RFR: 8333639: ubsan: cppVtables.cpp:81:55: runtime error: index 14 out of bounds for type 'long int [1]' [v2]

[Kim Barrett]

On Mon, 10 Jun 2024 19:31:02 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Flexible array members are not specified by C++.
>
> There are a number of "fake" VLA usage in HotSpot.  Some of them have come up in recent ubsan cleanups for similar
> reasons as here.  There's a pattern that has been used in at least some of those places.  See the class BufferNode in
> share/gc/shared/bufferNode.hpp, for example.  It would be nice to only have one pattern for this "feature".

> @kimbarrett: Thanks for taking a look! It makes sense to unify all VLA emulations. The implementation in `BufferNode` looks wrong, too. I believe specifying a length of 1 and accessing beyond it implies UB. Should I change that, too?

I _think_ it's not UB.  Or if it is, then I don't see how the mechanism used in the current version of this change isn't
as well.  The rationale for that belief is somewhat involved, and I think was discussed elsewhere.  I'll see if I can find
or reconstruct it.  Part of it is that the single-element `_buffer` is only used to provide the appropriately aligned
offset, with no accesses through that member.

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PR Comment: https://git.openjdk.org/jdk/pull/19623#issuecomment-2159199585