-      Comdat *C = G->getComdat();
-      if (!C) {
-        if (!G->hasName()) {
-          // If G is unnamed, it must be internal. Give it an artificial name
-          // so we can put it in a comdat.
-          assert(G->hasLocalLinkage());
-          G->setName(Twine(kAsanGenPrefix) + "_anon_global");
-        }
-        C = M.getOrInsertComdat(G->getName());
-        // Make this IMAGE_COMDAT_SELECT_NODUPLICATES on COFF.
-        if (TargetTriple.isOSBinFormatCOFF())
-          C->setSelectionKind(Comdat::NoDuplicates);
-        G->setComdat(C);
-      }
-    }
+    NewGlobals[i] = NewGlobal;
 
     Constant *SourceLoc;
     if (!MD.SourceLoc.empty()) {
       auto SourceLocGlobal = createPrivateGlobalForSourceLoc(M, MD.SourceLoc);
       SourceLoc = ConstantExpr::getPointerCast(SourceLocGlobal, IntptrTy);
     } else {
       SourceLoc = ConstantInt::get(IntptrTy, 0);
     }
Constant *Initializer = ConstantStruct::get(
         ConstantExpr::getPointerCast(ModuleName, IntptrTy),
         ConstantInt::get(IntptrTy, MD.IsDynInit), SourceLoc,
         ConstantExpr::getPointerCast(ODRIndicator, IntptrTy), nullptr);
 
     if (ClInitializers && MD.IsDynInit) HasDynamicallyInitializedGlobals = true;
 
     DEBUG(dbgs() << "NEW GLOBAL: " << *NewGlobal << "\n");
 
-    // If we aren't using separate metadata globals, add it to the initializer
-    // list and continue.
-    if (UseMetadataArray) {
-      Initializers[i] = Initializer;
-      continue;
-    }
 
-    // Create a separate metadata global and put it in the appropriate ASan
-    // global registration section.
-    GlobalVariable *Metadata = new GlobalVariable(
-        M, GlobalStructTy, false, GlobalVariable::InternalLinkage,
-        Initializer, Twine("__asan_global_") +
-                             GlobalValue::getRealLinkageName(G->getName()));
-    Metadata->setSection(getGlobalMetadataSection());
-
-    // We don't want any padding, but we also need a reasonable alignment.
-    // The MSVC linker always inserts padding when linking incrementally. We
-    // cope with that by aligning each struct to its size, which must be a power
-    // of two.
-    Metadata->setAlignment(SizeOfGlobalStruct);
+  // Even though comdat keys in ELF are symbols, they are treated as strings,
+  // ignoring the symbol linkage. As a result, if two translation units contain
+  // comdats keyed off similarly named internal symbols, one of them will be
+  // discarded. To avoid this, we append a module-unique string to such comdat
+  // names; and when this is not possible, we fall back to the old metadata
+  // array method.
+  std::string ELFUniqueModuleId =
+      TargetTriple.isOSBinFormatELF() ? getUniqueModuleId(&M) : "";
+  bool UseCOFFComdatMetadata = TargetTriple.isOSBinFormatCOFF();
+  bool UseELFComdatMetadata = !ELFUniqueModuleId.empty();
+  bool UseComdatMetadata = UseCOFFComdatMetadata || UseELFComdatMetadata;
 
-    // On platforms that support comdats, put the metadata and the
-    // instrumented global in the same group. This ensures that the metadata
-    // is discarded if the instrumented global is discarded.
-    if (UseComdatMetadata) {
-      assert(G->hasComdat());
-      Metadata->setComdat(G->getComdat());
-      continue;
-    }
-    assert(UseMachOGlobalsSection);
+  bool UseMachOGlobalsSection = ShouldUseMachOGlobalsSection();
+  bool UseMetadataArray = !(UseComdatMetadata || UseMachOGlobalsSection);
 
-    // On recent Mach-O platforms, we emit the global metadata in a way that
-    // allows the linker to properly strip dead globals.
-    auto LivenessBinder = ConstantStruct::get(
-        LivenessTy, Initializer->getAggregateElement(0u),
-        ConstantExpr::getPointerCast(Metadata, IntptrTy), nullptr);
-    GlobalVariable *Liveness = new GlobalVariable(
-        M, LivenessTy, false, GlobalVariable::InternalLinkage, LivenessBinder,
-        Twine("__asan_binder_") + G->getName());
-    Liveness->setSection("__DATA,__asan_liveness,regular,live_support");
-    LivenessGlobals[i] = Liveness;
+  if (UseCOFFComdatMetadata) {
+    InstrumentGlobalsCOFF(IRB, M, NewGlobals, Initializers);
+  } else if (UseELFComdatMetadata) {
+    InstrumentGlobalsELF(IRB, M, NewGlobals, Initializers, ELFUniqueModuleId);
+  } else if (UseMachOGlobalsSection) {
+    InstrumentGlobalsMachO(IRB, M, NewGlobals, Initializers);
+  } else if (UseMetadataArray) {
+    InstrumentGlobalsWithMetadataArray(IRB, M, NewGlobals, Initializers);
+  } else {
+    llvm_unreachable("");
   }
 
