.\" Copyright (c) 1983, 1991 The Regents of the University of California.
.\" and Copyright (C) 2009, 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"     @(#)setregid.2	6.4 (Berkeley) 3/10/91
.\"
.\" Modified Sat Jul 24 09:08:49 1993 by Rik Faith <faith@cs.unc.edu>
.\" Portions extracted from linux/kernel/sys.c:
.\"             Copyright (C) 1991, 1992  Linus Torvalds
.\"             May be distributed under the GNU General Public License
.\" Changes: 1994-07-29 by Wilf <G.Wilford@ee.surrey.ac.uk>
.\"          1994-08-02 by Wilf due to change in kernel.
.\"          2004-07-04 by aeb
.\"          2004-05-27 by Michael Kerrisk
.\"
.TH setreuid 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
setreuid, setregid \- set real and/or effective user or group ID
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.P
.BI "int setreuid(uid_t " ruid ", uid_t " euid );
.BI "int setregid(gid_t " rgid ", gid_t " egid );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR setreuid (),
.BR setregid ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE
.fi
.SH DESCRIPTION
.BR setreuid ()
sets real and effective user IDs of the calling process.
.P
Supplying a value of \-1 for either the real or effective user ID forces
the system to leave that ID unchanged.
.P
Unprivileged processes may only set the effective user ID to the real user ID,
the effective user ID, or the saved set-user-ID.
.P
Unprivileged users may only set the real user ID to
the real user ID or the effective user ID.
.P
If the real user ID is set (i.e.,
.I ruid
is not \-1) or the effective user ID is set to a value
not equal to the previous real user ID,
the saved set-user-ID will be set to the new effective user ID.
.P
Completely analogously,
.BR setregid ()
sets real and effective group ID's of the calling process,
and all of the above holds with "group" instead of "user".
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.P
.IR Note :
there are cases where
.BR setreuid ()
can fail even when the caller is UID 0;
it is a grave security error to omit checking for a failure return from
.BR setreuid ().
.SH ERRORS
.TP
.B EAGAIN
The call would change the caller's real UID (i.e.,
.I ruid
does not match the caller's real UID),
but there was a temporary failure allocating the
necessary kernel data structures.
.TP
.B EAGAIN
.I ruid
does not match the caller's real UID and this call would
bring the number of processes belonging to the real user ID
.I ruid
over the caller's
.B RLIMIT_NPROC
resource limit.
Since Linux 3.1, this error case no longer occurs
(but robust applications should check for this error);
see the description of
.B EAGAIN
in
.BR execve (2).
.TP
.B EINVAL
One or more of the target user or group IDs
is not valid in this user namespace.
.TP
.B EPERM
The calling process is not privileged
(on Linux, does not have the necessary capability in its user namespace:
.B CAP_SETUID
in the case of
.BR setreuid (),
or
.B CAP_SETGID
in the case of
.BR setregid ())
and a change other than (i)
swapping the effective user (group) ID with the real user (group) ID,
or (ii) setting one to the value of the other or (iii) setting the
effective user (group) ID to the value of the
saved set-user-ID (saved set-group-ID) was specified.
.SH VERSIONS
POSIX.1 does not specify all of the UID changes that Linux permits
for an unprivileged process.
For
.BR setreuid (),
the effective user ID can be made the same as the
real user ID or the saved set-user-ID,
and it is unspecified whether unprivileged processes may set the
real user ID to the real user ID, the effective user ID, or the
saved set-user-ID.
For
.BR setregid (),
the real group ID can be changed to the value of the saved set-group-ID,
and the effective group ID can be changed to the value of
the real group ID or the saved set-group-ID.
The precise details of what ID changes are permitted vary
across implementations.
.P
POSIX.1 makes no specification about the effect of these calls
on the saved set-user-ID and saved set-group-ID.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, 4.3BSD (first appeared in 4.2BSD).
.P
Setting the effective user (group) ID to the
saved set-user-ID (saved set-group-ID) is
possible since Linux 1.1.37 (1.1.38).
.P
The original Linux
.BR setreuid ()
and
.BR setregid ()
system calls supported only 16-bit user and group IDs.
Subsequently, Linux 2.4 added
.BR setreuid32 ()
and
.BR setregid32 (),
supporting 32-bit IDs.
The glibc
.BR setreuid ()
and
.BR setregid ()
wrapper functions transparently deal with the variations across kernel versions.
.\"
.SS C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process
share the same credentials.
The NPTL threading implementation handles the POSIX requirements by
providing wrapper functions for
the various system calls that change process UIDs and GIDs.
These wrapper functions (including those for
.BR setreuid ()
and
.BR setregid ())
employ a signal-based technique to ensure
that when one thread changes credentials,
all of the other threads in the process also change their credentials.
For details, see
.BR nptl (7).
.SH SEE ALSO
.BR getgid (2),
.BR getuid (2),
.BR seteuid (2),
.BR setgid (2),
.BR setresuid (2),
.BR setuid (2),
.BR capabilities (7),
.BR credentials (7),
.BR user_namespaces (7)