lib/mgetgroups.c

/* mgetgroups.c -- return a list of the groups a user or current process is in

   Copyright (C) 2007-2026 Free Software Foundation, Inc.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* Extracted from coreutils' src/id.c. */

#include <config.h>

#include "mgetgroups.h"

#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
#if HAVE_GETGROUPLIST
# include <grp.h>
#endif

#include "getugroups.h"
#include "xalloc-oversized.h"

/* Work around an incompatibility of OS X 10.11: getgrouplist
   accepts int *, not gid_t *, and int and gid_t differ in sign.  */
#if 4 < __GNUC__ + (3 <= __GNUC_MINOR__) || defined __clang__
# pragma GCC diagnostic ignored "-Wpointer-sign"
#endif

static gid_t *
realloc_groupbuf (gid_t *g, size_t num)
{
  if (xalloc_oversized (num, sizeof *g))
    {
      errno = ENOMEM;
      return NULL;
    }

  return realloc (g, num * sizeof *g);
}

/* Like getugroups, but store the result in malloc'd storage.
   Set *GROUPS to the malloc'd list of all group IDs of which USERNAME
   is a member.  If GID is not -1, store it first.  GID should be the
   group ID (pw_gid) obtained from getpwuid, in case USERNAME is not
   listed in the groups database (e.g., /etc/groups).  If USERNAME is
   NULL, store the supplementary groups of the current process, and GID
   should be -1 or the effective group ID (getegid).  Upon failure,
   don't modify *GROUPS, set errno, and return -1.  Otherwise, return
   the number of groups.  The resulting list may contain duplicates,
   but adjacent members will be distinct.  */

int
mgetgroups (char const *username, gid_t gid, gid_t **groups)
{
#if HAVE_GETGROUPLIST
  /* We prefer to use getgrouplist if available, because it has better
     performance characteristics.

     In glibc 2.3.2, getgrouplist is buggy.  If you pass a zero as the
     length of the output buffer, getgrouplist will still write to the
     buffer.  Contrary to what some versions of the getgrouplist
     manpage say, this doesn't happen with nonzero buffer sizes.
     Therefore our usage here just avoids a zero sized buffer.  */
  if (username)
    {
      enum { N_GROUPS_INIT = 10 };
      int max_n_groups = N_GROUPS_INIT;

      gid_t *g = realloc_groupbuf (NULL, max_n_groups);
      if (g == NULL)
        return -1;

      while (1)
        {
          int last_n_groups = max_n_groups;

          /* getgrouplist updates max_n_groups to num required.  */
          int ng = getgrouplist (username, gid, g, &max_n_groups);

          /* Some systems (like Darwin) have a bug where they
             never increase max_n_groups.  */
          if (ng < 0 && last_n_groups == max_n_groups)
            max_n_groups *= 2;

          gid_t *h = realloc_groupbuf (g, max_n_groups);
          if (h == NULL)
            {
              free (g);
              return -1;
            }
          g = h;

          if (0 <= ng)
            {
              *groups = g;
              /* On success some systems just return 0 from getgrouplist,
                 so return max_n_groups rather than ng.  */
              return max_n_groups;
            }
        }
    }
  /* else no username, so fall through and use getgroups. */
#endif

  int max_n_groups = (username
                      ? getugroups (0, NULL, username, gid)
                      : getgroups (0, NULL));

  /* If we failed to count groups because there is no supplemental
     group support, then return an array containing just GID.
     Otherwise, we fail for the same reason.  */
  if (max_n_groups < 0)
    {
      if (errno == ENOSYS)
        {
          gid_t *g = realloc_groupbuf (NULL, 1);
          if (g)
            {
              *groups = g;
              *g = gid;
              return gid != (gid_t) -1;
            }
        }
      return -1;
    }

  if (max_n_groups == 0 || (!username && gid != (gid_t) -1))
    max_n_groups++;
  gid_t *g = realloc_groupbuf (NULL, max_n_groups);
  if (g == NULL)
    return -1;

  int ng = (username
            ? getugroups (max_n_groups, g, username, gid)
            : getgroups (max_n_groups - (gid != (gid_t) -1),
                                    g + (gid != (gid_t) -1)));

  if (ng < 0)
    {
      /* Failure is unexpected, but handle it anyway.  */
      free (g);
      return -1;
    }

  if (!username && gid != (gid_t) -1)
    {
      *g = gid;
      ng++;
    }
  *groups = g;

  /* Reduce the number of duplicates.  On some systems, getgroups
     returns the effective gid twice: once as the first element, and
     once in its position within the supplementary groups.  On other
     systems, getgroups does not return the effective gid at all,
     which is why we provide a GID argument.  Meanwhile, the GID
     argument, if provided, is typically any member of the
     supplementary groups, and not necessarily the effective gid.  So,
     the most likely duplicates are the first element with an
     arbitrary other element, or pair-wise duplication between the
     first and second elements returned by getgroups.  It is possible
     that this O(n) pass will not remove all duplicates, but it is not
     worth the effort to slow down to an O(n log n) algorithm that
     sorts the array in place, nor the extra memory needed for
     duplicate removal via an O(n) hash-table.  Hence, this function
     is only documented as guaranteeing no pair-wise duplicates,
     rather than returning the minimal set.  */
  if (1 < ng)
    {
      gid_t first = *g;
      gid_t *groups_end = g + ng;

      for (gid_t *next = g + 1; next < groups_end; next++)
        {
          if (*next == first || *next == *g)
            ng--;
          else
            *++g = *next;
        }
    }

  return ng;
}