1SETFSGID(2)                Linux Programmer's Manual               SETFSGID(2)
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NAME

6       setfsgid - set group identity used for file system checks
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SYNOPSIS

9       #include <unistd.h> /* glibc uses <sys/fsuid.h> */
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11       int setfsgid(uid_t fsgid);
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DESCRIPTION

14       The system call setfsgid() sets the group ID that the Linux kernel uses
15       to check for all accesses to the file system.  Normally, the  value  of
16       fsgid  will shadow the value of the effective group ID.  In fact, when‐
17       ever the effective group ID is changed, fsgid will also be  changed  to
18       the new value of the effective group ID.
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20       Explicit  calls  to setfsuid(2) and setfsgid() are usually used only by
21       programs such as the Linux NFS server that need to change what user and
22       group  ID is used for file access without a corresponding change in the
23       real and effective user and group IDs.  A change in the normal user IDs
24       for a program such as the NFS server is a security hole that can expose
25       it to unwanted signals.  (But see below.)
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27       setfsgid() will succeed only if the caller is the superuser or if fsgid
28       matches  either the real group ID, effective group ID, saved set-group-
29       ID, or the current value of fsgid.
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RETURN VALUE

32       On success, the previous value of fsgid is  returned.   On  error,  the
33       current value of fsgid is returned.
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VERSIONS

36       This system call is present in Linux since version 1.2.
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CONFORMING TO

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

43       When glibc determines that the argument is not a  valid  group  ID,  it
44       will  return  -1  and set errno to EINVAL without attempting the system
45       call.
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47       Note that at the time this system call was introduced, a process  could
48       send a signal to a process with the same effective user ID.  Today sig‐
49       nal permission handling is slightly different.
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51       The original Linux setfsgid() system call supported only  16-bit  group
52       IDs.  Subsequently, Linux 2.4 added setfsgid32() supporting 32-bit IDs.
53       The glibc setfsgid() wrapper  function  transparently  deals  with  the
54       variation across kernel versions.
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BUGS

57       No  error messages of any kind are returned to the caller.  At the very
58       least, EPERM should be returned when the call fails (because the caller
59       lacks the CAP_SETGID capability).
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SEE ALSO

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

65       This  page  is  part of release 3.53 of the Linux man-pages project.  A
66       description of the project, and information about reporting  bugs,  can
67       be found at http://www.kernel.org/doc/man-pages/.
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71Linux                             2010-11-22                       SETFSGID(2)
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