select.2: Rewrote text describing feature test macro requirements for pselect()

[thirdparty/man-pages.git] / man2 / setfsgid.2

.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
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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 2004-05-27 "Linux" "Linux Programmer's Manual"
.SH NAME
setfsgid \- set group identity used for file system checks
.SH SYNOPSIS
.B #include <unistd.h>
/* glibc uses <sys/fsuid.h> */
.sp
.BI "int setfsgid(uid_t " fsgid );
.SH DESCRIPTION
The system call
.BR setfsgid ()
sets the group ID that the Linux kernel uses to check for all accesses
to the file system.
Normally, the value of
.I fsgid
will shadow the value of the effective group ID.
In fact, whenever the
effective group ID is changed,
.I fsgid
will also be changed to the new value of the effective group ID.

Explicit calls to
.BR setfsuid (2)
and
.BR setfsgid ()
are usually only used 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.)

.BR setfsgid ()
will only succeed if the caller is the superuser or if
.I fsgid
matches either the real group ID, effective group ID,
saved set-group-ID, or the current value of
.IR fsgid .
.SH "RETURN VALUE"
On success, the previous value of
.I fsgid
is returned.
On error, the current value of
.I fsgid
is returned.
.SH "CONFORMING TO"
.BR setfsgid ()
is Linux-specific and should not be used in programs intended
to be portable.
It is present since Linux 1.1.44 and in libc since libc 4.7.6.
.SH NOTES
When glibc determines that the argument is not a valid group ID,
it will return \-1 and set \fIerrno\fP to
.B EINVAL
without attempting
the system call.
.LP
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.
.SH BUGS
No error messages of any kind are returned to the caller.
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)