Как CRON исполняет процессы от имени пользователя?

Смена uid/gid происходит посредством setgid():

[pid 5591] read(5,
[pid 5593] <... stat resumed> {st_mode=S_IFREG|0644, st_size=2194, ...}) = 0
[pid 5593] getpid() = 5593
[pid 5593] sendto(7, "<78>Jan 30 19:27:01 /USR/SBIN/CRO"..., 59, MSG_NOSIGNAL, NULL, 0) = 59
[pid 5593] close(7) = 0
[pid 5593] setsid() = 5593
[pid 5593] close(4) = 0
[pid 5593] close(5) = 0
[pid 5593] dup2(3, 0) = 0
[pid 5593] dup2(6, 1) = 1
[pid 5593] dup2(1, 2) = 2
[pid 5593] close(3) = 0
[pid 5593] close(6) = 0
[pid 5593] setgid(1001) = 0
[pid 5593] open("/proc/sys/kernel/ngroups_max", O_RDONLY) = 3