SETUID(3P) POSIX Programmer's Manual SETUID(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
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 cor-
responding 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 effec-
tive 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 per-
missions of the calling process properly. In System V, these capabili-
ties have traditionally been implemented only via the setuid() and set-
gid() functions for non-privileged processes. The fact that the behav-
ior of those functions was different for privileged processes made them
difficult to use. The POSIX.1-1990 standard defined the setuid() func-
tion 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 appro-
priate 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 out-
side 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 priv-
ilege. 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 imple-
mentors 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 dis-
tinct 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 revers-
ing 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 privi-
leged calls to setuid().
* Add the seteuid() and setegid() functions as portable alternatives
to setuid() and setgid() for non-privileged and privileged pro-
cesses.
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 gen-
erally 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 alter-
native 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.1-2008, <sys_types.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SETUID(3P)
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