Unioning file systems: Architecture, features, and design choices

Ah, no. I mis-explained. I expect opendir() to do just what it does now:
but readdir()ing will remember the contents. (This is fine, because you
can't seekdir() to somewhere that you haven't telldir()ed, and you can't
telldir() something you haven't readdir()ed already.)

There's no DoS problem, because the application can keep an eye on the
amount of readdir()ing it's done, and stop if need be. It makes seekdir(),
even over NFS, a doddle, and retrieving dirents is no more expensive than
it is now.

I don't understand why glibc doesn't *already* implement this. Why on
earth is seekdir() the kernel's job?

(And, yes, we'd have to preserve the old syscall, but given the number of
users --- none on my system, two in the *entire* Debian source tree when I
counted it a few years ago --- I don't think anyone would care much, or
even notice, if it rotted gently into brokenness, or completely failed to
work on new filesystems.)