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.\" Copyright (C) 1995, Thomas K. Dyas <>
.\" and Copyright (C) 2013, 2019, Michael Kerrisk <>
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.\" Created 1995-08-06 Thomas K. Dyas <>
.\" Modified 2000-07-01 aeb
.\" Modified 2002-07-23 aeb
.\" Modified, 27 May 2004, Michael Kerrisk <>
.\" Added notes on capability requirements
.TH SETFSUID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
setfsuid \- set user identity used for filesystem checks
.B #include <sys/fsuid.h>
.BI "int setfsuid(uid_t " fsuid );
On Linux, a process has both a filesystem user ID and an effective user ID.
The (Linux-specific) filesystem user ID is used
for permissions checking when accessing filesystem objects,
while the effective user ID is used for various other kinds
of permissions checks (see
.BR credentials (7)).
Normally, the value of the process's filesystem user ID
is the same as the value of its effective user ID.
This is so, because whenever a process's effective user ID is changed,
the kernel also changes the filesystem user ID to be the same as
the new value of the effective user ID.
A process can cause the value of its filesystem user ID to diverge
from its effective user ID by using
.BR setfsuid ()
to change its filesystem user ID to the value given in
.IR fsuid .
Explicit calls to
.BR setfsuid ()
.BR setfsgid (2)
are (were) 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 (was) a security hole that can expose it to unwanted signals.
(However, this issue is historical; 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.
On both success and failure,
this call returns the previous filesystem user ID of the caller.
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.
.BR setfsuid ()
is Linux-specific and should not be used in programs intended
to be portable.
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.
.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.
.SS C library/kernel differences
In glibc 2.15 and earlier,
when the wrapper for this system call determines that the argument can't be
passed to the kernel without integer truncation (because the kernel
is old and does not support 32-bit user IDs),
it will return \-1 and set \fIerrno\fP to
without attempting
the system call.
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
should be returned when the call fails (because the caller lacks the
.BR kill (2),
.BR setfsgid (2),
.BR capabilities (7),
.BR credentials (7)