Furthermore, I'll go ahead and propose the following (simple) semantics:

1) birthtime is initialized to the current time when a new inode is
created

2) it's settable via the xattr to an arbitrary value

Either way, the xattr for this ought to be named the same on all
filesystems. Samba shouldn't need to know or care what the underlying
filesystem is, as long as it presents the correct xattr.

That should make samba happy, and be reasonably simple to implement.

Is there any reason to allow birthtime to be set in advance of the
current birthtime?

ie restore / copy tools clearly need to backdate it, but I'd prefer to
see it not advance-able.

Greg