setuid sets the effective user ID of the current process. If the
effective userid of the caller is root, the real and saved user ID's are also
Under Linux, setuid is implemented like the POSIX version with the
_POSIX_SAVED_IDS feature. This allows a setuid (other than root) program to
drop all of its user privileges, do some un-privileged work, and then
re-engage the original effective user ID in a secure manner.
If the user is root or the program is setuid root, special care must be taken.
The setuid function checks the effective user ID of the caller and if
it is the superuser, all process related user ID's are set to uid.
After this has occurred, it is impossible for the program to regain root
Thus, a setuid-root program wishing to temporarily drop root privileges, assume
the identity of a non-root user, and then regain root privileges afterwards
cannot use setuid. You can accomplish this with the (non-POSIX, BSD)
Linux has the concept of filesystem user ID, normally equal to the effective
user ID. The setuid call also sets the filesystem user ID of the
current process. See setfsuid(2).
If uid is different from the old effective uid, the process will be
forbidden from leaving core dumps.