The time subsystem in the Linux kernel through 4.9.9, when CONFIG_TIMER_STATS is enabled, allows local users to discover real PID values (as distinguished from PID values inside a PID namespace) by reading the /proc/timer_list file, related to the print_timer function in kernel/time/timer_list.c and the __timer_stats_timer_set_start_info function in kernel/time/timer.c.