[analyzer] Improve handling of placement new in PointerArith (#155855)

This pull improves the handling of placement new in`PointerArith`,
fixing one family of false positives, and one of negatives:

### False Positives

```cpp
  Buffer buffer;
  int* array = new (&buffer) int[10];
  ++array; // there should be no warning
```

The code above should flag the memory region `buffer` as reinterpreted,
very much as `reinterpret_cast` would do. Note that in this particular
case the placement new is inlined so the engine can track that `*array`
points to the same region as `buffer`.

This is no-op if the placement new is opaque.

### False Negatives

```cpp
  Buffer buffer;
  int* array = new (&buffer) int;
  ++array; // there should be a warning
```

In this case, there is an implicit cast to `void*` when calling
placement new. The memory region was marked as reinterpreted, and
therefore later pointer arithmetic will not raise. I have added a
condition to not consider a cast to `void*` as a reinterpretation, as an
array of voids does not make much sense.


There are still some limitations, of course. For starters, if a single
`int` is created in place of an array of `unsigned char` of exactly the
same size, it will still be considered as an array. A convoluted example
to make the point that I think it makes sense *not* to raise in this
situation is in the test `checkPlacementNewSlices`.

CPP-6868

Author: alejandro-alvarez-sonarsource

clang/lib/StaticAnalyzer/Checkers/PointerArithChecker.cpp

@@ -74,6 +74,22 @@ class PointerArithChecker
 
 REGISTER_MAP_WITH_PROGRAMSTATE(RegionState, const MemRegion *, AllocKind)
 
+static bool isArrayPlacementNew(const CXXNewExpr *NE) {
+  return NE->isArray() && NE->getNumPlacementArgs() > 0;
+}
+
+static ProgramStateRef markSuperRegionReinterpreted(ProgramStateRef State,
+                                                    const MemRegion *Region) {
+  while (const auto *BaseRegion = dyn_cast<CXXBaseObjectRegion>(Region)) {
+    Region = BaseRegion->getSuperRegion();
+  }
+  if (const auto *ElemRegion = dyn_cast<ElementRegion>(Region)) {
+    State = State->set<RegionState>(ElemRegion->getSuperRegion(),
+                                    AllocKind::Reinterpreted);
+  }
+  return State;
+}
+
 void PointerArithChecker::checkDeadSymbols(SymbolReaper &SR,
                                            CheckerContext &C) const {
   // TODO: intentional leak. Some information is garbage collected too early,
@@ -244,13 +260,23 @@ void PointerArithChecker::checkPostStmt(const CXXNewExpr *NE,
   const MemRegion *Region = AllocedVal.getAsRegion();
   if (!Region)
     return;
+
+  // For array placement-new, mark the original region as reinterpreted
+  if (isArrayPlacementNew(NE)) {
+    State = markSuperRegionReinterpreted(State, Region);
+  }
+
   State = State->set<RegionState>(Region, Kind);
   C.addTransition(State);
 }
 
 void PointerArithChecker::checkPostStmt(const CastExpr *CE,
                                         CheckerContext &C) const {
-  if (CE->getCastKind() != CastKind::CK_BitCast)
+  // Casts to `void*` happen, for instance, on placement new calls.
+  // We consider `void*` not to erase the type information about the underlying
+  // region.
+  if (CE->getCastKind() != CastKind::CK_BitCast ||
+      CE->getType()->isVoidPointerType())
     return;
 
   const Expr *CastedExpr = CE->getSubExpr();

clang/test/Analysis/PR24184.cpp

@@ -56,7 +56,7 @@ void fn1_1(void *p1, const void *p2) { p1 != p2; }
 void fn2_1(uint32_t *p1, unsigned char *p2, uint32_t p3) {
   unsigned i = 0;
   for (0; i < p3; i++)
-    fn1_1(p1 + i, p2 + i * 0);
+    fn1_1(p1 + i, p2 + i * 0); // expected-warning {{Pointer arithmetic on non-array variables relies on memory layout, which is dangerous}}
 }
 
