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.\" Created   1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>

.\" Modified  2000-07-01 aeb

.\" Modified  2002-07-23 aeb

.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added notes on capability requirements

.\"

.TH SETFSGID 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

setfsgid \- set group identity used for filesystem checks

.SH SYNOPSIS

.B #include <sys/fsuid.h>

.PP

.BI "int setfsgid(uid_t " fsgid );

.SH DESCRIPTION

The system call

.BR setfsgid ()

changes the value of the caller's filesystem group ID\(emthe

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.

.PP

Explicit calls to

.BR setfsuid (2)

and

.BR 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.)

.PP

.BR setfsgid ()

will succeed only if the caller is the superuser or if

.I fsgid

matches either the caller's real group ID, effective group ID,

saved set-group-ID, or current the filesystem user ID.

.SH RETURN VALUE

On both success and failure,

this call returns the previous filesystem group ID of the caller.

.SH VERSIONS

This system call is present in Linux since version 1.2.

.\" This system call is present since Linux 1.1.44

.\" and in libc since libc 4.7.6.

.SH CONFORMING TO

.BR setfsgid ()

is Linux-specific and should not be used in programs intended

to be portable.

.SH NOTES

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

.BR setfsuid (2)

for a discussion of why the use of both

.BR setfsuid (2)

and

.BR setfsgid ()

is nowadays unneeded.

.PP

The original Linux

.BR setfsgid ()

system call supported only 16-bit group IDs.

Subsequently, Linux 2.4 added

.BR setfsgid32 ()

supporting 32-bit IDs.

The glibc

.BR setfsgid ()

wrapper function transparently deals with the variation across kernel versions.

.SS 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),

they will return \-1 and set \fIerrno\fP to

.B EINVAL

without attempting

the system call.

.SH 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

.IR setfsgid(\-1)

(which will always fail), in order to determine if a preceding call to

.BR setfsgid ()

changed the filesystem group ID.

At the very

least,

.B EPERM

should be returned when the call fails (because the caller lacks the

.B CAP_SETGID

capability).

.SH SEE ALSO

.BR kill (2),

.BR setfsuid (2),

.BR capabilities (7),

.BR credentials (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.