.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
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.\" 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 2013-08-08 "Linux" "Linux Programmer's Manual"
.SH NAME
setfsuid \- set user identity used for filesystem checks
.SH SYNOPSIS
.B #include <sys/fsuid.h>
.sp
.BI "int setfsuid(uid_t " fsuid );
.SH DESCRIPTION
The system call
.BR setfsuid ()
changes the value of the caller's filesystem user ID\(emthe
user ID that the Linux kernel uses to check for all accesses
to the filesystem.
Normally, the value of
the filesystem user ID
will shadow the value of the effective user ID.
In fact, whenever the
effective user ID is changed,
the filesystem user ID
will also be changed to the new value of the effective user ID.

Explicit calls to
.BR setfsuid ()
and
.BR setfsgid (2)
are usually used only 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 succeed only if the caller is the superuser or if
.I fsuid
matches either the caller's real user ID, effective user ID,
saved set-user-ID, or current filesystem user ID.
.SH RETURN VALUE
On both success and failure,
this call returns the previous filesystem user ID of the caller.
.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
At the time when this system call was introduced, one process
could send a signal to another process with the same effective user ID.
This meant that if a privileged process changed its effective user ID
for the purpose of file permission checking,
then it could become vulnerable to receiving signals
sent by another (unprivileged) process with the same user ID.
The filesystem user ID attribute was thus added to allow a process to
change its user ID for the purposes of file permission checking without
at the same time becoming vulnerable to receiving unwanted signals.
Since Linux 2.0, signal permission handling is different (see
.BR kill (2)),
with the result that a process change can change its effective user ID
without being vulnerable to receiving signals from unwanted processes.
Thus,
.BR setfsuid ()
is nowadays unneeded and should be avoided in new applications
(likewise for
.BR setfsgid (2)).

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 indications of any kind are returned to the caller,
and the fact that both successful and unsuccessful calls return
the same value makes it impossible to directly determine
whether the call succeeded or failed.
Instead, the caller must resort to looking at the return value
from a further call such as
.IR setfsuid(\-1)
(which will always fail), in order to determine if a preceding call to
.BR setfsuid ()
changed the filesystem user ID.
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)