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

       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 caller.

       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 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.

   C library/kernel differences
       In glibc 2.15 and earlier, when the wrapper for this system call deter-
       mines  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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       description of the project, information about reporting bugs,  and  the
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Linux				  2016-07-17			   SETFSGID(2)