Section (2) setfsgid


Linux manual pages Section 2  

Name

setfsgid — set group identity used for filesystem checks

Synopsis

#include <sys/fsuid.h>
int setfsgid( uid_t fsgid);
 

DESCRIPTION

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_zsingle_quotesz_s filesystem group ID is the same as the value of its effective group ID. This is so, because whenever a process_zsingle_quotesz_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_zsingle_quotesz_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

The filesystem group ID concept and the setfsgid() system call were invented for historical reasons that are no longer applicable on modern Linux kernels.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_zsingle_quotesz_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

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

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

COLOPHON

This page is part of release 5.04 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 https://www.kernel.org/doc/man−pages/.


  Copyright (C) 1995, Thomas K. Dyas <tdyaseden.rutgers.edu>
and Copyright (C) 2019, Michael Kerrisk <mtk.manpagesgmail.com>

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

Created   1995-08-06 Thomas K. Dyas <tdyaseden.rutgers.edu>
Modified  2000-07-01 aeb
Modified  2002-07-23 aeb
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com>
    Added notes on capability requirements