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

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.

CONFORMING TO

       setfsgid() is Linux-specific and should not be used in programs intended to be portable.

NOTES

       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  attempt-
       ing the system call.

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 capabil-
       ity).

SEE ALSO

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

COLOPHON

       This  page is part of release 4.15 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 https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15							       SETFSGID(2)