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.
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).
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