1 /* group-member.c -- determine whether group id is in calling user's group list
2
3 Copyright (C) 1994, 1997-1998, 2003, 2005-2006, 2009-2021 Free
4 Software Foundation, Inc.
5
6 This program is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
10
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <https://www.gnu.org/licenses/>. */
18
19 #include <config.h>
20
21 /* Specification. */
22 #include <unistd.h>
23
24 #include <stdio.h>
25 #include <sys/types.h>
26 #include <stdlib.h>
27
28 #include "xalloc-oversized.h"
29
30 /* Most processes have no more than this many groups, and for these
31 processes we can avoid using malloc. */
32 enum { GROUPBUF_SIZE = 100 };
33
34 struct group_info
35 {
36 gid_t *group;
37 gid_t groupbuf[GROUPBUF_SIZE];
38 };
39
40 static void
free_group_info(struct group_info const * g)41 free_group_info (struct group_info const *g)
42 {
43 if (g->group != g->groupbuf)
44 free (g->group);
45 }
46
47 static int
get_group_info(struct group_info * gi)48 get_group_info (struct group_info *gi)
49 {
50 int n_groups = getgroups (GROUPBUF_SIZE, gi->groupbuf);
51 gi->group = gi->groupbuf;
52
53 if (n_groups < 0)
54 {
55 int n_group_slots = getgroups (0, NULL);
56 if (0 <= n_group_slots
57 && ! xalloc_oversized (n_group_slots, sizeof *gi->group))
58 {
59 gi->group = malloc (n_group_slots * sizeof *gi->group);
60 if (gi->group)
61 n_groups = getgroups (n_group_slots, gi->group);
62 }
63 }
64
65 /* In case of error, the user loses. */
66 return n_groups;
67 }
68
69 /* Return non-zero if GID is one that we have in our groups list.
70 Note that the groups list is not guaranteed to contain the current
71 or effective group ID, so they should generally be checked
72 separately. */
73
74 int
group_member(gid_t gid)75 group_member (gid_t gid)
76 {
77 int i;
78 int found;
79 struct group_info gi;
80 int n_groups = get_group_info (&gi);
81
82 /* Search through the list looking for GID. */
83 found = 0;
84 for (i = 0; i < n_groups; i++)
85 {
86 if (gid == gi.group[i])
87 {
88 found = 1;
89 break;
90 }
91 }
92
93 free_group_info (&gi);
94
95 return found;
96 }
97
98 #ifdef TEST
99
100 int
main(int argc,char ** argv)101 main (int argc, char **argv)
102 {
103 int i;
104
105 for (i = 1; i < argc; i++)
106 {
107 gid_t gid;
108
109 gid = atoi (argv[i]);
110 printf ("%d: %s\n", gid, group_member (gid) ? "yes" : "no");
111 }
112 exit (0);
113 }
114
115 #endif /* TEST */
116