setuid - set user identity
int setuid(uid_t uid))
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 set.
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
uid 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 privileges.
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) call seteuid.
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
The user is not the super-user, and uid does not match
the effective or saved user ID of the calling process.
SVr4, SVID, POSIX.1. Not quite compatible with the 4.4BSD
call, which sets all of the real, saved, and effective user
IDs. SVr4 documents an additional EINVAL error condition.
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.
getuid(2), setreuid(2), seteuid(2), setfsuid(2)