Race-free process creation in the GNU C Library

subset=pids has no effect on the mount_too_revealing() check because all of the "are the flags the same" checks are based on the generic VFS flags not FS-specific ones. So if you only have an overmounted procfs you cannot mount subset=pids even if the overmounts are paths that don't exist with subset=pids.

In fact this also means you can bypass the check entirely -- if you have a "safe" subset=pids mount in your namespace, the kernel will allow you to mount an unmasked (fully-fledged) procfs.