setfsgid.2, setfsuid.2: Fix note about errors from the syscall wrapper

[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 2013-08-08 "Linux" "Linux Programmer's Manual"
.SH NAME
setfsgid \- set group identity used for filesystem checks
.SH SYNOPSIS
.B #include <sys/fsuid.h>
.sp
.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.

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.)

.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
When old versions of glibc determine that the argument can not 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.
.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.
See
.BR setfsuid (2)
for a discussion of why the use of both
.BR setfsuid (2)
and
.BR setfsgid ()
is nowadays unneeded.

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.
.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)