.\" Copyright (C), 1994, Graeme W. Wilford (Wilf).

.\" and Copyright (C) 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

.\"

.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.\"

.\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>

.\" Changes inspired by patch from Richard Kettlewell

.\"   <richard@greenend.org.uk>, aeb 970616.

.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added notes on capability requirements

.TH SETUID 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

setuid \- set user identity

.SH SYNOPSIS

.B #include <sys/types.h>

.br

.B #include <unistd.h>

.PP

.BI "int setuid(uid_t " uid );

.SH DESCRIPTION

.BR setuid ()

sets the effective user ID of the calling process.

If the calling process is privileged

(more precisely: if the process has the

.BR CAP_SETUID

capability in its user namespace),

the real UID and saved set-user-ID are also set.

.PP

Under Linux,

.BR setuid ()

is implemented like the POSIX version with the

.B _POSIX_SAVED_IDS

feature.

This allows a set-user-ID (other than root) program to drop all of its user

privileges, do some un-privileged work, and then reengage the original

effective user ID in a secure manner.

.PP

If the user is root or the program is set-user-ID-root, special care must be

taken:

.BR setuid ()

checks the effective user ID of the caller and if it is

the superuser, all process-related user ID's are set to

.IR uid .

After this has occurred, it is impossible for the program to regain root

privileges.

.PP

Thus, a set-user-ID-root program wishing to temporarily drop root

privileges, assume the identity of an unprivileged user, and then regain

root privileges afterward cannot use

.BR setuid ().

You can accomplish this with

.BR seteuid (2).

.SH RETURN VALUE

On success, zero is returned.

On error, \-1 is returned, and

.I errno

is set appropriately.

.PP

.IR Note :

there are cases where

.BR setuid ()

can fail even when the caller is UID 0;

it is a grave security error to omit checking for a failure return from

.BR setuid ().

.SH ERRORS

.TP

.B EAGAIN

The call would change the caller's real UID (i.e.,

.I uid

does not match the caller's real UID),

but there was a temporary failure allocating the

necessary kernel data structures.

.TP

.B EAGAIN

.I uid

does not match the real user ID of the caller and this call would

bring the number of processes belonging to the real user ID

.I uid

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

The user ID specified in

.I uid

is not valid in this user namespace.

.TP

.B EPERM

The user is not privileged (Linux: does not have the

.B CAP_SETUID

capability) and

.I uid

does not match the real UID or saved set-user-ID of the calling process.

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

Not quite compatible with the 4.4BSD call, which

sets all of the real, saved, and effective user IDs.

.\" SVr4 documents an additional EINVAL error condition.

.SH NOTES

Linux has the concept of the filesystem user ID, normally equal to the

effective user ID.

The

.BR setuid ()

call also sets the filesystem user ID of the calling process.

See

.BR setfsuid (2).

.PP

If

.I uid

is different from the old effective UID, the process will

be forbidden from leaving core dumps.

.PP

The original Linux

.BR setuid ()

system call supported only 16-bit user IDs.

Subsequently, Linux 2.4 added

.BR setuid32 ()

supporting 32-bit IDs.

The glibc

.BR setuid ()

wrapper function transparently deals with the variation 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 the one for

.BR setuid ())

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 getuid (2),

.BR seteuid (2),

.BR setfsuid (2),

.BR setreuid (2),

.BR capabilities (7),

.BR credentials (7),

.BR user_namespaces (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.