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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SETUID(3P) POSIX Programmer's Manual SETUID(3P)
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.
setuid — set user ID
#include <unistd.h>
int setuid(uid_t uid);
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.
Upon successful completion, 0 shall be returned. Otherwise, -1
shall be returned and errno set to indicate the error.
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.
None.
None.
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.
None.
exec(1p), getegid(3p), geteuid(3p), getgid(3p), getuid(3p),
setegid(3p), seteuid(3p), setgid(3p), setregid(3p), setreuid(3p)
The Base Definitions volume of POSIX.1‐2017, sys_types.h(0p),
unistd.h(0p)
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(3P)
Pages that refer to this page: unistd.h(0p), getegid(3p), geteuid(3p), getgid(3p), getuid(3p), posix_spawn(3p), setegid(3p), seteuid(3p), setgid(3p), setregid(3p), setreuid(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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