resolv: Support an exactly sized buffer in ns_name_pack [BZ #21359]

This bug did not affect name resolution because those functions
indirectly call ns_name_pack with a buffer which is always larger
than the generated query packet, even in the case of the
longest-possible domain name.

resolv/tst-ns_name_compress.c

@@ -0,0 +1,75 @@
+/* Test ns_name_compress corner cases.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library 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.
+
+   The GNU C Library 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 the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <resolv.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/support.h>
+
+/* Check that we can process names which fit into the destination
+   buffer exactly.  See bug 21359.  */
+static void
+test_exact_fit (const char *name, size_t length)
+{
+  unsigned char *buf = xmalloc (length + 1);
+  memset (buf, '$', length + 1);
+  enum { ptr_count = 5 };
+  const unsigned char *dnptrs[ptr_count] = { buf, };
+  int ret = ns_name_compress (name, buf, length,
+                          dnptrs, dnptrs + ptr_count);
+  if (ret < 0)
+    {
+      support_record_failure ();
+      printf ("error: ns_name_compress for %s/%zu failed\n", name, length);
+      return;
+    }
+  if ((size_t) ret != length)
+    {
+      support_record_failure ();
+      printf ("error: ns_name_compress for %s/%zu result mismatch: %d\n",
+              name, length, ret);
+    }
+  if (buf[length] != '$')
+    {
+      support_record_failure ();
+      printf ("error: ns_name_compress for %s/%zu padding write\n",
+              name, length);
+    }
+  free (buf);
+}
+
+static int
+do_test (void)
+{
+  test_exact_fit ("abc", 5);
+  test_exact_fit ("abc.", 5);
+  {
+    char long_name[]
+      = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa."
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa."
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa."
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.";
+    TEST_VERIFY (strlen (long_name) == NS_MAXCDNAME - 1);
+    test_exact_fit (long_name, NS_MAXCDNAME);
+    long_name[sizeof (long_name) - 1] = '\0';
+    test_exact_fit (long_name, NS_MAXCDNAME);
+  }
+  return 0;
+}
+
+#include <support/test-driver.c>