I think it would make more sense to define a new buffer size constant than modifying BUFSIZ.
Unconditionally allowing absolute paths with O_BENEATH seems a bit strange, as I expect this to be used for partially untrusted paths. An application can easily check for absolute paths and cause either behaviour (allowing or disallowing).
experimenting with the pastbin.. ignore..
iflib_admin_intr_deferred() called with STATE_LOCK() in a few places.
I've moved STATE_UNLOCK before iflib_admin_intr_defereed call although releasing lock just to immediately grab it again makes me a bit uneasy. Maybe it would be better idea to use mtx_owned in iflib_in_detach to check if this lock is already aquired?
Addressed feedback I received here and elsewhere on the previous patch. Please consider this alternative approach.