blob: 1fec339be2029bf0c554065731187a113d38af92 [file] [log] [blame]
.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\" and Copyright (C) 2013, 2019, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" 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.\" manual page may be incorrect or out-of-date.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.
.\" %%%LICENSE_END
.\"
.\" 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 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
setfsuid \- set user identity used for filesystem checks
.SH SYNOPSIS
.nf
.B #include <sys/fsuid.h>
.PP
.BI "int setfsuid(uid_t " fsuid );
.fi
.SH DESCRIPTION
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)).
.PP
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 .
.PP
Explicit calls to
.BR setfsuid ()
and
.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.)
.PP
.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
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 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)).
.PP
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
.B EINVAL
without attempting
the system call.
.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)