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

.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\"
.\" %%%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.\" manual page may be incorrect or out-of-date.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 <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),
it 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)
Commit	Line	Data
	1	.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
	2	.\"
	3	.\" %%%LICENSE_START(VERBATIM)
	4	.\" Permission is granted to make and distribute verbatim copies of this
	5	.\" manual provided the copyright notice and this permission notice are
	6	.\" preserved on all copies.
	7	.\"
	8	.\" Permission is granted to copy and distribute modified versions of this
	9	.\" manual under the conditions for verbatim copying, provided that the
	10	.\" entire resulting derived work is distributed under the terms of a
	11	.\" permission notice identical to this one.
	12	.\"
	13	.\" Since the Linux kernel and libraries are constantly changing, this
	14	.\" manual page may be incorrect or out-of-date. The author(s) assume no
	15	.\" responsibility for errors or omissions, or for damages resulting from
	16	.\" the use of the information contained herein. The author(s) may not
	17	.\" have taken the same level of care in the production of this manual,
	18	.\" which is licensed free of charge, as they might when working
	19	.\" professionally.
	20	.\"
	21	.\" Formatted or processed versions of this manual, if unaccompanied by
	22	.\" the source, must acknowledge the copyright and authors of this work.
	23	.\" %%%LICENSE_END
	24	.\"
	25	.\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
	26	.\" Modified 2000-07-01 aeb
	27	.\" Modified 2002-07-23 aeb
	28	.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
	29	.\" Added notes on capability requirements
	30	.\"
	31	.TH SETFSGID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
	32	.SH NAME
	33	setfsgid \- set group identity used for filesystem checks
	34	.SH SYNOPSIS
	35	.B #include <sys/fsuid.h>
	36	.PP
	37	.BI "int setfsgid(uid_t " fsgid );
	38	.SH DESCRIPTION
	39	The system call
	40	.BR setfsgid ()
	41	changes the value of the caller's filesystem group ID\(emthe
	42	group ID that the Linux kernel uses to check for all accesses
	43	to the filesystem.
	44	Normally, the value of
	45	the filesystem group ID
	46	will shadow the value of the effective group ID.
	47	In fact, whenever the
	48	effective group ID is changed,
	49	the filesystem group ID
	50	will also be changed to the new value of the effective group ID.
	51	.PP
	52	Explicit calls to
	53	.BR setfsuid (2)
	54	and
	55	.BR setfsgid ()
	56	are usually used only by programs such as the Linux NFS server that
	57	need to change what user and group ID is used for file access without a
	58	corresponding change in the real and effective user and group IDs.
	59	A change in the normal user IDs for a program such as the NFS server
	60	is a security hole that can expose it to unwanted signals.
	61	(But see below.)
	62	.PP
	63	.BR setfsgid ()
	64	will succeed only if the caller is the superuser or if
	65	.I fsgid
	66	matches either the caller's real group ID, effective group ID,
	67	saved set-group-ID, or current the filesystem user ID.
	68	.SH RETURN VALUE
	69	On both success and failure,
	70	this call returns the previous filesystem group ID of the caller.
	71	.SH VERSIONS
	72	This system call is present in Linux since version 1.2.
	73	.\" This system call is present since Linux 1.1.44
	74	.\" and in libc since libc 4.7.6.
	75	.SH CONFORMING TO
	76	.BR setfsgid ()
	77	is Linux-specific and should not be used in programs intended
	78	to be portable.
	79	.SH NOTES
	80	Note that at the time this system call was introduced, a process
	81	could send a signal to a process with the same effective user ID.
	82	Today signal permission handling is slightly different.
	83	See
	84	.BR setfsuid (2)
	85	for a discussion of why the use of both
	86	.BR setfsuid (2)
	87	and
	88	.BR setfsgid ()
	89	is nowadays unneeded.
	90	.PP
	91	The original Linux
	92	.BR setfsgid ()
	93	system call supported only 16-bit group IDs.
	94	Subsequently, Linux 2.4 added
	95	.BR setfsgid32 ()
	96	supporting 32-bit IDs.
	97	The glibc
	98	.BR setfsgid ()
	99	wrapper function transparently deals with the variation across kernel versions.
	100	.SS C library/kernel differences
	101	In glibc 2.15 and earlier,
	102	when the wrapper for this system call determines that the argument can't be
	103	passed to the kernel without integer truncation (because the kernel
	104	is old and does not support 32-bit group IDs),
	105	it will return \-1 and set \fIerrno\fP to
	106	.B EINVAL
	107	without attempting
	108	the system call.
	109	.SH BUGS
	110	No error indications of any kind are returned to the caller,
	111	and the fact that both successful and unsuccessful calls return
	112	the same value makes it impossible to directly determine
	113	whether the call succeeded or failed.
	114	Instead, the caller must resort to looking at the return value
	115	from a further call such as
	116	.IR setfsgid(\-1)
	117	(which will always fail), in order to determine if a preceding call to
	118	.BR setfsgid ()
	119	changed the filesystem group ID.
	120	At the very
	121	least,
	122	.B EPERM
	123	should be returned when the call fails (because the caller lacks the
	124	.B CAP_SETGID
	125	capability).
	126	.SH SEE ALSO
	127	.BR kill (2),
	128	.BR setfsuid (2),
	129	.BR capabilities (7),
	130	.BR credentials (7)