setfsgid32(2) — Linux manual page


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

       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         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

       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       invented for historical reasons that are no longer applicable on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         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 5.09 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

Linux                            2019-05-09                      SETFSGID(2)

Pages that refer to this page: syscalls(2)