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, 2009-2023 Free Software
       4     Foundation, Inc.
       5  
       6     This file is free software: you can redistribute it and/or modify
       7     it under the terms of the GNU Lesser General Public License as
       8     published by the Free Software Foundation; either version 2.1 of the
       9     License, or (at your option) any later version.
      10  
      11     This file 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 Lesser General Public License for more details.
      15  
      16     You should have received a copy of the GNU Lesser 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 <stdckdint.h>
      25  #include <stdio.h>
      26  #include <sys/types.h>
      27  #include <stdlib.h>
      28  
      29  /* Most processes have no more than this many groups, and for these
      30     processes we can avoid using malloc.  */
      31  enum { GROUPBUF_SIZE = 100 };
      32  
      33  struct group_info
      34    {
      35      gid_t *group;
      36      gid_t groupbuf[GROUPBUF_SIZE];
      37    };
      38  
      39  static void
      40  free_group_info (struct group_info const *g)
      41  {
      42    if (g->group != g->groupbuf)
      43      free (g->group);
      44  }
      45  
      46  static int
      47  get_group_info (struct group_info *gi)
      48  {
      49    int n_groups = getgroups (GROUPBUF_SIZE, gi->groupbuf);
      50    gi->group = gi->groupbuf;
      51  
      52    if (n_groups < 0)
      53      {
      54        int n_group_slots = getgroups (0, NULL);
      55        size_t nbytes;
      56        if (! ckd_mul (&nbytes, n_group_slots, sizeof *gi->group))
      57          {
      58            gi->group = malloc (nbytes);
      59            if (gi->group)
      60              n_groups = getgroups (n_group_slots, gi->group);
      61          }
      62      }
      63  
      64    /* In case of error, the user loses.  */
      65    return n_groups;
      66  }
      67  
      68  /* Return non-zero if GID is one that we have in our groups list.
      69     Note that the groups list is not guaranteed to contain the current
      70     or effective group ID, so they should generally be checked
      71     separately.  */
      72  
      73  int
      74  group_member (gid_t gid)
      75  {
      76    int i;
      77    int found;
      78    struct group_info gi;
      79    int n_groups = get_group_info (&gi);
      80  
      81    /* Search through the list looking for GID. */
      82    found = 0;
      83    for (i = 0; i < n_groups; i++)
      84      {
      85        if (gid == gi.group[i])
      86          {
      87            found = 1;
      88            break;
      89          }
      90      }
      91  
      92    free_group_info (&gi);
      93  
      94    return found;
      95  }
      96  
      97  #ifdef TEST
      98  
      99  int
     100  main (int argc, char **argv)
     101  {
     102    int i;
     103  
     104    for (i = 1; i < argc; i++)
     105      {
     106        gid_t gid;
     107  
     108        gid = atoi (argv[i]);
     109        printf ("%d: %s\n", gid, group_member (gid) ? "yes" : "no");
     110      }
     111    exit (0);
     112  }
     113  
     114  #endif /* TEST */