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       setfsgid - set group identity used for filesystem checks


       #include <sys/fsuid.h>

       int setfsgid(uid_t fsgid);


       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.


       On  both  success  and  failure, this call returns the previous filesystem group ID of the


       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.


       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.

   C library/kernel differences
       In  glibc  2.15  and  earlier,  when  the wrapper for this system call determines 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), they will return -1 and set errno to EINVAL
       without attempting the system call.


       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).


       kill(2), setfsuid(2), capabilities(7), credentials(7)


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