[ruby-core:112634] [Ruby master Bug#19469] Crash when resizing generic iv list

Issue #19469 has been reported by peterzhu2118 (Peter Zhu).

----------------------------------------
Bug #19469: Crash when resizing generic iv list
https://bugs.ruby-lang.org/issues/19469

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Backport: 2.7: DONTNEED, 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED
----------------------------------------
GitHub PR: https://github.com/ruby/ruby/pull/7407

The following script can sometimes trigger a crash:

```ruby
GC.stress = true

class Array
  def foo(bool)
    if bool
      @a = 1
      @b = 2
      @c = 1
    else
      @c = 1
    end
  end
end

obj = []
obj.foo(true)

obj2 = []
obj2.foo(false)

obj3 = []
obj3.foo(true)
```

This is because vm_setivar_default calls rb_ensure_generic_iv_list_size to resize the iv list. However, the call to gen_ivtbl_resize reallocs the iv list, and then inserts into the generic iv table. If the st_insert triggers a GC then the old iv list will be read during marking, causing a use-after-free bug.



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