[Runtime] Fix UB in layout string runtime functions (#66112)

rdar://109790722

Layout strings are not guaranteed to be properly aligned, so we can't directly read from and write to it, but have to use memcpy instead.

Author: drexin

stdlib/public/runtime/BytecodeLayouts.cpp

@@ -42,16 +42,18 @@ static const size_t layoutStringHeaderSize = sizeof(uint64_t) + sizeof(size_t);
 /// offset
 template <typename T>
 T readBytes(const uint8_t *typeLayout, size_t &i) {
-  T returnVal = *(const T *)(typeLayout + i);
+  T returnVal;
+  memcpy(&returnVal, typeLayout + i, sizeof(T));
   i += sizeof(T);
   return returnVal;
 }
 
 /// Given a pointer, a value, and an offset, write the value at the given
-/// offset in big-endian order
+/// offset and increment offset by the size of T
 template <typename T>
-void writeBytes(uint8_t *typeLayout, size_t i, T value) {
-  *((T*)(typeLayout + i)) = value;
+void writeBytes(uint8_t *typeLayout, size_t &i, T value) {
+  memcpy(typeLayout + i, &value, sizeof(T));
+  i += sizeof(T);
 }
 
 Metadata *getExistentialTypeMetadata(OpaqueValue *object) {
@@ -64,7 +66,8 @@ const Metadata *getResilientTypeMetadata(const Metadata* metadata,
                                          const uint8_t *layoutStr,
                                          size_t &offset) {
   auto absolute = layoutStr + offset;
-  auto relativeOffset = (uintptr_t)(intptr_t)(int32_t)readBytes<intptr_t>(layoutStr, offset);
+  auto relativeOffset =
+      (uintptr_t)(intptr_t)(int32_t)readBytes<intptr_t>(layoutStr, offset);
   MetadataAccessor fn;
 
 #if SWIFT_PTRAUTH
@@ -404,12 +407,13 @@ void swift::swift_resolve_resilientAccessors(
 
     switch (tag) {
     case RefCountingKind::Resilient: {
-      auto *type = getResilientTypeMetadata(fieldType, fieldLayoutStr,
-                                            i);
-      uint8_t *curPos = (layoutStr + layoutStrOffset + currentOffset - layoutStringHeaderSize);
-      *((uint64_t*)curPos) =
+      auto *type = getResilientTypeMetadata(fieldType, fieldLayoutStr, i);
+      size_t writeOffset = layoutStrOffset + currentOffset -
+                           layoutStringHeaderSize;
+      uint64_t tagAndOffset =
           (((uint64_t)RefCountingKind::Metatype) << 56) | size;
-      *((Metadata const* *)(curPos + sizeof(uint64_t))) = type;
+      writeBytes(layoutStr, writeOffset, tagAndOffset);
+      writeBytes(layoutStr, writeOffset, type);
       break;
     }
     case RefCountingKind::Metatype:

stdlib/public/runtime/Metadata.cpp

@@ -2641,24 +2641,40 @@ void swift::swift_initStructMetadata(StructMetadata *structType,
   vwtable->publishLayout(layout);
 }
 
-enum LayoutStringFlags : uint64_t {
-  Empty = 0,
-  // TODO: Track other useful information tha can be used to optimize layout
-  //       strings, like different reference kinds contained in the string
-  //       number of ref counting operations (maybe up to 4), so we can
-  //       use witness functions optimized for these cases.
-  HasRelativePointers = (1ULL << 63),
-};
+namespace {
+  enum LayoutStringFlags : uint64_t {
+    Empty = 0,
+    // TODO: Track other useful information tha can be used to optimize layout
+    //       strings, like different reference kinds contained in the string
+    //       number of ref counting operations (maybe up to 4), so we can
+    //       use witness functions optimized for these cases.
+    HasRelativePointers = (1ULL << 63),
+  };
 
-inline bool operator&(LayoutStringFlags a, LayoutStringFlags b) {
-  return (uint64_t(a) & uint64_t(b)) != 0;
-}
-inline LayoutStringFlags operator|(LayoutStringFlags a, LayoutStringFlags b) {
-  return LayoutStringFlags(uint64_t(a) | uint64_t(b));
-}
-inline LayoutStringFlags &operator|=(LayoutStringFlags &a, LayoutStringFlags b) {
-  return a = (a | b);
-}
+  inline bool operator&(LayoutStringFlags a, LayoutStringFlags b) {
+    return (uint64_t(a) & uint64_t(b)) != 0;
+  }
+  inline LayoutStringFlags operator|(LayoutStringFlags a, LayoutStringFlags b) {
+    return LayoutStringFlags(uint64_t(a) | uint64_t(b));
+  }
+  inline LayoutStringFlags &operator|=(LayoutStringFlags &a, LayoutStringFlags b) {
+    return a = (a | b);
+  }
+
+  template <typename T>
+  inline T readBytes(const uint8_t *layoutStr, size_t &i) {
+    T returnVal;
+    memcpy(&returnVal, layoutStr + i, sizeof(T));
+    i += sizeof(T);
+    return returnVal;
+  }
+
+  template <typename T>
+  inline void writeBytes(uint8_t *layoutStr, size_t &i, T value) {
+    memcpy(layoutStr + i, &value, sizeof(T));
+    i += sizeof(T);
+  }
+} // end anonymous namespace
 
 void swift::swift_initStructMetadataWithLayoutString(
     StructMetadata *structType, StructLayoutFlags layoutFlags, size_t numFields,
@@ -2732,9 +2748,9 @@ void swift::swift_initStructMetadataWithLayoutString(
   uint8_t *layoutStr = (uint8_t *)MetadataAllocator(LayoutStringTag)
       .Allocate(fixedLayoutStringSize + refCountBytes, alignof(uint8_t));
 
