lib/group-member.c

   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, 2009-2017 Free Software
   4    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
  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
  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
  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
 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 */