Christoph Hellwig [off-list ref] wrote:

It's not a userspace ABI, so by defintion it does not break an
existing user program.

That's an invalid assumption.  It is a de facto userspace ABI as it has been
exposed in /usr/include/linux/uuid.h for some time.

If someone was using it they should be using uuid_t from libuuid instead, as
that gives them the routines to deal with it.

Yes, they should - but that doesn't mean they do.

David