256 lines
6.2 KiB
HTML
256 lines
6.2 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SETUID</TITLE>
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</HEAD><BODY>
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<H1>SETUID</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-03-06<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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setuid - set user identity
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></B>
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<BR>
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<B>#include <<A HREF="file:///usr/include/unistd.h">unistd.h</A>></B>
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<P>
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<B>int setuid(uid_t </B><I>uid</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>setuid</B>()
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sets the effective user ID of the calling process.
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If the calling process is privileged
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(more precisely: if the process has the
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<B>CAP_SETUID</B>
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capability in its user namespace),
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the real UID and saved set-user-ID are also set.
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<P>
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Under Linux,
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<B>setuid</B>()
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is implemented like the POSIX version with the
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<B>_POSIX_SAVED_IDS</B>
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feature.
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This allows a set-user-ID (other than root) program to drop all of its user
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privileges, do some un-privileged work, and then reengage the original
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effective user ID in a secure manner.
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<P>
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If the user is root or the program is set-user-ID-root, special care must be
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taken:
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<B>setuid</B>()
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checks the effective user ID of the caller and if it is
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the superuser, all process-related user ID's are set to
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<I>uid</I>.
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After this has occurred, it is impossible for the program to regain root
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privileges.
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<P>
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Thus, a set-user-ID-root program wishing to temporarily drop root
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privileges, assume the identity of an unprivileged user, and then regain
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root privileges afterward cannot use
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<B>setuid</B>().
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You can accomplish this with
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<B><A HREF="/cgi-bin/man/man2html?2+seteuid">seteuid</A></B>(2).
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On success, zero is returned.
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On error, -1 is returned, and
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<I>errno</I>
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is set appropriately.
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<P>
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<I>Note</I>:
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there are cases where
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<B>setuid</B>()
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can fail even when the caller is UID 0;
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it is a grave security error to omit checking for a failure return from
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<B>setuid</B>().
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="1"><B>EAGAIN</B>
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<DD>
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The call would change the caller's real UID (i.e.,
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<I>uid</I>
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does not match the caller's real UID),
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but there was a temporary failure allocating the
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necessary kernel data structures.
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<DT id="2"><B>EAGAIN</B>
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<DD>
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<I>uid</I>
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does not match the real user ID of the caller and this call would
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bring the number of processes belonging to the real user ID
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<I>uid</I>
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over the caller's
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<B>RLIMIT_NPROC</B>
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resource limit.
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Since Linux 3.1, this error case no longer occurs
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(but robust applications should check for this error);
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see the description of
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<B>EAGAIN</B>
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in
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<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2).
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<DT id="3"><B>EINVAL</B>
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<DD>
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The user ID specified in
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<I>uid</I>
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is not valid in this user namespace.
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<DT id="4"><B>EPERM</B>
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<DD>
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The user is not privileged (Linux: does not have the
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<B>CAP_SETUID</B>
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capability in its user namespace) and
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<I>uid</I>
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does not match the real UID or saved set-user-ID of the calling process.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, SVr4.
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Not quite compatible with the 4.4BSD call, which
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sets all of the real, saved, and effective user IDs.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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Linux has the concept of the filesystem user ID, normally equal to the
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effective user ID.
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The
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<B>setuid</B>()
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call also sets the filesystem user ID of the calling process.
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See
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<B><A HREF="/cgi-bin/man/man2html?2+setfsuid">setfsuid</A></B>(2).
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<P>
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If
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<I>uid</I>
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is different from the old effective UID, the process will
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be forbidden from leaving core dumps.
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<P>
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The original Linux
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<B>setuid</B>()
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system call supported only 16-bit user IDs.
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Subsequently, Linux 2.4 added
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<B>setuid32</B>()
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supporting 32-bit IDs.
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The glibc
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<B>setuid</B>()
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wrapper function transparently deals with the variation across kernel versions.
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<A NAME="lbAI"> </A>
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<H3>C library/kernel differences</H3>
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At the kernel level, user IDs and group IDs are a per-thread attribute.
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However, POSIX requires that all threads in a process
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share the same credentials.
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The NPTL threading implementation handles the POSIX requirements by
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providing wrapper functions for
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the various system calls that change process UIDs and GIDs.
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These wrapper functions (including the one for
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<B>setuid</B>())
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employ a signal-based technique to ensure
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that when one thread changes credentials,
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all of the other threads in the process also change their credentials.
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For details, see
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<B><A HREF="/cgi-bin/man/man2html?7+nptl">nptl</A></B>(7).
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+getuid">getuid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+seteuid">seteuid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+setfsuid">setfsuid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+setreuid">setreuid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+capabilities">capabilities</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+credentials">credentials</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+user_namespaces">user_namespaces</A></B>(7)
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<A NAME="lbAK"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="5"><A HREF="#lbAB">NAME</A><DD>
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<DT id="6"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="7"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="8"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="9"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="10"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="11"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="12"><A HREF="#lbAI">C library/kernel differences</A><DD>
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</DL>
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<DT id="13"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="14"><A HREF="#lbAK">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:34 GMT, March 31, 2021
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</BODY>
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</HTML>
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