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 re-engage 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 a non-root user, and then regain
root privileges afterwards cannot use
setuid().
You can accomplish this with the (non-POSIX, BSD) call
seteuid(2).
RETURN VALUE
On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
EAGAIN
The
uid
does not match the current uid and
uid
brings process over its
RLIMIT_NPROC
resource limit.
EPERM
The user is not privileged (Linux: does not have the
CAP_SETUID
capability) and
uid
does not match the real UID or saved set-user-ID of the calling process.
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 Notes
Linux has the concept of 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.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
