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



NAME
       setfsgid - set group identity used for filesystem checks

SYNOPSIS
       #include <sys/fsuid.h>

       int setfsgid(uid_t fsgid);

DESCRIPTION
       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  with‐
       out  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
       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
       When glibc determines that the argument is not a valid group ID, it will return -1 and set
       errno to EINVAL without attempting the system call.

       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 setfs‐
       gid() 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.

BUGS
       No error indications of any kind are returned to the caller, and the fact that  both  suc‐
       cessful  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  look‐
       ing 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
       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2013-08-08                                SETFSGID(2)