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utilities is done by calling gr_addgid() for each group to be
added (usually found by traversing /etc/group) then calling the
setgroups() system call after the group set has been created.
The gr_addgid() function (helpfully?) deduplicates the addition
of group members. So, if you call it to add a group member that
already exists, it is just dropped. Because group[0] is the
effective group-ID and is over-written when a setgid program
is run, The value in group[0] is usually duplicated so that
group value is not lost when a setgid program is run.
Historically this happened because the group value indicated
in the password file also appears in /etc/group (e.g., if you
are group staff in the password file, you will also appear in
the staff line in /etc/group). But, with the addition of the
deduplication, the attempt to add group staff was lost because
it already appeared in group[0]. So, the fix is to deduplicate
starting from group[1] which allows a duplicate of the entry in
group[0], but not in later entries.
There is some confusion about the setgroups system call because in
BSD it has (always) set the entire group including the egid group
(in group[0]). However, in Linux, it skips over group[0] and starts
setting from group[1]. See this comment from linux_setgroups:
/*
* cr_groups[0] holds egid. Setting the whole set from
* the supplied set will cause egid to be changed too.
* Keep cr_groups[0] unchanged to prevent that.
*/
To make it clear what the BSD setgroups system call does, I
added the following paragraph to the setgroups(2) manual page:
The first entry of the group array (gidset[0]) is used as the effective
group-ID for the process. This entry is over-written when a setgid
program is run. To avoid losing access to the privileges of the
gidset[0] entry, it should be duplicated later in the group array.
By convention, this happens because the group value indicated in the
password file also appears in /etc/group. The group value in the
password file is placed in gidset[0] and that value then gets added a
second time when the /etc/group file is scanned to create the group set.
Reported by: Paul McMath paulm at tetrardus.net
Reviewed by: kib
MFC after: 2 weeks
104 lines
3.2 KiB
Groff
104 lines
3.2 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)setgroups.2 8.2 (Berkeley) 4/16/94
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.\" $FreeBSD$
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.\"
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.Dd April 16, 1994
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.Dt SETGROUPS 2
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.Os
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.Sh NAME
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.Nm setgroups
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.Nd set group access list
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/param.h
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.In unistd.h
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.Ft int
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.Fn setgroups "int ngroups" "const gid_t *gidset"
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.Sh DESCRIPTION
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The
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.Fn setgroups
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system call
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sets the group access list of the current user process
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according to the array
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.Fa gidset .
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The
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.Fa ngroups
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argument
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indicates the number of entries in the array and must be no
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more than
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.Dv {NGROUPS_MAX}+1 .
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.Pp
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Only the super-user may set a new group list.
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.Pp
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The first entry of the group array
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.Pq Va gidset[0]
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is used as the effective group-ID for the process.
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This entry is over-written when a setgid program is run.
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To avoid losing access to the privileges of the
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.Va gidset[0]
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entry, it should be duplicated later in the group array.
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By convention,
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this happens because the group value indicated
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in the password file also appears in
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.Pa /etc/group .
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The group value in the password file is placed in
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.Va gidset[0]
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and that value then gets added a second time when the
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.Pa /etc/group
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file is scanned to create the group set.
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.Sh RETURN VALUES
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.Rv -std setgroups
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.Sh ERRORS
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The
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.Fn setgroups
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system call will fail if:
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.Bl -tag -width Er
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.It Bq Er EPERM
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The caller is not the super-user.
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.It Bq Er EINVAL
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The number specified in the
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.Fa ngroups
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argument is larger than the
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.Dv {NGROUPS_MAX}+1
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limit.
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.It Bq Er EFAULT
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The address specified for
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.Fa gidset
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is outside the process
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address space.
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.El
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.Sh SEE ALSO
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.Xr getgroups 2 ,
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.Xr initgroups 3
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.Sh HISTORY
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The
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.Fn setgroups
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system call appeared in
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.Bx 4.2 .
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