/usr/share/man/man2/rt_sigprocmask.2.gz is in manpages-dev 3.35-0.1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man2/sigprocmask.2.gz
/usr/share/man/man2/sigprocmask.2.gz