/usr/share/postgresql/9.6/extension/plv8.control is in postgresql-9.6-plv8 1:1.4.10.ds-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | ## plv8 extension
comment = 'PL/JavaScript (v8) trusted procedural language'
default_version = '1.4.10'
module_pathname = '$libdir/plv8'
relocatable = false
schema = pg_catalog
superuser = true
|