setfsgid — set group identity used for filesystem checks
#include <sys/fsuid.h>
int
setfsgid( |
uid_t fsgid); |
The system call setfsgid()
changes the value of the caller's filesystem group
ID—the group ID that the Linux kernel uses to check for
all accesses to the filesystem. Normally, the value of the
filesystem group ID will shadow the value of the effective
group ID. In fact, whenever the effective group ID is
changed, the filesystem group ID will also be changed to the
new value of the effective group ID.
Explicit calls to setfsuid(2) and
setfsgid() 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.)
setfsgid() will succeed only
if the caller is the superuser or if fsgid matches either the
caller's real group ID, effective group ID, saved
set-group-ID, or current the filesystem user ID.
On both success and failure, this call returns the previous filesystem group ID of the caller.
setfsgid() is Linux-specific
and should not be used in programs intended to be
portable.
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. See setfsuid(2) for a
discussion of why the use of both setfsuid(2) and
setfsgid() is nowadays
unneeded.
The original Linux setfsgid() system call supported only
16-bit group IDs. Subsequently, Linux 2.4 added setfsgid32() supporting 32-bit IDs. The
glibc setfsgid() wrapper
function transparently deals with the variation across kernel
versions.
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 group IDs), they will
return −1 and set errno
to EINVAL 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 setfsgid(−1) (which
will always fail), in order to determine if a preceding call
to setfsgid() changed the
filesystem group ID. At the very least, EPERM should be returned when the call
fails (because the caller lacks the CAP_SETGID capability).
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright (C) 1995, Thomas K. Dyas <tdyaseden.rutgers.edu> %%%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 <tdyaseden.rutgers.edu> Modified 2000-07-01 aeb Modified 2002-07-23 aeb Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements |