209 lines
6.1 KiB
HTML
209 lines
6.1 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 SETFSUID</TITLE>
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</HEAD><BODY>
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<H1>SETFSUID</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-05-09<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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setfsuid - set user identity used for filesystem checks
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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/fsuid.h">sys/fsuid.h</A>></B>
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<P>
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<B>int setfsuid(uid_t </B><I>fsuid</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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On Linux, a process has both a filesystem user ID and an effective user ID.
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The (Linux-specific) filesystem user ID is used
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for permissions checking when accessing filesystem objects,
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while the effective user ID is used for various other kinds
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of permissions checks (see
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<B><A HREF="/cgi-bin/man/man2html?7+credentials">credentials</A></B>(7)).
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<P>
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Normally, the value of the process's filesystem user ID
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is the same as the value of its effective user ID.
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This is so, because whenever a process's effective user ID is changed,
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the kernel also changes the filesystem user ID to be the same as
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the new value of the effective user ID.
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A process can cause the value of its filesystem user ID to diverge
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from its effective user ID by using
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<B>setfsuid</B>()
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to change its filesystem user ID to the value given in
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<I>fsuid</I>.
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<P>
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Explicit calls to
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<B>setfsuid</B>()
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and
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<B><A HREF="/cgi-bin/man/man2html?2+setfsgid">setfsgid</A></B>(2)
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are (were) usually used only by programs such as the Linux NFS server that
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need to change what user and group ID is used for file access without a
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corresponding change in the real and effective user and group IDs.
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A change in the normal user IDs for a program such as the NFS server
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is (was) a security hole that can expose it to unwanted signals.
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(However, this issue is historical; see below.)
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<P>
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<B>setfsuid</B>()
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will succeed only if the caller is the superuser or if
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<I>fsuid</I>
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matches either the caller's real user ID, effective user ID,
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saved set-user-ID, or current filesystem user ID.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On both success and failure,
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this call returns the previous filesystem user ID of the caller.
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<A NAME="lbAF"> </A>
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<H2>VERSIONS</H2>
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This system call is present in Linux since version 1.2.
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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<B>setfsuid</B>()
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is Linux-specific and should not be used in programs intended
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to be portable.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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At the time when this system call was introduced, one process
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could send a signal to another process with the same effective user ID.
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This meant that if a privileged process changed its effective user ID
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for the purpose of file permission checking,
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then it could become vulnerable to receiving signals
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sent by another (unprivileged) process with the same user ID.
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The filesystem user ID attribute was thus added to allow a process to
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change its user ID for the purposes of file permission checking without
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at the same time becoming vulnerable to receiving unwanted signals.
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Since Linux 2.0, signal permission handling is different (see
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<B><A HREF="/cgi-bin/man/man2html?2+kill">kill</A></B>(2)),
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with the result that a process can change its effective user ID
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without being vulnerable to receiving signals from unwanted processes.
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Thus,
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<B>setfsuid</B>()
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is nowadays unneeded and should be avoided in new applications
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(likewise for
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<B><A HREF="/cgi-bin/man/man2html?2+setfsgid">setfsgid</A></B>(2)).
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<P>
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The original Linux
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<B>setfsuid</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>setfsuid32</B>()
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supporting 32-bit IDs.
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The glibc
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<B>setfsuid</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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In glibc 2.15 and earlier,
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when the wrapper for this system call determines that the argument can't be
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passed to the kernel without integer truncation (because the kernel
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is old and does not support 32-bit user IDs),
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it will return -1 and set <I>errno</I> to
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<B>EINVAL</B>
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without attempting
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the system call.
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<A NAME="lbAJ"> </A>
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<H2>BUGS</H2>
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No error indications of any kind are returned to the caller,
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and the fact that both successful and unsuccessful calls return
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the same value makes it impossible to directly determine
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whether the call succeeded or failed.
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Instead, the caller must resort to looking at the return value
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from a further call such as
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<I>setfsuid(-1)</I>
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(which will always fail), in order to determine if a preceding call to
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<B>setfsuid</B>()
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changed the filesystem user ID.
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At the very
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least,
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<B>EPERM</B>
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should be returned when the call fails (because the caller lacks the
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<B>CAP_SETUID</B>
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capability).
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+kill">kill</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+setfsgid">setfsgid</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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<A NAME="lbAL"> </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="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="5"><A HREF="#lbAF">VERSIONS</A><DD>
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<DT id="6"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="7"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="8"><A HREF="#lbAI">C library/kernel differences</A><DD>
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</DL>
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<DT id="9"><A HREF="#lbAJ">BUGS</A><DD>
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<DT id="10"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="11"><A HREF="#lbAL">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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