On discussion with chs@, I realize that:
(a) you could just use uao_create(INT64_MAX - PAGE_SIZE), like tmpfs
does, and not bother with defining a uao_resize (just .pgo_put the
pages when you truncate and update the length under the lock);
but
(b) this is kind of duplicating the existing functionality of tmpfs,
and it's a shame to have to do that.
That said, I suspect it'll be easier to:
1. add .fo_truncate (which I think is reasonable to do anyway), and
2. duplicate the tmpfs functionality anyway just for memfd_create,
than to go through the effort of finding a way to incorporate the
Linux-style O_TMPFILE and linkat(..., AT_EMPTY_PATH) into NetBSD VFS
(although it might be worthwhile to do that in the long run).
It would also be technically possible to use vn_open("/var/shm/...")
with a randomly generated pathname and then unlink it immediately, but
that would likely introduce an exploitable race unless there's a
clever way I haven't thought of to avoid the race.
