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.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\"
.\" 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.\" manual page may be incorrect or out-of-date.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.
.\"
.\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\" Modified 2000-07-01 aeb
.\" Modified 2002-07-23 aeb
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Added notes on capability requirements
.\"
.TH SETFSUID 2 2010-11-22 "Linux" "Linux Programmer's Manual"
.SH NAME
setfsuid \- set user identity used for file system checks
.SH SYNOPSIS
.B #include <unistd.h>
/* glibc uses <sys/fsuid.h> */
.sp
.BI "int setfsuid(uid_t " fsuid );
.SH DESCRIPTION
The system call
.BR setfsuid ()
sets the user ID that the Linux kernel uses to check for all accesses
to the file system.
Normally, the value of
.I fsuid
will shadow the value of the effective user ID.
In fact, whenever the
effective user ID is changed,
.I fsuid
will also be changed to the new value of the effective user ID.
Explicit calls to
.BR setfsuid ()
and
.BR setfsgid (2)
are usually only used by programs such as the Linux NFS server that
need to change what user and group ID is used for file access without a
corresponding change in the real and effective user and group IDs.
A change in the normal user IDs for a program such as the NFS server
is a security hole that can expose it to unwanted signals.
(But see below.)
.BR setfsuid ()
will only succeed if the caller is the superuser or if
.I fsuid
matches either the real user ID, effective user ID, saved set-user-ID, or
the current value of
.IR fsuid .
.SH "RETURN VALUE"
On success, the previous value of
.I fsuid
is returned.
On error, the current value of
.I fsuid
is returned.
.SH VERSIONS
This system call is present in Linux since version 1.2.
.\" This system call is present since Linux 1.1.44
.\" and in libc since libc 4.7.6.
.SH "CONFORMING TO"
.BR setfsuid ()
is Linux-specific and should not be used in programs intended
to be portable.
.SH NOTES
When glibc determines that the argument is not a valid user ID,
it will return \-1 and set \fIerrno\fP to
.B EINVAL
without attempting
the system call.
.LP
Note that at the time this system call was introduced, a process
could send a signal to a process with the same effective user ID.
Today signal permission handling is slightly different.
The original Linux
.BR setfsuid ()
system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added
.BR setfsuid32 ()
supporting 32-bit IDs.
The glibc
.BR setfsuid ()
wrapper function transparently deals with the variation across kernel versions.
.SH BUGS
No error messages of any kind are returned to the caller.
At the very
least,
.B EPERM
should be returned when the call fails (because the caller lacks the
.B CAP_SETUID
capability).
.SH "SEE ALSO"
.BR kill (2),
.BR setfsgid (2),
.BR capabilities (7),
.BR credentials (7)
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