[ruby-core:122518] [Ruby Bug#21438] use-after-free when resizing exivars

Issue #21438 has been reported by byroot (Jean Boussier).

----------------------------------------
Bug #21438: use-after-free when resizing exivars
https://bugs.ruby-lang.org/issues/21438

* Author: byroot (Jean Boussier)
* Status: Open
* Backport: 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED
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Here's a semi-reliable reproduction: 

```ruby
objs = 10_000.times.map do
  a = []
  a.instance_variable_set(:@a, 1)
  a
end

GC.stress = true
GC.auto_compact = true

steps = 1000.times.map do
  a = []
  a.instance_variable_set(:@a, 1)
  a.instance_variable_set(:@b, 2)
  a.instance_variable_set(:@c, 3)
  a.instance_variable_set(:@d, 4)
  a.instance_variable_set(:@e, 5)
  a.instance_variable_set(:@f, 6)
  a.instance_variable_set(:@g, 7)
  a.instance_variable_set(:@h, 8)
  # resize
  a.instance_variable_set(:@i, 9)
  a.instance_variable_set(:@j, 10)
  a
end
objs.clear

GC.stress = false
GC.auto_compact = false
```

The Exivar codepath uses `st_update` and allocate within the codebase.

If GC trigger, it may remove entires from the table, or delete+insert in case of compaction, and this can trigger a table rebuild of the generic fields st_table in the middle of calling the st_update callback.
This can cause entries to be reallocated or rearranged and the update to be for the wrong entry.

Auto compaction isn't strictly required to trigger the bug, but makes it more likely.





-- 
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