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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
   attempting 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 capability).

SEE ALSO

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

COLOPHON

   This  page  is  part of release 4.09 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/.



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