-  *((size_t*)(layoutStr + sizeof(uint64_t))) = refCountBytes;
+  size_t layoutStrOffset = sizeof(uint64_t);
 
-  size_t layoutStrOffset = layoutStringHeaderSize;
+  writeBytes(layoutStr, layoutStrOffset, refCountBytes);
   size_t fullOffset = 0;
   size_t previousFieldOffset = 0;
   LayoutStringFlags flags = LayoutStringFlags::Empty;
@@ -2753,9 +2769,8 @@ void swift::swift_initStructMetadataWithLayoutString(
         auto tag = fieldTag <= 0x2 ? RefCountingKind::UnknownUnowned :
                                      RefCountingKind::UnknownWeak;
 
-        *(uint64_t *)(layoutStr + layoutStrOffset) =
-            ((uint64_t)tag << 56) | offset;
-        layoutStrOffset += sizeof(uint64_t);
+        auto tagAndOffset = ((uint64_t)tag << 56) | offset;
+        writeBytes(layoutStr, layoutStrOffset, tagAndOffset);
       }
 
       fullOffset += fieldType->size;
@@ -2793,10 +2808,18 @@ void swift::swift_initStructMetadataWithLayoutString(
         }
 
         if (offset) {
-          *(uint64_t *)(layoutStr + layoutStrOffset) += offset;
+          auto layoutStrOffsetCopy = layoutStrOffset;
+          auto firstTagAndOffset =
+              readBytes<uint64_t>(layoutStr, layoutStrOffsetCopy);
+          layoutStrOffsetCopy = layoutStrOffset;
+          firstTagAndOffset += offset;
+          writeBytes(layoutStr, layoutStrOffsetCopy, firstTagAndOffset);
         }
 
-        previousFieldOffset = *(const uint64_t*)(fieldLayoutStr + layoutStringHeaderSize + fieldRefCountBytes);
+        auto previousFieldOffsetOffset =
+            layoutStringHeaderSize + fieldRefCountBytes;
+        previousFieldOffset = readBytes<uint64_t>(fieldLayoutStr,
+                                                  previousFieldOffsetOffset);
         layoutStrOffset += fieldRefCountBytes;
       } else {
         previousFieldOffset += fieldType->vw_size();
@@ -2827,39 +2850,35 @@ void swift::swift_initStructMetadataWithLayoutString(
         };
       }
 
-      *(uint64_t*)(layoutStr + layoutStrOffset) =
-        ((uint64_t)tag << 56) | offset;
-      layoutStrOffset += sizeof(uint64_t);
+      writeBytes(layoutStr, layoutStrOffset, ((uint64_t)tag << 56) | offset);
       previousFieldOffset = fieldType->vw_size();
       fullOffset += previousFieldOffset;
     } else if (fieldType->isAnyExistentialType()) {
       auto *existential = dyn_cast<ExistentialTypeMetadata>(fieldType);
       assert(existential);
       auto tag = existential->isClassBounded() ? RefCountingKind::Unknown
                                                : RefCountingKind::Existential;
-      *(uint64_t*)(layoutStr + layoutStrOffset) =
-        ((uint64_t)tag << 56) | offset;
-      layoutStrOffset += sizeof(uint64_t);
+      writeBytes(layoutStr, layoutStrOffset, ((uint64_t)tag << 56) | offset);
       previousFieldOffset = fieldType->vw_size();
       fullOffset += previousFieldOffset;
     } else {
 metadata:
-      *(uint64_t*)(layoutStr + layoutStrOffset) =
-        ((uint64_t)RefCountingKind::Metatype << 56) | offset;
-      *(uintptr_t*)(layoutStr + layoutStrOffset + sizeof(uint64_t)) =
-          (uintptr_t)fieldType;
-      layoutStrOffset += sizeof(uint64_t) + sizeof(uintptr_t);
+      writeBytes(layoutStr, layoutStrOffset,
+                 ((uint64_t)RefCountingKind::Metatype << 56) | offset);
+      writeBytes(layoutStr, layoutStrOffset, fieldType);
       previousFieldOffset = fieldType->vw_size();
       fullOffset += previousFieldOffset;
     }
   }
 
-  *(uint64_t *)(layoutStr + layoutStrOffset) = previousFieldOffset;
-  *(uint64_t *)(layoutStr + layoutStrOffset + sizeof(uint64_t)) = 0;
+  writeBytes(layoutStr, layoutStrOffset, previousFieldOffset);
+  writeBytes(layoutStr, layoutStrOffset, 0);
 
   // we mask out HasRelativePointers, because at this point they have all been
   // resolved to metadata pointers
-  *(uint64_t *)(layoutStr) = ((uint64_t)flags) & ~((uint64_t)LayoutStringFlags::HasRelativePointers);
+  layoutStrOffset = 0;
+  writeBytes(layoutStr, layoutStrOffset,
+      ((uint64_t)flags) & ~((uint64_t)LayoutStringFlags::HasRelativePointers));
 
   structType->setLayoutString(layoutStr);