[Runtime] Scribble on metadata allocations.

Previously, when NDEBUG was not defined, the allocations made for value
metadata records were filled with 0xAA bytes.  Here, that behavior is
both expanded to all metadata records and enabled in release builds when
the environment variable SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE is set.

Author: nate-chandler

stdlib/public/runtime/EnvironmentVariables.def

@@ -41,4 +41,7 @@ VARIABLE(SWIFT_ENABLE_MANGLED_NAME_VERIFICATION, bool, false,
          "Enable verification that metadata can roundtrip through a mangled "
          "name each time metadata is instantiated.")
 
+VARIABLE(SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE, bool, false,
+         "Scribble on runtime allocations such as metadata allocations.")
+
 #undef VARIABLE

stdlib/public/runtime/Metadata.cpp

@@ -662,11 +662,6 @@ swift::swift_allocateGenericValueMetadata(const ValueTypeDescriptor *description
   auto bytes = (char*) cache.getAllocator().withTag(GenericValueMetadataTag)
     .Allocate(totalSize, alignof(void*));
 
-#ifndef NDEBUG
-  // Fill the metadata record with garbage.
-  memset(bytes, 0xAA, totalSize);
-#endif
-
   auto addressPoint = bytes + sizeof(ValueMetadata::HeaderType);
   auto metadata = reinterpret_cast<ValueMetadata *>(addressPoint);
 
@@ -5547,6 +5542,21 @@ static void recordBacktrace(void *allocation) {
   });
 }
 
+template <typename Pointee>
+static inline void memsetScribble(Pointee *bytes, size_t totalSize) {
+#ifndef NDEBUG
+  // When DEBUG is defined, always scribble.
+  memset(bytes, 0xAA, totalSize);
+#else
+  // When DEBUG is not defined, only scribble when the
+  // SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE environment variable is set.
+  if (SWIFT_UNLIKELY(
+          runtime::environment::SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE())) {
+    memset(bytes, 0xAA, totalSize);
+  }
+#endif
+}
+
 void *MetadataAllocator::Allocate(size_t size, size_t alignment) {
   assert(Tag != 0);
   assert(alignment <= alignof(void*));
@@ -5556,8 +5566,11 @@ void *MetadataAllocator::Allocate(size_t size, size_t alignment) {
   SWIFT_ONCE_F(getenvToken, checkAllocatorDebugEnvironmentVariable, nullptr);
 
   // If the size is larger than the maximum, just use malloc.
-  if (size > PoolRange::MaxPoolAllocationSize)
-    return malloc(size);
+  if (size > PoolRange::MaxPoolAllocationSize) {
+    void *allocation = malloc(size);
+    memsetScribble(allocation, size);
+    return allocation;
+  }
 
   // Allocate out of the pool.
   auto sizeWithHeader = size;
@@ -5601,7 +5614,8 @@ void *MetadataAllocator::Allocate(size_t size, size_t alignment) {
       // If that succeeded, we've successfully allocated.
       __msan_allocated_memory(allocation, sizeWithHeader);
       __asan_unpoison_memory_region(allocation, sizeWithHeader);
-      
+      memsetScribble(allocation, sizeWithHeader);
+
       if (SWIFT_UNLIKELY(_swift_debug_metadataAllocationIterationEnabled)) {
         AllocationHeader *header = (AllocationHeader *)allocation;
         header->Size = size;

test/Runtime/environment_variables.swift

@@ -5,7 +5,7 @@
 // REQUIRES: executable_test
 // UNSUPPORTED: use_os_stdlib
 
-// RUN: env %env-SWIFT_DEBUG_HELP=YES %env-SWIFT_DEBUG_SOME_UNKNOWN_VARIABLE=42 %env-SWIFT_DEBUG_ENABLE_METADATA_ALLOCATION_ITERATION=YES %env-SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT=abc %env-SWIFT_DETERMINISTIC_HASHING=whatever %env-SWIFT_ENABLE_MANGLED_NAME_VERIFICATION=YES %target-run %t/main 2>&1 | %FileCheck %s --dump-input fail
+// RUN: env %env-SWIFT_DEBUG_HELP=YES %env-SWIFT_DEBUG_SOME_UNKNOWN_VARIABLE=42 %env-SWIFT_DEBUG_ENABLE_METADATA_ALLOCATION_ITERATION=YES %env-SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT=abc %env-SWIFT_DETERMINISTIC_HASHING=whatever %env-SWIFT_ENABLE_MANGLED_NAME_VERIFICATION=YES %env-SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE=YES %target-run %t/main 2>&1 | %FileCheck %s --dump-input fail
 
 // CHECK-DAG: {{Warning: unknown environment variable SWIFT_DEBUG_SOME_UNKNOWN_VARIABLE|Using getenv to read variables. Unknown SWIFT_DEBUG_ variables will not be flagged.}}
 // CHECK-DAG: Warning: cannot parse value SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT=abc, defaulting to 2.
@@ -15,6 +15,7 @@
 // CHECK-DAG: uint8_t SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT [default: 2] - Print warnings when using implicit @objc entrypoints. Set to desired reporting level, 0-3.
 // CHECK-DAG:    bool SWIFT_DETERMINISTIC_HASHING [default: false] - Disable randomized hash seeding.
 // CHECK-DAG:    bool SWIFT_ENABLE_MANGLED_NAME_VERIFICATION [default: false] - Enable verification that metadata can roundtrip through a mangled name each time metadata is instantiated.
+// CHECK-DAG:    bool SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE [default: false] - Scribble on runtime allocations such as metadata allocations.
 
 print("Hello, world")
 // CHECK: Hello, world