.\" 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.
.\"
.\" 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@gmx.net>
+.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Added notes on capability requirements
-.TH SETUID 2 2004-05-27 "Linux" "Linux Programmer's Manual"
+.TH SETUID 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
setuid \- set user identity
.SH SYNOPSIS
.BI "int setuid(uid_t " uid );
.SH DESCRIPTION
.BR setuid ()
-sets the effective user ID of the current process.
-If the effective UID of the caller is root,
+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 _POSIX_SAVED_IDS feature.
+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 re-engage the original
+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
The
.BR setuid ()
function checks the effective user ID of the caller and if it is
-the superuser, all process related user ID's are set to
+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 a non-root user, and then regain
-root privileges afterwards cannot use
+privileges, assume the identity of an unprivileged user, and then regain
+root privileges afterward cannot use
.BR setuid ().
-You can accomplish this with the (non-POSIX, BSD) call
+You can accomplish this with
.BR seteuid (2).
-.SH "RETURN VALUE"
+.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
+
+.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
+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 current uid and
+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
-brings process over its NPROC rlimit.
+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
capability) and
.I uid
does not match the real UID or saved set-user-ID of the calling process.
-.SH "CONFORMING TO"
-SVr4, POSIX.1-2001.
+.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
-.SS Linux Notes
-Linux has the concept of filesystem user ID, normally equal to the
+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 current process.
+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
+is different from the old effective UID, the process will
be forbidden from leaving core dumps.
-.SH "SEE ALSO"
+
+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 capabilities (7),
+.BR credentials (7),
+.BR user_namespaces (7)
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