Merge pull request #63380 from aschwaighofer/remove_ub_in_relative_witness_table_description

Remove undefined behavior in RelativeWitnessTable::getDescription()

Author: aschwaighofer

include/swift/ABI/Metadata.h

@@ -1661,13 +1661,6 @@ class TargetRelativeWitnessTable {
 
 public:
   const TargetProtocolConformanceDescriptor<Runtime> *getDescription() const {
-    // "Look through" dynamically allocated witness tables.
-    uintptr_t selfValue = (uintptr_t)this;
-    if (selfValue & 0x1) {
-      selfValue = selfValue & ~((uintptr_t)0x1);
-      auto selfAddr = (TargetRelativeWitnessTable<Runtime> **)selfValue;
-      return (*selfAddr)->Description;
-    }
     return Description;
   }
 };

stdlib/public/runtime/AnyHashableSupport.cpp

@@ -99,8 +99,9 @@ findHashableBaseTypeImpl(const Metadata *type) {
   }
   // By this point, `type` is known to conform to `Hashable`.
 #if SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES
-  const auto *conformance = reinterpret_cast<const RelativeWitnessTable*>(
-    witnessTable)->getDescription();
+  const auto *conformance = lookThroughOptionalConditionalWitnessTable(
+    reinterpret_cast<const RelativeWitnessTable*>(witnessTable))
+    ->getDescription();
 #else
   const auto *conformance = witnessTable->getDescription();
 #endif

stdlib/public/runtime/Metadata.cpp

@@ -5847,15 +5847,15 @@ swift::swift_getAssociatedTypeWitness(MetadataRequest request,
                                             reqBase, assocType);
 }
 
-RelativeWitnessTable *
-lookThroughOptionalConditionalWitnessTable(RelativeWitnessTable *wtable) {
+RelativeWitnessTable *swift::lookThroughOptionalConditionalWitnessTable(
+    const RelativeWitnessTable *wtable) {
   uintptr_t conditional_wtable = (uintptr_t)wtable;
   if (conditional_wtable & 0x1) {
     conditional_wtable = conditional_wtable & ~(uintptr_t)(0x1);
     conditional_wtable = (uintptr_t)*(void**)conditional_wtable;
   }
-  wtable = (RelativeWitnessTable*)conditional_wtable;
-  return wtable;
+  auto table = (RelativeWitnessTable*)conditional_wtable;
+  return table;
 }
 
 SWIFT_CC(swift)

stdlib/public/runtime/MetadataCache.h

@@ -31,6 +31,8 @@
 
 namespace swift {
 
+RelativeWitnessTable *lookThroughOptionalConditionalWitnessTable(const RelativeWitnessTable *);
+
 #if !SWIFT_STDLIB_PASSTHROUGH_METADATA_ALLOCATOR
 
 class MetadataAllocator : public llvm::AllocatorBase<MetadataAllocator> {
@@ -450,10 +452,10 @@ class MetadataCacheKey {
     if (awt == bwt)
       return 0;
 #if SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES
-    auto *aDescription =
-      reinterpret_cast<const RelativeWitnessTable*>(awt)->getDescription();
-    auto *bDescription =
-      reinterpret_cast<const RelativeWitnessTable*>(bwt)->getDescription();
+    auto *aDescription = lookThroughOptionalConditionalWitnessTable(
+      reinterpret_cast<const RelativeWitnessTable*>(awt))->getDescription();
+    auto *bDescription = lookThroughOptionalConditionalWitnessTable(
+      reinterpret_cast<const RelativeWitnessTable*>(bwt))->getDescription();
 #else
     auto *aDescription = awt->getDescription();
     auto *bDescription = bwt->getDescription();

stdlib/public/runtime/Private.h

@@ -663,6 +663,9 @@ class TypeInfo {
                                   const ProtocolRequirement *reqBase,
                                   const ProtocolRequirement *assocConformance);
 
+  RelativeWitnessTable *
+  lookThroughOptionalConditionalWitnessTable(const RelativeWitnessTable *wtable);
+
 #if SWIFT_OBJC_INTEROP
   /// Returns a retained Quick Look representation object an Obj-C object.
   SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_INTERNAL