RFR: 8339242: Fix overflow issues in AdlArena [v2]

[Casper Norrbin]

On Mon, 2 Sep 2024 11:54:40 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Casper Norrbin has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   arena realloc overflow check
>
> src/hotspot/share/memory/arena.cpp line 339:
> 
>> 337:   // See if we can resize in-place
>> 338:   if( (c_old+old_size == _hwm) &&                      // Adjusting recent thing
>> 339:       ((size_t)(_max-c_old) >= corrected_new_size) ) { // Still fits where it sits, safe from overflow
> 
> This change is correct, but it hides an important finding behind a reshuffling of parameters that someone else may innocently reshape later. It also makes the code less readable. Can we use something like saturated_add()?
> 
> I would also add an explicit assert for a reasonable max size. Arena allocations should be small. Nobody should hand in sizes larger than a few MB, so asserting for size >= 2^31 (2g) would make sense. Anything as large as that is almost certainly an error we should trap on.

@tstuefe I've now added asserts and changed this check to use saturated adds.

The already existing `saturated_add()` only works on `int`s and `uint`s, so I made a version working on offsetting pointers. We still have to check for overflow (if the pointer addition was saturated), but the code becomes more readable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20774#discussion_r1742111702