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       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 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.

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 .