 struct A_1 {

clang/test/Analysis/ptr-arith.cpp

@@ -165,3 +165,113 @@ void LValueToRValueBitCast_dumps(void *p, char (*array)[8]) {
 unsigned long ptr_arithmetic(void *p) {
   return __builtin_bit_cast(unsigned long, p) + 1; // no-crash
 }
+
+struct AllocOpaqueFlag {};
+
+void *operator new(unsigned long, void *ptr) noexcept { return ptr; }
+void *operator new(unsigned long, void *ptr, AllocOpaqueFlag const &) noexcept;
+
+void *operator new[](unsigned long, void *ptr) noexcept { return ptr; }
+void *operator new[](unsigned long, void *ptr,
+                     AllocOpaqueFlag const &) noexcept;
+
+struct Buffer {
+  char buf[100];
+  int padding;
+};
+
+void checkPlacementNewArryInObject() {
+  Buffer buffer;
+  int *array = new (&buffer) int[10];
+  ++array; // no warning
+  (void)*array;
+}
+
+void checkPlacementNewArrayInObjectOpaque() {
+  Buffer buffer;
+  int *array = new (&buffer, AllocOpaqueFlag{}) int[10];
+  ++array; // no warning
+  (void)*array;
+}
+
+void checkPlacementNewArrayInArray() {
+  char buffer[100];
+  int *array = new (buffer) int[10];
+  ++array; // no warning
+  (void)*array;
+}
+
+void checkPlacementNewArrayInArrayOpaque() {
+  char buffer[100];
+  int *array = new (buffer, AllocOpaqueFlag{}) int;
+  ++array; // no warning
+  (void)*array;
+}
+
+void checkPlacementNewObjectInObject() {
+  Buffer buffer;
+  int *array = new (&buffer) int;
+  ++array; // expected-warning{{Pointer arithmetic on non-array variables relies on memory layout, which is dangerous}}
+  (void)*array;
+}
+
+void checkPlacementNewObjectInObjectOpaque() {
+  Buffer buffer;
+  int *array = new (&buffer, AllocOpaqueFlag{}) int;
+  ++array; // no warning (allocator is opaque)
+  (void)*array;
+}
+
+void checkPlacementNewObjectInArray() {
+  char buffer[sizeof(int)];
+  int *array = new (buffer) int;
+  ++array; // no warning (FN)
+  (void)*array;
+}
+
+void checkPlacementNewObjectInArrayOpaque() {
+  char buffer[sizeof(int)];
+  int *array = new (buffer, AllocOpaqueFlag{}) int;
+  ++array; // no warning (FN)
+  (void)*array;
+}
+
+void checkPlacementNewSlices() {
+  const int N = 10;
+  char buffer[sizeof(int) * N] = {0};
+  int *start = new (buffer) int{0};
+  for (int i = 1; i < N; i++) {
+    auto *ptr = new int(buffer[i * sizeof(int)]);
+    *ptr = i;
+  }
+  ++start; // no warning
+  (void *)start;
+}
+
+class BumpAlloc {
+  char *buffer;
+  char *offset;
+
+public:
+  BumpAlloc(int n) : buffer(new char[n]), offset{buffer} {}
+  ~BumpAlloc() { delete[] buffer; }
+
+  void *alloc(unsigned long size) {
+    auto *ptr = offset;
+    offset += size;
+    return ptr;
+  }
+};
+
+void *operator new(unsigned long size, BumpAlloc &ba) { return ba.alloc(size); }
+
+void checkPlacementSlab() {
+  BumpAlloc bump{10};
+
+  int *ptr = new (bump) int{0};
+  ++ptr; // no warning
+  (void)*ptr;
+
+  BumpAlloc *why = &bump;
+  ++why; // expected-warning {{Pointer arithmetic on non-array variables relies on memory layout, which is dangerous [alpha.core.PointerArithm]}}
+}