NAME¶
setfsgid - set group identity used for filesystem checks
SYNOPSIS¶
#include <sys/fsuid.h>
int setfsgid(uid_t fsgid);
DESCRIPTION¶
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.
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.
setfsgid() is Linux-specific and should not be used in programs intended
to be portable.
NOTES¶
When glibc determines that the argument is not a valid group ID, it will return
-1 and set
errno to
EINVAL without attempting the system call.
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.
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 3.74 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
http://www.kernel.org/doc/man-pages/.
