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

NAME         top

       setfsgid - set group identity used for filesystem checks

SYNOPSIS         top

       #include <sys/fsuid.h>

       int setfsgid(uid_t fsgid);

DESCRIPTION         top

       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

       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         top

       On both success and failure, this call returns the previous
       filesystem group ID of the caller.

VERSIONS         top

       This system call is present in Linux since version 1.2.

CONFORMING TO         top

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

NOTES         top

       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), it will return -1 and set errno to EINVAL without
       attempting the system call.

BUGS         top

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.00 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)

Pages that refer to this page: setfsuid(2)setresuid(2)syscalls(2)credentials(7)path_resolution(7)user_namespaces(7)