DESCRIPTION

setuid() sets the effective user ID of the calling process. If the effective UID of the caller is root, the real UID and saved set-user-ID are also set.

Under Linux, setuid() is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a set-user-ID (other than root) program to drop all of its user privileges, do some un-privileged work, and then reengage the original effective user ID in a secure manner.

If the user is root or the program is set-user-ID-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 privileges.

Thus, a set-user-ID-root program wishing to temporarily drop root privileges, assume the identity of an unprivileged user, and then regain root privileges afterward cannot use setuid(). You can accomplish this with seteuid(2).

RETURN VALUE

On success, zero is returned. On error, −1 is returned, and errno is set appropriately.

ERRORS

CONFORMING TO

SVr4, POSIX.1-2001. Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs.

NOTES

Linux has the concept of the file system user ID, normally equal to the effective user ID. The setuid() call also sets the file system user ID of the calling process. See setfsuid(2).

If uid is different from the old effective UID, the process will be forbidden from leaving core dumps.

The original Linux setuid() system call supported only 16-bit user IDs. Subsequently, Linux 2.4 added setuid32() supporting 32-bit IDs. The glibc setuid() wrapper function transparently deals with the variation across kernel versions.

SEE ALSO

getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)