.\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright (C) 2005, 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
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.\" Modified, 2003-05-26, Michael Kerrisk, <mtk.manpages@gmail.com>
.TH setresuid 2 2023-03-30 "Linux man-pages 6.04"
.SH NAME
setresuid, setresgid \- set real, effective, and saved user or group ID
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <unistd.h>
.PP
.BI "int setresuid(uid_t " ruid ", uid_t " euid ", uid_t " suid );
.BI "int setresgid(gid_t " rgid ", gid_t " egid ", gid_t " sgid );
.fi
.SH DESCRIPTION
.BR setresuid ()
sets the real user ID, the effective user ID, and the
saved set-user-ID of the calling process.
.PP
An unprivileged process may change its real UID,
effective UID, and saved set-user-ID, each to one of:
the current real UID, the current effective UID, or the
current saved set-user-ID.
.PP
A privileged process (on Linux, one having the \fBCAP_SETUID\fP capability)
may set its real UID, effective UID, and
saved set-user-ID to arbitrary values.
.PP
If one of the arguments equals \-1, the corresponding value is not changed.
.PP
Regardless of what changes are made to the real UID, effective UID,
and saved set-user-ID, the filesystem UID is always set to the same
value as the (possibly new) effective UID.
.PP
Completely analogously,
.BR setresgid ()
sets the real GID, effective GID, and saved set-group-ID
of the calling process (and always modifies the filesystem GID
to be the same as the effective GID),
with the same restrictions for unprivileged processes.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.PP
.IR Note :
there are cases where
.BR setresuid ()
can fail even when the caller is UID 0;
it is a grave security error to omit checking for a failure return from
.BR setresuid ().
.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 (did not have the necessary
capability in its user namespace)
and tried to change the IDs to values that are not permitted.
For
.BR setresuid (),
the necessary capability is
.BR CAP_SETUID ;
for
.BR setresgid (),
it is
.BR CAP_SETGID .
.SH 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 setresuid ()
and
.BR setresgid ())
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 STANDARDS
None.
.SH HISTORY
Linux 2.1.44,
glibc 2.3.2.
HP-UX, FreeBSD.
.PP
The original Linux
.BR setresuid ()
and
.BR setresgid ()
system calls supported only 16-bit user and group IDs.
Subsequently, Linux 2.4 added
.BR setresuid32 ()
and
.BR setresgid32 (),
supporting 32-bit IDs.
The glibc
.BR setresuid ()
and
.BR setresgid ()
wrapper functions transparently deal with the variations across kernel versions.
.SH SEE ALSO
.BR getresuid (2),
.BR getuid (2),
.BR setfsgid (2),
.BR setfsuid (2),
.BR setreuid (2),
.BR setuid (2),
.BR capabilities (7),
.BR credentials (7),
.BR user_namespaces (7)