While uid_t and gid_t are both unsigned, the values (uid_t) -1 and
(gid_t) -1 are reserved.  A uid or gid argument of -1 to the chown(2)
system call means to leave the uid/gid unchanged.  Catch this case
so that trying to set a uid or gid to -1 will result in an error.

Test cases:

  chown 4294967295 file
  chown :4294967295 file
  chgrp 4294967295 file

Andreas Gruenbacher <agruen@suse.de>

Index: lib/userspec.c
===================================================================
--- lib/userspec.c.orig	2010-05-04 17:27:48.479359439 +0200
+++ lib/userspec.c	2010-05-04 17:29:12.439359267 +0200
@@ -169,7 +169,7 @@ parse_with_separator (char const *spec,
 	    {
 	      unsigned long int tmp;
 	      if (xstrtoul (u, NULL, 10, &tmp, "") == LONGINT_OK
-		  && tmp <= MAXUID)
+		  && tmp <= MAXUID && tmp != (uid_t) -1)
 		unum = tmp;
 	      else
 		error_msg = E_invalid_user;
@@ -200,7 +200,8 @@ parse_with_separator (char const *spec,
       if (grp == NULL)
 	{
 	  unsigned long int tmp;
-	  if (xstrtoul (g, NULL, 10, &tmp, "") == LONGINT_OK && tmp <= MAXGID)
+	  if (xstrtoul (g, NULL, 10, &tmp, "") == LONGINT_OK && tmp <= MAXGID
+	      && tmp != (gid_t) -1)
 	    gnum = tmp;
 	  else
 	    error_msg = E_invalid_group;
Index: src/chgrp.c
===================================================================
--- src/chgrp.c.orig	2010-05-04 17:27:48.479359439 +0200
+++ src/chgrp.c	2010-05-04 17:29:12.443359269 +0200
@@ -89,7 +89,7 @@ parse_group (const char *name)
 	{
 	  unsigned long int tmp;
 	  if (! (xstrtoul (name, NULL, 10, &tmp, "") == LONGINT_OK
-		 && tmp <= GID_T_MAX))
+		 && tmp <= GID_T_MAX && tmp != (gid_t) -1))
 	    error (EXIT_FAILURE, 0, _("invalid group: %s"), quote (name));
 	  gid = tmp;
 	}