userns: add argument sanity checking

In find_new_sub_{u,g}ids, check for min, count and max values.

In idmapping.c:get_map_ranges(), make sure that the value passed
in for ranges did not overflow.  Couldn't happen with the current
code, but this is a sanity check for any future potential mis-uses.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

libmisc/find_new_sub_uids.c

@@ -58,6 +58,14 @@ int find_new_sub_uids (const char *owner,
 	max = getdef_ulong ("SUB_UID_MAX", 600100000UL);
 	count = getdef_ulong ("SUB_UID_COUNT", 10000);
 
+	if (min >= max || count >= max || (min + count) >= max) {
+		(void) fprintf (stderr,
+				_("%s: Invalid configuration: SUB_UID_MIN (%lu),"
+				  " SUB_UID_MAX (%lu), SUB_UID_COUNT (%lu)\n"),
+			Prog, min, max, count);
+		return -1;
+	}
+
 	/* Is there a preferred range that works? */
 	if ((*range_count != 0) &&
 	    (*range_start >= min) &&