RFR: 8342504: Remove NMT header and footer canaries

[Afshin Zafari]

On Thu, 17 Oct 2024 10:04:16 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> Today NMT has two canaries: A header and a footer canary. These enable mainly two things:
> 
> 1. For NMT to aid in describing a pointer
> 2. A basic form of out-of-bounds protection
> 
> With the introduction of UBSan and Asan into OpenJDK we have gained stronger tools for this sort of analysis, without requiring NMT to be activated. Therefore, I believe that point 2 is no longer something that NMT needs to support. For point number one, we will unfortunately be losing this ability.
> 
> I want to delete these canaries to open up a few free bytes. These can allow us to have "practically unlimited" (4 bytes) of memory tags.
> 
> tier1-tier2 tests succeeded.
> 
> I am awaiting discussion on the Hotspot-dev mailing list, but keeping this PR open for review.

Thanks for this work. 
Some `flags` vs `MemTag` found.

test/hotspot/gtest/nmt/test_nmt_buffer_overflow_detection.cpp line 67:

> 65: // realloc is the trickiest of the bunch. Test that realloc works and correctly takes over
> 66: // NMT header and footer to the resized block. We just test that nothing crashes - if the
> 67: // header/footer get corrupted, NMT heap corruption checker will trigger alert on os::free()).

After removing canary, can NMT still check memory corruption?

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PR Review: https://git.openjdk.org/jdk/pull/21560#pullrequestreview-2442419323
PR Review Comment: https://git.openjdk.org/jdk/pull/21560#discussion_r1846462308