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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SETUID" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" setuid 
.SH NAME
setuid \- set user ID
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
int setuid(uid_t\fP \fIuid\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
If the process has appropriate privileges, \fIsetuid\fP() shall set
the real user ID, effective user ID, and the saved
set-user-ID of the calling process to \fIuid\fP.
.LP
If the process does not have appropriate privileges, but \fIuid\fP
is equal to the real user ID or the saved set-user-ID,
\fIsetuid\fP() shall set the effective user ID to \fIuid\fP; the real
user ID and saved set-user-ID shall remain unchanged.
.LP
The \fIsetuid\fP() function shall not affect the supplementary group
list in any way.
.SH RETURN VALUE
.LP
Upon successful completion, 0 shall be returned. Otherwise, -1 shall
be returned and \fIerrno\fP set to indicate the error.
.SH ERRORS
.LP
The \fIsetuid\fP() function shall fail, return -1, and set \fIerrno\fP
to the corresponding value if one or more of the
following are true:
.TP 7
.B EINVAL
The value of the \fIuid\fP argument is invalid and not supported by
the implementation.
.TP 7
.B EPERM
The process does not have appropriate privileges and \fIuid\fP does
not match the real user ID or the saved set-user-ID.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
The various behaviors of the \fIsetuid\fP() and \fIsetgid\fP() 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 \fIseteuid\fP() and \fIsetegid\fP()
functions instead.
.LP
The saved set-user-ID capability allows a program to regain the effective
user ID established at the last \fIexec\fP call. Similarly, the saved
set-group-ID capability allows a program to regain the effective
group ID established at the last \fIexec\fP 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 \fIsetuid\fP() and \fIsetgid\fP() 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 \fIsetuid\fP() function to behave differently for privileged
and unprivileged users. When the caller had the
appropriate privilege, the function set the calling process' real
user ID, effective user ID, and saved set-user ID on
implementations that supported it. When the caller did not have the
appropriate privilege, the function set only the effective user
ID, subject to permission checks. The former use is generally needed
for utilities like \fIlogin\fP and \fIsu\fP, which are not
conforming applications and thus outside the scope of IEEE\ Std\ 1003.1-2001.
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.
.LP
IEEE\ Std\ 1003.1-2001 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 \fIexec\fP-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 the
(implementation-defined) appropriate privilege. 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 IEEE\ Std\ 1003.1-2001. However,
there are implementors who have been reluctant to
support it given the limitation described above.
.LP
The 4.3BSD system handles the problem by supporting separate functions:
\fIsetuid\fP() (which always sets both the real and
effective user IDs, like \fIsetuid\fP() in IEEE\ Std\ 1003.1-2001
for privileged users), and \fIseteuid\fP() (which always sets just
the effective user ID, like \fIsetuid\fP() in
IEEE\ Std\ 1003.1-2001 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 \fIsetreuid\fP(), 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 \fIseteuid\fP() and
\fIsetegid\fP() as described above. The \fIsetreuid\fP()
and \fIsetregid\fP() functions will be deprecated or removed.
.LP
The solution here is:
.IP " *" 3
Require that all implementations support the functionality of the
saved set-user-ID, which is set by the \fIexec\fP functions and by
privileged calls to \fIsetuid\fP().
.LP
.IP " *" 3
Add the \fIseteuid\fP() and \fIsetegid\fP()
functions as portable alternatives to \fIsetuid\fP() and \fIsetgid\fP()
for non-privileged
and privileged processes.
.LP
.LP
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 \fIsetuid\fP() function in the
presence of a saved set-user-ID, or the use of the BSD \fIsetreuid\fP()
function, which
was able to swap the real and effective user IDs. The changes included
in IEEE\ Std\ 1003.1-2001 provide a new mechanism
using \fIseteuid\fP() in conjunction with a saved set-user-ID. Thus,
all implementations
with the new \fIseteuid\fP() 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 \fIkill\fP()
function is an exception. Implementors of the new \fIseteuid\fP()
mechanism will generally be required to maintain compatibility with
the older
mechanisms previously supported by their systems. However, compatibility
with this use of \fIsetreuid\fP() and with the _POSIX_SAVED_IDS behavior
of \fIkill\fP() is unfortunately complicated. If an implementation
with a saved set-user-ID allows a
process to use \fIsetreuid\fP() 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 \fIkill\fP() 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 \fIsetreuid\fP(). The standard developers considered that alternative
to be less
correct than the retention of the old behavior of \fIkill\fP() in
such systems. Current
conforming applications shall accommodate either behavior from \fIkill\fP(),
and there
appears to be no strong reason for \fIkill\fP() to check the saved
set-user-ID rather than
the effective user ID.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIexec\fP() , \fIgetegid\fP() , \fIgeteuid\fP() , \fIgetgid\fP()
, \fIgetuid\fP() ,
\fIsetegid\fP() , \fIseteuid\fP() , \fIsetgid\fP() , \fIsetregid\fP()
, \fIsetreuid\fP() ,
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<sys/types.h>\fP,
\fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .