SETUID(3)                  POSIX Programmer's Manual                 SETUID(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       setuid -- set user ID

SYNOPSIS
       #include <unistd.h>

       int setuid(uid_t uid);

DESCRIPTION
       If the process has appropriate privileges, setuid() shall set the real
       user ID, effective user ID, and the saved set-user-ID of the calling
       process to uid.

       If the process does not have appropriate privileges, but uid is equal
       to the real user ID or the saved set-user-ID, setuid() shall set the
       effective user ID to uid; the real user ID and saved set-user-ID shall
       remain unchanged.

       The setuid() function shall not affect the supplementary group list in
       any way.

RETURN VALUE
       Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
       returned and errno set to indicate the error.

ERRORS
       The setuid() function shall fail, return -1, and set errno to the
       corresponding value if one or more of the following are true:

       EINVAL The value of the uid argument is invalid and not supported by
              the implementation.

       EPERM  The process does not have appropriate privileges and uid does
              not match the real user ID or the saved set-user-ID.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       The various behaviors of the setuid() and setgid() functions when
       called by non-privileged processes reflect the behavior of different
       historical implementations. For portability, it is recommended that new
       non-privileged applications use the seteuid() and setegid() functions
       instead.

       The saved set-user-ID capability allows a program to regain the
       effective user ID established at the last exec call. Similarly, the
       saved set-group-ID capability allows a program to regain the effective
       group ID established at the last exec call. These capabilities are
       derived from System V. Without them, a program might have to run as
       superuser in order to perform the same functions, because superuser can
       write on the user's files. This is a problem because such a program can
       write on any user's files, and so must be carefully written to emulate
       the permissions of the calling process properly. In System V, these
       capabilities have traditionally been implemented only via the setuid()
       and setgid() functions for non-privileged processes. The fact that the
       behavior of those functions was different for privileged processes made
       them difficult to use. The POSIX.1-1990 standard defined the setuid()
       function to behave differently for privileged and unprivileged users.
       When the caller had appropriate privileges, the function set the real
       user ID, effective user ID, and saved set-user ID of the calling
       process on implementations that supported it. When the caller did not
       have appropriate privileges, the function set only the effective user
       ID, subject to permission checks. The former use is generally needed
       for utilities like login and su, which are not conforming applications
       and thus outside the scope of POSIX.1-2008. These utilities wish to
       change the user ID irrevocably to a new value, generally that of an
       unprivileged user. The latter use is needed for conforming applications
       that are installed with the set-user-ID bit and need to perform
       operations using the real user ID.

       POSIX.1-2008 augments the latter functionality with a mandatory feature
       named _POSIX_SAVED_IDS. This feature permits a set-user-ID application
       to switch its effective user ID back and forth between the values of
       its exec-time real user ID and effective user ID. Unfortunately, the
       POSIX.1-1990 standard did not permit a conforming application using
       this feature to work properly when it happened to be executed with
       (implementation-defined) appropriate privileges. Furthermore, the
       application did not even have a means to tell whether it had this
       privilege. Since the saved set-user-ID feature is quite desirable for
       applications, as evidenced by the fact that NIST required it in FIPS
       151-2, it has been mandated by POSIX.1-2008. However, there are
       implementors who have been reluctant to support it given the limitation
       described above.

       The 4.3BSD system handles the problem by supporting separate functions:
       setuid() (which always sets both the real and effective user IDs, like
       setuid() in POSIX.1-2008 for privileged users), and seteuid() (which
       always sets just the effective user ID, like setuid() in POSIX.1-2008
       for non-privileged users). This separation of functionality into
       distinct functions seems desirable. 4.3BSD does not support the saved
       set-user-ID feature. It supports similar functionality of switching the
       effective user ID back and forth via setreuid(), which permits
       reversing the real and effective user IDs. This model seems less
       desirable than the saved set-user-ID because the real user ID changes
       as a side-effect. The current 4.4BSD includes saved effective IDs and
       uses them for seteuid() and setegid() as described above. The
       setreuid() and setregid() functions will be deprecated or removed.

       The solution here is:

        *  Require that all implementations support the functionality of the
           saved set-user-ID, which is set by the exec functions and by
           privileged calls to setuid().

        *  Add the seteuid() and setegid() functions as portable alternatives
           to setuid() and setgid() for non-privileged and privileged
           processes.

       Historical systems have provided two mechanisms for a set-user-ID
       process to change its effective user ID to be the same as its real user
       ID in such a way that it could return to the original effective user
       ID: the use of the setuid() function in the presence of a saved set-
       user-ID, or the use of the BSD setreuid() function, which was able to
       swap the real and effective user IDs. The changes included in
       POSIX.1-2008 provide a new mechanism using seteuid() in conjunction
       with a saved set-user-ID. Thus, all implementations with the new
       seteuid() mechanism will have a saved set-user-ID for each process, and
       most of the behavior controlled by _POSIX_SAVED_IDS has been changed to
       agree with the case where the option was defined. The kill() function
       is an exception. Implementors of the new seteuid() mechanism will
       generally be required to maintain compatibility with the older
       mechanisms previously supported by their systems. However,
       compatibility with this use of setreuid() and with the _POSIX_SAVED_IDS
       behavior of kill() is unfortunately complicated. If an implementation
       with a saved set-user-ID allows a process to use setreuid() to swap its
       real and effective user IDs, but were to leave the saved set-user-ID
       unmodified, the process would then have an effective user ID equal to
       the original real user ID, and both real and saved set-user-ID would be
       equal to the original effective user ID. In that state, the real user
       would be unable to kill the process, even though the effective user ID
       of the process matches that of the real user, if the kill() behavior of
       _POSIX_SAVED_IDS was used. This is obviously not acceptable. The
       alternative choice, which is used in at least one implementation, is to
       change the saved set-user-ID to the effective user ID during most calls
       to setreuid().  The standard developers considered that alternative to
       be less correct than the retention of the old behavior of kill() in
       such systems. Current conforming applications shall accommodate either
       behavior from kill(), and there appears to be no strong reason for
       kill() to check the saved set-user-ID rather than the effective user
       ID.

FUTURE DIRECTIONS
       None.

SEE ALSO
       exec, getegid(), geteuid(), getgid(), getuid(), setegid(), seteuid(),
       setgid(), setregid(), setreuid()

       The Base Definitions volume of POSIX.1-2017, <sys_types.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                            SETUID(3)
