[analyzer] Don't crash when dynamic type of a concrete region is hard-set with placement new.

[NoQ]

Summary

This is assertion removal that i find valid. With placement new (which isn't even need to be inlined, we used to model it conservatively well enough), anything of any type can have any dynamic type. Even if we have a concrete region of a variable, its dynamic type and its static type can be completely unrelated. This might be UB due to strict aliasing rules, but we shouldn't crash. This patch introduces a relatively sane behavior in this scenario that consists of evalCast()ing this-region to the assumed dynamic type during virtual calls.

In the common case when the dynamic type is a sub-class of the static type, this is worse than calling attemptDownCast() because it adds element region instead of removing base regions (which is not incorrect but produces a non-canonical representation of the SVal). But when the common-case approach is known to have failed, there doesn't seem to be a better option.

A lot of these crashes have suddenly shown up when i was testing temporaries. They have nothing to do with temporaries though, but with a weird implementation detail of std::function that suddenly got some if its methods inlined.

[george.karpenkov]

LGTM, though in general I still think we should do something on soft-failure-modes in order to aid future debugging...

Is the failure mode in this case always a UB? In this case, could we emit a warning? If not from CallEvent, then from where?

lib/StaticAnalyzer/Core/CallEvent.cpp

593 ↗

(On Diff #135563)

nit: auto?

[NoQ]

• Add a definitely-not-UB example (char buffers are special).
• Bring back an accidentally deleted blank line.

[NoQ]

lib/StaticAnalyzer/Core/CallEvent.cpp

593 ↗

(On Diff #135563)

Dunno, i somehow like the symmetry with the next line. "Take method decl, take record decl". Maybe it's just me.

[NoQ]

In this case, could we emit a warning? If not from CallEvent, then from where?

(1) This might result in a buffer overflow, so i home that alpha.ArrayBound may eventually catch it.
(2) It might be a good idea to make a fairly generic checker for the strict aliasing rule.

cfe/trunk/lib/StaticAnalyzer/Core/CallEvent.cpp

if (!ThisVal.isUnknown()) {
     if (MD->getCanonicalDecl() != getDecl()->getCanonicalDecl()) {
       ASTContext &Ctx = SVB.getContext();
       const CXXRecordDecl *Class = MD->getParent();
       QualType Ty = Ctx.getPointerType(Ctx.getRecordType(Class));
 
       // FIXME: CallEvent maybe shouldn't be directly accessing StoreManager.
       bool Failed;
       ThisVal = StateMgr.getStoreManager().attemptDownCast(ThisVal, Ty, Failed);
-      assert(!Failed && "Calling an incorrectly devirtualized method");
+      if (Failed) {
+        // We might have suffered some sort of placement new earlier, so
+        // we're constructing in a completely unexpected storage.
+        // Fall back to a generic pointer cast for this-value.
+        const CXXMethodDecl *StaticMD = cast<CXXMethodDecl>(getDecl());
+        const CXXRecordDecl *StaticClass = StaticMD->getParent();
+        QualType StaticTy = Ctx.getPointerType(Ctx.getRecordType(StaticClass));
+        ThisVal = SVB.evalCast(ThisVal, Ty, StaticTy);
+      }
     }
 
     if (!ThisVal.isUnknown())
       Bindings.push_back(std::make_pair(ThisLoc, ThisVal));
   }
 }
 
 

cfe/trunk/test/Analysis/new-dynamic-types.cpp

+// RUN: %clang_analyze_cc1 -analyzer-checker=core -std=c++11 -verify %s
+
+// expected-no-diagnostics
+
+typedef __typeof(sizeof(int)) size_t;
+
+void *operator new(size_t size, void *ptr);
+
+struct B {
+  virtual void foo();
+};
+
+struct D : public B {
+  virtual void foo() override {}
+};
+
+void test_ub() {
+  // FIXME: Potentially warn because this code is pretty weird.
+  B b;
+  new (&b) D;
+  b.foo(); // no-crash
+}
+
+void test_non_ub() {
+  char c[sizeof(D)]; // Should be enough storage.
+  new (c) D;
+  ((B *)c)->foo(); // no-crash
+}