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Be that as it may, existing software that used to work will no longer work, and the developers might not be around/able to fix it.

There are also cases of custom suballocators or arrays of objects - Looking at an address makes it possibly to figure out which array it belongs to. This code would break.

Granted, it would still be possible to do all this if you just mask off the tag bits, but it requires a software change.

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The "suballocate out of some arrays" code should not break, because the whole array would be allocated at once and so would have the same tag for the whole range. Code that does a simple "is this pointer value inside the "array_base + size" range" continues to work, because array_base has whatever tag malloc() handed out for that array, and so do the pointer values that the suballocator handed out. I think for MTE to break your code you would have to be doing some pretty weird stuff with pointer arithmetic (beyond just the usual "technically maybe this is undefined behaviour but it works" level stuff).

It's always the case that some software that does things that are not valid-by-the-language-standard might break if run on a newer version of the OS or a newer system library version (remember the big flap about glibc memcpy() changing its behaviour when called for overlapping regions?). You don't want to break lots of software gratuitously, but sometimes the tradeoff is worth making.

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