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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)


     This  page  is  part of release 4.16 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

Linux 2017-09-15 SETFSGID(2)

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