SETFSGID(2) Linux Programmer's Manual SETFSGID(2)
NAME
setfsgid - set group identity used for filesystem checks
SYNOPSIS
#include <sys/fsuid.h>
int setfsgid(uid_t fsgid);
DESCRIPTION
On Linux, a process has both a filesystem group ID and an effective
group ID. The (Linux-specific) filesystem group ID is used for permis-
sions checking when accessing filesystem objects, while the effective
group ID is used for some other kinds of permissions checks (see cre-
dentials(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.
RETURN VALUE
On both success and failure, this call returns the previous filesystem
group ID of the caller.
VERSIONS
This system call is present in Linux since version 1.2.
CONFORMING TO
setfsgid() is Linux-specific and should not be used in programs in-
tended to be portable.
NOTES
The filesystem group ID concept and the setfsgid() system call were in-
vented for historical reasons that are no longer applicable on modern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.
C library/kernel differences
In glibc 2.15 and earlier, when the wrapper for this system call deter-
mines 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.
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 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).
SEE ALSO
kill(2), setfsuid(2), capabilities(7), credentials(7)
COLOPHON
This page is part of release 5.07 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/.
Linux 2019-05-09 SETFSGID(2)