pg_rewind and replication user

Hi!
Provided my replication user created with
CREATE USER repl_user REPLICATION LOGIN ENCRYPTED PASSWORD'....';

If I run pg_rewing referring to this user
postgres(at)host1:~: pg_rewind -D $PGDATA --source-server="host=nre_primary
port=5432 user=repl_user passfile='/var/lib/postgresql/.pgpass'
dbname=postgres" -P -R

I get
pg_rewind: connected to server
pg_rewind: error: could not fetch remote file "global/pg_control": ERROR:
permission denied for function pg_read_binary_file

My question/doubt is: i can need to run pg_rewind referring to a superuser
only? Or i am "just" forgetting some grant to the replication user?

Thanks!