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>

include/arpa/nameser.h

@@ -55,6 +55,12 @@ int __ns_name_ntop (const unsigned char *, char *, size_t) __THROW;
 int __ns_name_unpack (const unsigned char *, const unsigned char *,
 		      const unsigned char *, unsigned char *, size_t) __THROW;
 
+/* Like ns_samename, but for uncompressed binary names.  Return true
+   if the two arguments compare are equal as case-insensitive domain
+   names.  */
+_Bool __ns_samebinaryname (const unsigned char *, const unsigned char *)
+  attribute_hidden;
+
 #define ns_msg_getflag(handle, flag) \
   (((handle)._flags & _ns_flagdata[flag].mask) >> _ns_flagdata[flag].shift)
 

resolv/Makefile

@@ -46,6 +46,7 @@ routines := \
   ns_name_skip \
   ns_name_uncompress \
   ns_name_unpack \
+  ns_samebinaryname \
   ns_samename \
   nsap_addr \
   nss_dns_functions \
@@ -106,6 +107,10 @@ tests += \
 tests-internal += tst-resolv-txnid-collision
 tests-static += tst-resolv-txnid-collision
 
+# Likewise for __ns_samebinaryname.
+tests-internal += tst-ns_samebinaryname
+tests-static += tst-ns_samebinaryname
+
 # These tests need libdl.
 ifeq (yes,$(build-shared))
 tests += \

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;
+}

resolv/tst-ns_samebinaryname.c

@@ -0,0 +1,62 @@
+/* Test the __ns_samebinaryname function.
+   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 <array_length.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <support/check.h>
+
+/* First character denotes the comparison group: All names with the
+   same first character are expected to compare equal.  */
+static const char *const cases[] =
+  {
+    " ",
+    "1\001a", "1\001A",
+    "2\002ab", "2\002aB", "2\002Ab", "2\002AB",
+    "3\001a\002ab", "3\001A\002ab",
+    "w\003www\007example\003com", "w\003Www\007Example\003Com",
+    "w\003WWW\007EXAMPLE\003COM",
+    "W\003WWW", "W\003www",
+  };
+
+static int
+do_test (void)
+{
+  for (int i = 0; i < array_length (cases); ++i)
+    for (int j = 0; j < array_length (cases); ++j)
+      {
+        unsigned char *a = (unsigned char *) &cases[i][1];
+        unsigned char *b = (unsigned char *) &cases[j][1];
+        bool actual = __ns_samebinaryname (a, b);
+        bool expected = cases[i][0] == cases[j][0];
+        if (actual != expected)
+          {
+            char a1[NS_MAXDNAME];
+            TEST_VERIFY (ns_name_ntop (a, a1, sizeof (a1)) > 0);
+            char b1[NS_MAXDNAME];
+            TEST_VERIFY (ns_name_ntop (b, b1, sizeof (b1)) > 0);
+            printf ("error: \"%s\" \"%s\": expected %s\n",
+                    a1, b1, expected ? "equal" : "unqueal");
+            support_record_failure ();
+          }
+      }
+  return 0;
+}
+
+#include <support/test-driver.c>