.\" Copyright (C), 1994, Graeme W. Wilford (Wilf).
+.\" and Copyright (C) 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
-.\" %%%LICENSE_START(VERBATIM)
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-.\" manual provided the copyright notice and this permission notice are
-.\" preserved on all copies.
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-.\" manual under the conditions for verbatim copying, provided that the
-.\" entire resulting derived work is distributed under the terms of a
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+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 2014-06-13 "Linux" "Linux Programmer's Manual"
+.TH setuid 2 (date) "Linux man-pages (unreleased)"
.SH NAME
setuid \- set user identity
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
.SH SYNOPSIS
-.B #include <sys/types.h>
-.br
+.nf
.B #include <unistd.h>
-.sp
+.P
.BI "int setuid(uid_t " uid );
+.fi
.SH DESCRIPTION
.BR setuid ()
sets the effective user ID of the calling process.
-If the effective UID of the caller is root,
+If the calling process is privileged
+(more precisely: if the process has the
+.B CAP_SETUID
+capability in its user namespace),
the real UID and saved set-user-ID are also set.
-.PP
+.P
Under Linux,
.BR setuid ()
is implemented like the POSIX version with the
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
+.P
If the user is root or the program is set-user-ID-root, special care must be
-taken.
-The
+taken:
.BR setuid ()
-function checks the effective user ID of the caller and if it is
+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
+.P
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
On success, zero is returned.
On error, \-1 is returned, and
.I errno
-is set appropriately.
-
+is set to indicate the error.
+.P
.IR Note :
there are cases where
.BR setuid ()
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
+capability in its user namespace) 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 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 STANDARDS
+POSIX.1-2008.
+.SH HISTORY
+POSIX.1-2001, SVr4.
+.P
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.
+.P
+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.
.SH NOTES
Linux has the concept of the filesystem user ID, normally equal to the
effective user ID.
call also sets the filesystem user ID of the calling process.
See
.BR setfsuid (2).
-.PP
+.P
If
.I uid
is different from the old effective UID, the process will
be forbidden from leaving core dumps.
-
-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.
.SH SEE ALSO
.BR getuid (2),
.BR seteuid (2),
.BR setfsuid (2),
.BR setreuid (2),
.BR capabilities (7),
-.BR credentials (7)
+.BR credentials (7),
+.BR user_namespaces (7)