/proc/PID/io may be used for gathering private information. E.g. for
openssh and vsftpd daemons wchars/rchars may be used to learn the
precise password length. Restrict it to processes being able to ptrace
the target process.
ptrace_may_access() is needed to prevent keeping open file descriptor of
"io" file, executing setuid binary and gathering io information of the
setuid'ed process.