SETFSGID(2)                Linux Programmer's Manual               SETFSGID(2)

       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, when-
       ever 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 pro-
       grams 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.

       No  error  messages of any kind are returned to the caller. At the very
       least, EPERM should be returned when the call fails.

       When glibc determines that the argument is not a  valid  gid,  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 sig-
       nal permission handling is slightly different.

       kill(2), setfsuid(2)

Linux 1.3.15                      2002-07-23                       SETFSGID(2)