   // Create calls for poisoning before initializers run and unpoisoning after.
   if (HasDynamicallyInitializedGlobals)
     createInitializerPoisonCalls(M, ModuleName);
 
-  // Platforms with a dedicated metadata section don't need to emit any more
-  // code.
-  if (UseComdatMetadata)
-    return true;
-
-  GlobalVariable *AllGlobals = nullptr;
-  GlobalVariable *RegisteredFlag = nullptr;
-
-  if (UseMachOGlobalsSection) {
-    // RegisteredFlag serves two purposes. First, we can pass it to dladdr()
-    // to look up the loaded image that contains it. Second, we can store in it
-    // whether registration has already occurred, to prevent duplicate
-    // registration.
-    //
-    // common linkage ensures that there is only one global per shared library.
-    RegisteredFlag = new GlobalVariable(
-        M, IntptrTy, false, GlobalVariable::CommonLinkage,
-        ConstantInt::get(IntptrTy, 0), kAsanGlobalsRegisteredFlagName);
-    RegisteredFlag->setVisibility(GlobalVariable::HiddenVisibility);
-
-    // Update llvm.compiler.used, adding the new liveness globals. This is
-    // needed so that during LTO these variables stay alive. The alternative
-    // would be to have the linker handling the LTO symbols, but libLTO
-    // current API does not expose access to the section for each symbol.
-    if (!LivenessGlobals.empty())
-      appendToCompilerUsed(M, LivenessGlobals);
-  } else if (UseMetadataArray) {
-    // On platforms that don't have a custom metadata section, we emit an array
-    // of global metadata structures.
-    ArrayType *ArrayOfGlobalStructTy = ArrayType::get(GlobalStructTy, n);
-    AllGlobals = new GlobalVariable(
-        M, ArrayOfGlobalStructTy, false, GlobalVariable::InternalLinkage,
-        ConstantArray::get(ArrayOfGlobalStructTy, Initializers), "");
-  }
-
-  // Create a call to register the globals with the runtime.
-  if (UseMachOGlobalsSection) {
-    IRB.CreateCall(AsanRegisterImageGlobals,
-                   {IRB.CreatePointerCast(RegisteredFlag, IntptrTy)});
-  } else {
-    IRB.CreateCall(AsanRegisterGlobals,
-                   {IRB.CreatePointerCast(AllGlobals, IntptrTy),
-                    ConstantInt::get(IntptrTy, n)});
-  }
-
-  // We also need to unregister globals at the end, e.g., when a shared library
-  // gets closed.
-  Function *AsanDtorFunction =
-      Function::Create(FunctionType::get(Type::getVoidTy(*C), false),
-                       GlobalValue::InternalLinkage, kAsanModuleDtorName, &M);
-  BasicBlock *AsanDtorBB = BasicBlock::Create(*C, "", AsanDtorFunction);
-  IRBuilder<> IRB_Dtor(ReturnInst::Create(*C, AsanDtorBB));
-
-  if (UseMachOGlobalsSection) {
-    IRB_Dtor.CreateCall(AsanUnregisterImageGlobals,
-                        {IRB.CreatePointerCast(RegisteredFlag, IntptrTy)});
-  } else {
-    IRB_Dtor.CreateCall(AsanUnregisterGlobals,
-                        {IRB.CreatePointerCast(AllGlobals, IntptrTy),
-                         ConstantInt::get(IntptrTy, n)});
-  }
-
-  appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority);
-
   DEBUG(dbgs() << M);
   return true;
 }
 
 bool AddressSanitizerModule::runOnModule(Module &M) {
   C = &(M.getContext());
   int LongSize = M.getDataLayout().getPointerSizeInBits();
   IntptrTy = Type::getIntNTy(*C, LongSize);

lib/Transforms/Utils/ModuleUtils.cpp

void llvm::filterDeadComdatFunctions(
   }
 
   // Remove the entries that were not covering.
   erase_if(DeadComdatFunctions, [&](GlobalValue *GV) {
     return ComdatEntriesCovered.find(GV->getComdat()) ==
            ComdatEntriesCovered.end();
   });
 }
+
+std::string llvm::getUniqueModuleId(Module *M) {
+  MD5 Md5;
+  bool ExportsSymbols = false;
+  auto AddGlobal = [&](GlobalValue &GV) {
+    if (GV.isDeclaration() || GV.getName().startswith("llvm.") ||
+        !GV.hasExternalLinkage())
+      return;
+    ExportsSymbols = true;
+    Md5.update(GV.getName());
+    Md5.update(ArrayRef<uint8_t>{0});
+  };
+
+  for (auto &F : *M)
+    AddGlobal(F);
+  for (auto &GV : M->globals())
+    AddGlobal(GV);
+  for (auto &GA : M->aliases())
+    AddGlobal(GA);
+  for (auto &IF : M->ifuncs())
+    AddGlobal(IF);
+
+  if (!ExportsSymbols)
+    return "";
+
+  MD5::MD5Result R;
+  Md5.final(R);
+
+  SmallString<32> Str;
+  MD5::stringifyResult(R, Str);
+  return ("$" + Str).str();
+}

test/Instrumentation/AddressSanitizer/global_metadata.ll

 ; RUN: opt < %s -asan -asan-module -S | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Globals:
 @global = global i32 0, align 4
 @dyn_init_global = global i32 0, align 4
 @blacklisted_global = global i32 0, align 4
 @_ZZ4funcvE10static_var = internal global i32 0, align 4
 @.str = private unnamed_addr constant [14 x i8] c"Hello, world!\00", align 1
 @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 65535, void ()* @_GLOBAL__sub_I_asan_globals.cpp, i8* null }]
 
