[flang] Fixed write past allocated descriptor in PointerAssociateRema…

…pping. (#127000)

The pointer descriptor might be smaller than the target descriptor,
so `operator=` would write beyound the pointer descriptor.

flang/runtime/pointer.cpp

@@ -89,14 +89,18 @@ void RTDEF(PointerAssociateLowerBounds)(Descriptor &pointer,
 void RTDEF(PointerAssociateRemapping)(Descriptor &pointer,
     const Descriptor &target, const Descriptor &bounds, const char *sourceFile,
     int sourceLine) {
-  pointer = target;
-  pointer.raw().attribute = CFI_attribute_pointer;
   Terminator terminator{sourceFile, sourceLine};
   SubscriptValue byteStride{/*captured from first dimension*/};
   std::size_t boundElementBytes{bounds.ElementBytes()};
   std::size_t boundsRank{
       static_cast<std::size_t>(bounds.GetDimension(1).Extent())};
-  pointer.raw().rank = boundsRank;
+  // We cannot just assign target into pointer descriptor, because
+  // the ranks may mismatch. Use target as a mold for initializing
+  // the pointer descriptor.
+  INTERNAL_CHECK(static_cast<std::size_t>(pointer.rank()) == boundsRank);
+  pointer.ApplyMold(target, boundsRank);
+  pointer.set_base_addr(target.raw().base_addr);
+  pointer.raw().attribute = CFI_attribute_pointer;
   for (unsigned j{0}; j < boundsRank; ++j) {
     auto &dim{pointer.GetDimension(j)};
     dim.SetBounds(GetInt64(bounds.ZeroBasedIndexedElement<const char>(2 * j),
@@ -115,13 +119,6 @@ void RTDEF(PointerAssociateRemapping)(Descriptor &pointer,
                      "pointer (%zd > %zd)",
         pointer.Elements(), target.Elements());
   }
-  if (auto *pointerAddendum{pointer.Addendum()}) {
-    if (const auto *targetAddendum{target.Addendum()}) {
-      if (const auto *derived{targetAddendum->derivedType()}) {
-        pointerAddendum->set_derivedType(derived);
-      }
-    }
-  }
 }
 
 RT_API_ATTRS void *AllocateValidatedPointerPayload(std::size_t byteSize) {

flang/unittests/Runtime/Pointer.cpp

@@ -105,3 +105,47 @@ TEST(Pointer, AllocateSourceZeroSize) {
   EXPECT_EQ(p->GetDimension(0).UpperBound(), 0);
   p->Destroy();
 }
+
+TEST(Pointer, PointerAssociateRemapping) {
+  using Fortran::common::TypeCategory;
+  // REAL(4), POINTER :: p(:)
+  StaticDescriptor<Fortran::common::maxRank, true> staticDesc;
+  auto p{staticDesc.descriptor()};
+  SubscriptValue extent[1]{1};
+  p.Establish(TypeCode{Fortran::common::TypeCategory::Real, 4}, 4, nullptr, 1,
+      extent, CFI_attribute_pointer);
+  std::size_t descSize{p.SizeInBytes()};
+  EXPECT_LE(descSize, staticDesc.byteSize);
+  // REAL(4), CONTIGUOUS, POINTER :: t(:,:,:)
+  auto t{Descriptor::Create(TypeCode{Fortran::common::TypeCategory::Real, 4}, 4,
+      nullptr, 3, nullptr, CFI_attribute_pointer)};
+  RTNAME(PointerSetBounds)(*t, 0, 1, 1);
+  RTNAME(PointerSetBounds)(*t, 1, 1, 1);
+  RTNAME(PointerSetBounds)(*t, 2, 1, 1);
+  RTNAME(PointerAllocate)(
+      *t, /*hasStat=*/false, /*errMsg=*/nullptr, __FILE__, __LINE__);
+  EXPECT_TRUE(RTNAME(PointerIsAssociated)(*t));
+  // INTEGER(4) :: b(2,1) = [[1,1]]
+  auto b{MakeArray<TypeCategory::Integer, 4>(
+      std::vector<int>{2, 1}, std::vector<std::int32_t>{1, 1})};
+  // p(1:1) => t
+  RTNAME(PointerAssociateRemapping)(p, *t, *b, __FILE__, __LINE__);
+  EXPECT_TRUE(RTNAME(PointerIsAssociated)(p));
+  EXPECT_EQ(p.rank(), 1);
+  EXPECT_EQ(p.Elements(), 1u);
+
+  // Verify that the memory past the p's descriptor is not affected.
+  const char *addr = reinterpret_cast<const char *>(&staticDesc);
+  const char *ptr = addr + descSize;
+  const char *end = addr + staticDesc.byteSize;
+  while (ptr != end) {
+    if (*ptr != '\0') {
+      std::fprintf(stderr, "byte %zd after pointer descriptor was written\n",
+          ptr - addr);
+      EXPECT_EQ(*ptr, '\0');
+      break;
+    }
+    ++ptr;
+  }
+  p.Destroy();
+}