RFR: 8359200: Memory corruption in MStack::push

[Aleksey Shipilev]

On Wed, 11 Jun 2025 11:49:08 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> I found this by accident when running testing with non-default `-XX:OptoNodeListSize` (see JBS for details). The problem is that `MStack::push` assumes that `Node_Stack::grow` will always grow the stack by at least two (line 69) and it then proceeds to put two items in:
> https://github.com/openjdk/jdk/blob/db6fa5923cd0394dfb44c7e46c3e7ccc102a933a/src/hotspot/share/opto/matcher.hpp#L67-L74
> 
> But after [JDK-8336999](https://bugs.openjdk.org/browse/JDK-8336999), `Node_Stack::grow` will only grow the stack if needed:
> https://github.com/openjdk/jdk/blob/db6fa5923cd0394dfb44c7e46c3e7ccc102a933a/src/hotspot/share/opto/node.cpp#L3027-L3031
> 
> However, if there's **one** empty slot, the stack will not be grown and `MStack::push` then puts **two** items on the stack which leads to memory corruption.
> 
> I refactored the push method to delegate to `Node_Stack::push` which does the right thing and, similar to [JDK-8343056](https://bugs.openjdk.org/browse/JDK-8343056), also added `maybe_grow` methods for all the containers affected by the original change. For additional coverage, I moved the `_nesting.check` calls to before the check that determines if we grow.
> 
> I only ever observed this with a non-default and odd value for `-XX:OptoNodeListSize` but I'm not 100% convinced that it can't happen with the default value, so I'm treating this as P2 and will backport the fix to JDK 25.
> 
> @shipilev Since you worked on [JDK-8343056](https://bugs.openjdk.org/browse/JDK-8343056), could you please take a look at this?
> 
> Thanks,
> Tobias

Aw, fish, these bugs are annoying. I have a further suggestion:

src/hotspot/share/opto/block.cpp line 41:

> 39: 
> 40: void Block_Array::grow(uint i) {
> 41:   assert(i >= Max(), "must be an overflow");

Assert message here is misleading: it is more likely someone had called `grow` when they intended `maybe_grow`. See how it is done elsewhere:


void Node_Array::grow(uint i) {
  _nesting.check(_a); // Check if a potential reallocation in the arena is safe
  assert(i >= _max, "Should have been checked before, use maybe_grow?");

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PR Review: https://git.openjdk.org/jdk/pull/25751#pullrequestreview-2918411519
PR Review Comment: https://git.openjdk.org/jdk/pull/25751#discussion_r2140899439