Difference among RUID, EUID, SUID

□ RUID(Real User ID)

○ The actual owner of a process
○ It is used in signal transmission. A unprivileged process can signal to the another process when the RUID, EUID is the same as RUID, SUID of the another process

□ EUID(Effective User ID)

○ Generally, UID and EUID is the same
○ EUID is changed by executable file that is configured SetUID authority
○ EUID temporarily stores another account's UID
○ The authority of a process is determined according to the UID stored in the EUID

□ SUID(Saved set-user-ID)

○ SUID is used when a process's authority is recovered after lowered
○ When process's authority is changed to lower. previous EUID is stored at SUID
○ Then, when the lowered authority is recovered to original authority, the SUID is stored at EUID