setfsgid(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(gid_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       were invented for historical reasons that are no longer
       applicable on modern Linux kernels.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 capability).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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                          2021-03-22                    SETFSGID(2)

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