RFR: 8342504: Remove NMT header and footer canaries [v2]

[Thomas Stuefe]

On Wed, 26 Feb 2025 16:07:08 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

> > As I said in preparation of this work, I don't oppose it, but I am not happy.
> > ASAN is not a replacement. ASAN is a special build, slow, needs tons of additional memory, stops at the first (often false) positive, and is often bitrotted since, to my knowledge, no vendor builds ASAN-enabled JVMs regularly. More importantly, if you have a problem in the field, it is easy to convince a customer to switch on NMT. You will not convince a customer to switch their production JVMs against an ASAN-enabled one.
> > But okay, let's remove it. I hope the capabilities this will enable are worth the loss of this capability.
> 
> Do you have any recent examples of NMT canaries helping out "in the field"? I do feel a little uneasy about loosing this feature, assuming it still helps out in the field.

Nothing I can share, but I had two customers using Unsafe.AllocateMemory (which atm are the only allocations from outside the JVM that are tracked), and they overwrote the boundaries. Took a while to find this, but the canaries added by NMT (mainly the hex dump outputs) were helpful.

I wonder, do we have to completely throw this feature away? Even a single byte as canary would be better than nothing?

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PR Comment: https://git.openjdk.org/jdk/pull/21560#issuecomment-2685626377