[WebAssembly] Lower ASan constructor priority on Emscripten

[quantum]

Summary

This change gives Emscripten the ability to use more than one constructor
priorities that runs before ASan. By convention, constructor priorites 0-100
are reserved for use by the system. ASan on Emscripten now uses priority 50,
leaving plenty of room for use by Emscripten before and after ASan.

This change is done in response to:
https://github.com/emscripten-core/emscripten/pull/9076#discussion_r310323723

[tlively]

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

1780

You probably don't need a variable here since the value is only used once and the function name is plenty descriptive.

2444

How about renaming this Priority or similar since there is a lot of repeated information otherwise?

[sbc100]

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

132

Any reason not to change this for all platforms? In any case an explaining comment might be a good idea here.

[quantum]

Apply suggestions.

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

132

I suspect doing that will break things on other platforms. I shall add a comment.

[tlively]

It would be good to add a test here, too ;)

[quantum]

Add constructor priority test

[sbc100]

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

132

I think it might be a good idea to contact the original author to find out why 1 was chosen here and if they think it would be OK to change to 50 across the board.

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

 static const size_t kMinStackMallocSize = 1 << 6;   // 64B
 static const size_t kMaxStackMallocSize = 1 << 16;  // 64K
 static const uintptr_t kCurrentStackFrameMagic = 0x41B58AB3;
 static const uintptr_t kRetiredStackFrameMagic = 0x45E0360E;
 
 static const char *const kAsanModuleCtorName = "asan.module_ctor";
 static const char *const kAsanModuleDtorName = "asan.module_dtor";
 static const uint64_t kAsanCtorAndDtorPriority = 1;
+static const uint64_t kAsanEmscriptenCtorAndDtorPriority = 50;
 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 size_t RedzoneSizeForScale(int MappingScale) {
   // Redzone used for stack and globals is at least 32 bytes.
   // For scales 6 and 7, the redzone has to be 64 and 128 bytes respectively.
   return std::max(32U, 1U << MappingScale);
 }
 
+static uint64_t GetCtorAndDtorPriority(Triple &TargetTriple) {
+  if (TargetTriple.isOSEmscripten()) {
+    return kAsanEmscriptenCtorAndDtorPriority;
+  } else {
+    return kAsanCtorAndDtorPriority;
+  }
+}
+
 namespace {
 
 /// Module analysis for getting various metadata about the module.
 class ASanGlobalsMetadataWrapperPass : public ModulePass {
 public:
   static char ID;
 
   ASanGlobalsMetadataWrapperPass() : ModulePass(ID) {
void ModuleAddressSanitizer::createInitializerPoisonCalls(
   GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
   if (!GV)
     return;
 
   ConstantArray *CA = dyn_cast<ConstantArray>(GV->getInitializer());
   if (!CA)
     return;
 
+  const uint64_t CtorAndDtorPriority = GetCtorAndDtorPriority(TargetTriple);
+
   for (Use &OP : CA->operands()) {
     if (isa<ConstantAggregateZero>(OP)) continue;
     ConstantStruct *CS = cast<ConstantStruct>(OP);
 
     // Must have a function or null ptr.
     if (Function *F = dyn_cast<Function>(CS->getOperand(1))) {
       if (F->getName() == kAsanModuleCtorName) continue;
       ConstantInt *Priority = dyn_cast<ConstantInt>(CS->getOperand(0));
       // Don't instrument CTORs that will run before asan.module_ctor.
-      if (Priority->getLimitedValue() <= kAsanCtorAndDtorPriority) continue;
+      if (Priority->getLimitedValue() <= CtorAndDtorPriority)
+        continue;
       poisonOneInitializer(*F, ModuleName);
     }
   }
 }
 
 bool ModuleAddressSanitizer::ShouldInstrumentGlobal(GlobalVariable *G) {
   Type *Ty = G->getValueType();
   LLVM_DEBUG(dbgs() << "GLOBAL: " << *G << "\n");
bool ModuleAddressSanitizer::instrumentModule(Module &M) {
   bool CtorComdat = true;
   bool Changed = false;
   // TODO(glider): temporarily disabled globals instrumentation for KASan.
   if (ClGlobals) {
     IRBuilder<> IRB(AsanCtorFunction->getEntryBlock().getTerminator());
     Changed |= InstrumentGlobals(IRB, M, &CtorComdat);
   }
 
+  const uint64_t CtorAndDtorPriority = GetCtorAndDtorPriority(TargetTriple);
+
   // Put the constructor and destructor in comdat if both
   // (1) global instrumentation is not TU-specific
   // (2) target is ELF.
   if (UseCtorComdat && TargetTriple.isOSBinFormatELF() && CtorComdat) {
     AsanCtorFunction->setComdat(M.getOrInsertComdat(kAsanModuleCtorName));
-    appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority,
+    appendToGlobalCtors(M, AsanCtorFunction, CtorAndDtorPriority,
                         AsanCtorFunction);
     if (AsanDtorFunction) {
       AsanDtorFunction->setComdat(M.getOrInsertComdat(kAsanModuleDtorName));
-      appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority,
+      appendToGlobalDtors(M, AsanDtorFunction, CtorAndDtorPriority,
                           AsanDtorFunction);
     }
   } else {
-    appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority);
+    appendToGlobalCtors(M, AsanCtorFunction, CtorAndDtorPriority);
     if (AsanDtorFunction)
-      appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority);
+      appendToGlobalDtors(M, AsanDtorFunction, CtorAndDtorPriority);
   }
 
   return Changed;
 }
 
 void AddressSanitizer::initializeCallbacks(Module &M) {
   IRBuilder<> IRB(*C);
   // Create __asan_report* callbacks.