1 files changed, 115 insertions, 0 deletions
diff --git a/lib/group-member.c b/lib/group-member.c
new file mode 100644
index 0000000..6a6fc56
--- /dev/null
+++ b/lib/group-member.c
@@ -0,0 +1,115 @@
+/* group-member.c -- determine whether group id is in calling user's group list
+
+   Copyright (C) 1994, 1997-1998, 2003, 2005-2006, 2009-2020 Free Software
+   Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <unistd.h>
+
+#include <stdio.h>
+#include <sys/types.h>
+#include <stdlib.h>
+
+#include "xalloc-oversized.h"
+
+/* Most processes have no more than this many groups, and for these
+   processes we can avoid using malloc.  */
+enum { GROUPBUF_SIZE = 100 };
+
+struct group_info
+  {
+    gid_t *group;
+    gid_t groupbuf[GROUPBUF_SIZE];
+  };
+
+static void
+free_group_info (struct group_info const *g)
+{
+  if (g->group != g->groupbuf)
+    free (g->group);
+}
+
+static int
+get_group_info (struct group_info *gi)
+{
+  int n_groups = getgroups (GROUPBUF_SIZE, gi->groupbuf);
+  gi->group = gi->groupbuf;
+
+  if (n_groups < 0)
+    {
+      int n_group_slots = getgroups (0, NULL);
+      if (0 <= n_group_slots
+          && ! xalloc_oversized (n_group_slots, sizeof *gi->group))
+        {
+          gi->group = malloc (n_group_slots * sizeof *gi->group);
+          if (gi->group)
+            n_groups = getgroups (n_group_slots, gi->group);
+        }
+    }
+
+  /* In case of error, the user loses.  */
+  return n_groups;
+}
+
+/* Return non-zero if GID is one that we have in our groups list.
+   Note that the groups list is not guaranteed to contain the current
+   or effective group ID, so they should generally be checked
+   separately.  */
+
+int
+group_member (gid_t gid)
+{
+  int i;
+  int found;
+  struct group_info gi;
+  int n_groups = get_group_info (&gi);
+
+  /* Search through the list looking for GID. */
+  found = 0;
+  for (i = 0; i < n_groups; i++)
+    {
+      if (gid == gi.group[i])
+        {
+          found = 1;
+          break;
+        }
+    }
+
+  free_group_info (&gi);
+
+  return found;
+}
+
+#ifdef TEST
+
+int
+main (int argc, char **argv)
+{
+  int i;
+
+  for (i = 1; i < argc; i++)
+    {
+      gid_t gid;
+
+      gid = atoi (argv[i]);
+      printf ("%d: %s\n", gid, group_member (gid) ? "yes" : "no");
+    }
+  exit (0);
+}
+
+#endif /* TEST */