resolv: Add the __ns_samebinaryname function

During packet parsing, only the binary name is available.  If the name
equality check is performed before conversion to text, we can sometimes
skip the last step.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

resolv/ns_samebinaryname.c

@@ -0,0 +1,55 @@
+/* Compare two binary domain names for quality.
+   Copyright (C) 2022 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <arpa/nameser.h>
+#include <stdbool.h>
+
+/* Convert ASCII letters to upper case.  */
+static inline int
+ascii_toupper (unsigned char ch)
+{
+  if (ch >= 'a' && ch <= 'z')
+    return ch - 'a' + 'A';
+  else
+    return ch;
+}
+
+bool
+__ns_samebinaryname (const unsigned char *a, const unsigned char *b)
+{
+  while (*a != 0 && *b != 0)
+    {
+      if (*a != *b)
+        /* Different label length.  */
+        return false;
+      int labellen = *a;
+      ++a;
+      ++b;
+      for (int i = 0; i < labellen; ++i)
+        {
+          if (*a != *b && ascii_toupper (*a) != ascii_toupper (*b))
+            /* Different character in label.  */
+            return false;
+          ++a;
+          ++b;
+        }
+    }
+
+  /* Match if both names are at the root label.  */
+  return *a == 0 && *b == 0;
+}