Null dereference when using AppCDS with old classes and VM_RedefineClasses

[Justin Du]

Hi everyone,

I recently reported JDK-8342303 [0], where a rare interaction
between AppCDS and VM_RedefineClasses leads to a null pointer
dereference. I believe the cause is the following:

1. At AppCDS dump time, old classes (bytecode version < 50)
   aren't linked, so their vtables contain all null pointers.
2. If we load old class A before the first class redefinition,
   A's vtable is still uninitialized.
3. If we load old class B after the first class redefinition,
   JvmtiExport::has_redefined_a_class() is true. Then
   InstanceKlass::restore_unshareable_info reinitializes B's
   vtable, in case its superclass was redefined.
4. If B extends A, then A's uninitialized vtable is copied into
   B's vtable, leading to a null pointer dereference.

Notably, if B's superclass is already linked when B is loaded,
then the vtable reinitialization still succeeds.

To prevent this, maybe we only need to reinitialize vtables if
the class is already linked?

Thanks!
Justin

[0] https://bugs.openjdk.org/browse/JDK-8342303