NAME

       setfsgid - set group identity used for filesystem checks

SYNOPSIS

       #include <sys/fsuid.h>

       int setfsgid(uid_t fsgid);

DESCRIPTION

       On  Linux,  a  process  has  both  a filesystem group ID and an effective group ID.  The (Linux-specific)
       filesystem group ID is used for  permissions  checking  when  accessing  filesystem  objects,  while  the
       effective group ID is used for some other kinds of permissions checks (see credentials(7)).

       Normally,  the value of the process's filesystem group ID is the same as the value of its effective group
       ID.  This is so, because whenever a process's effective group ID is changed, the kernel also changes  the
       filesystem  group  ID to be the same as the new value of the effective group ID.  A process can cause the
       value of its filesystem group ID to diverge from its effective group ID by using setfsgid() to change its
       filesystem group ID to the value given in fsgid.

       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

       The filesystem group ID concept and the setfsgid() system call were invented for historical reasons  that
       are  no  longer  applicable  on modern Linux kernels.  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), it 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

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