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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  ac-
       cesses  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  in-
       tended to be portable.

NOTES
       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 sig-
       nal  permission	handling is slightly different.	 See setfsuid(2) for a
       discussion of why the use of both setfsuid(2) and setfsgid()  is	 nowa-
       days 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.

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 capability).

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

COLOPHON
       This  page  is  part of release 3.74 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
       http://www.kernel.org/doc/man-pages/.

Linux				  2013-08-08			   SETFSGID(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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