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 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.
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.