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Linux 2.6 - man page for setfsgid (linux section 2)

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.   Nor-
       mally,  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  with-
       out  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.

       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 signal  permission  handling  is  slightly
       different.  See setfsuid(2) for a discussion of why the use of both setfsuid(2) and setfs-
       gid() 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.

       No error indications of any kind are returned to the caller, and the fact that  both  suc-
       cessful	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  look-
       ing 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)

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2013-08-08				      SETFSGID(2)

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