setfsgid — set group identity used for filesystem checks
#include <sys/fsuid.h>
int
setfsgid( |
gid_t fsgid); |
On Linux, a process has both a filesystem group ID and an effective group ID. The (Linux-specific) filesystem group ID is used for permissions checking when accessing filesystem objects, while the effective group ID is used for some other kinds of permissions checks (see credentials(7)).
Normally, the value of the process's filesystem group ID
is the same as the value of its effective group ID. This is
so, because whenever a process's effective group ID is
changed, the kernel also changes the filesystem group ID to
be the same as the new value of the effective group ID. A
process can cause the value of its filesystem group ID to
diverge from its effective group ID by using setfsgid() to change its filesystem group
ID to the value given in fsgid.
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.
The filesystem group ID concept and the setfsgid() system call were invented for
historical reasons that are no longer applicable on modern
Linux kernels. 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), it 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 5.11 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> and Copyright (C) 2019, Michael Kerrisk <mtk.manpagesgmail.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 <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 |