setfsgid — set group identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int
setfsgid( |
uid_t | fsgid); |
The system call setfsgid()
sets the group ID that the Linux kernel uses to check for all
accesses to the file system. Normally, the value of
fsgid will shadow the
value of the effective group ID. In fact, whenever the
effective group ID is changed, fsgid will also be changed to
the new value of the effective group ID.
Explicit calls to setfsuid(2) and
setfsgid() are usually only
used 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 only succeed
if the caller is the superuser or if fsgid matches either the real
group ID, effective group ID, saved set-group-ID, or the
current value of fsgid.
On success, the previous value of fsgid is returned. On error,
the current value of fsgid is returned.
setfsgid() is Linux-specific
and should not be used in programs intended to be portable.
It is present since Linux 1.1.44 and in libc since libc
4.7.6.
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.
No error messages of any kind are returned to the caller.
At the very least, EPERM
should be returned when the call fails (because the caller
lacks the CAP_SETGID
capability).