 ; Check that globals were instrumented:
-; CHECK: @global = global { i32, [60 x i8] } zeroinitializer, align 32
-; CHECK: @.str = internal unnamed_addr constant { [14 x i8], [50 x i8] } { [14 x i8] c"Hello, world!\00", [50 x i8] zeroinitializer }, align 32
+
+; CHECK: $global = comdat any
+; CHECK: $".str$[[ID:.*]]" = comdat any
+; CHECK: @global = global { i32, [60 x i8] } zeroinitializer, comdat, align 32
+; CHECK: @.str = internal unnamed_addr constant { [14 x i8], [50 x i8] } { [14 x i8] c"Hello, world!\00", [50 x i8] zeroinitializer }, comdat($".str$[[ID]]"), align 32
 
 ; Check emitted location descriptions:
 ; CHECK: [[VARNAME:@__asan_gen_.[0-9]+]] = private unnamed_addr constant [7 x i8] c"global\00", align 1
 ; CHECK: [[FILENAME:@__asan_gen_.[0-9]+]] = private unnamed_addr constant [22 x i8] c"/tmp/asan-globals.cpp\00", align 1
 ; CHECK: [[LOCDESCR:@__asan_gen_.[0-9]+]] = private unnamed_addr constant { [22 x i8]*, i32, i32 } { [22 x i8]* [[FILENAME]], i32 5, i32 5 }
+; CHECK: @__asan_global_global = {{.*}}, section "asan_globals"
 
 ; Check that location descriptors and global names were passed into __asan_register_globals:
-; CHECK: i64 ptrtoint ([7 x i8]* [[VARNAME]] to i64)
-; CHECK: i64 ptrtoint ({ [22 x i8]*, i32, i32 }* [[LOCDESCR]] to i64)
+; CHECK: call void @__asan_register_elf_globals(i64 ptrtoint (i64* @__asan_globals_registered to i64), i64 ptrtoint (i64* @__start_asan_globals to i64), i64 ptrtoint (i64* @__stop_asan_globals to i64))
 
 ; Function Attrs: nounwind sanitize_address
 define internal void @__cxx_global_var_init() #0 section ".text.startup" {
 entry:
   %0 = load i32, i32* @global, align 4
   store i32 %0, i32* @dyn_init_global, align 4
   ret void
 }

test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll

 
 
 ; Find the metadata for @global:
 ; CHECK: [[METADATA:@.+]] = internal global {{.*}} @global {{.*}} section "__DATA,__asan_globals,regular", align 64
 
 ; Find the liveness binder for @global and its metadata:
 ; CHECK: @__asan_binder_global = internal global {{.*}} @global {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
 
-; Test that there is the flag global variable:
-; CHECK: @__asan_globals_registered = common hidden global i64 0
-
 ; The binder has to be inserted to llvm.compiler.used to avoid being stripped
 ; during LTO.
 ; CHECK: @llvm.compiler.used {{.*}} @__asan_binder_global {{.*}} section "llvm.metadata"
 
+; Test that there is the flag global variable:
+; CHECK: @__asan_globals_registered = common hidden global i64 0
 
 ; Test that __asan_register_image_globals is invoked from the constructor:
 ; CHECK-LABEL: define internal void @asan.module_ctor
 ; CHECK-NOT: ret
 ; CHECK: call void @__asan_register_image_globals(i64 ptrtoint (i64* @__asan_globals_registered to i64))
 ; CHECK: ret
 
 ; Test that __asan_unregister_image_globals is invoked from the destructor:
 ; CHECK-LABEL: define internal void @asan.module_dtor
 ; CHECK-NOT: ret
 ; CHECK: call void @__asan_unregister_image_globals(i64 ptrtoint (i64* @__asan_globals_registered to i64))
 ; CHECK: ret

test/Instrumentation/AddressSanitizer/instrument_global.ll

 }
 
 
 !llvm.asan.globals = !{!0}
 !0 = !{[10 x i32]* @GlobDy, null, null, i1 true, i1 false}
 
 ; CHECK-LABEL: define internal void @asan.module_ctor
 ; CHECK-NOT: ret
-; CHECK: call void @__asan_register_globals
+; CHECK: call void @__asan_register_elf_globals
 ; CHECK: ret
 
 ; CHECK-LABEL: define internal void @asan.module_dtor
 ; CHECK-NOT: ret
-; CHECK: call void @__asan_unregister_globals
+; CHECK: call void @__asan_unregister_elf_globals
 ; CHECK: ret