resolv: Add internal __ns_name_length_uncompressed function

This function is useful for checking that the question name is
uncompressed (as it should be).

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 78b1a4f0e49064e5dfb686c7cd87bd4df2640b29)

include/arpa/nameser.h

@@ -95,5 +95,13 @@ libc_hidden_proto (__ns_name_unpack)
 extern __typeof (ns_samename) __libc_ns_samename;
 libc_hidden_proto (__libc_ns_samename)
 
+/* Packet parser helper functions.  */
+
+/* Verify that P points to an uncompressed domain name in wire format.
+   On success, return the length of the encoded name, including the
+   terminating null byte.  On failure, return -1 and set errno.  EOM
+   must point one past the last byte in the packet.  */
+int __ns_name_length_uncompressed (const unsigned char *p,
+				   const unsigned char *eom) attribute_hidden;
 # endif /* !_ISOMAC */
 #endif

resolv/Makefile

@@ -40,6 +40,7 @@ routines := \
   inet_pton \
   ns_makecanon \
   ns_name_compress \
+  ns_name_length_uncompressed \
   ns_name_ntop \
   ns_name_pack \
   ns_name_pton \
@@ -111,6 +112,10 @@ tests-static += tst-resolv-txnid-collision
 tests-internal += tst-ns_samebinaryname
 tests-static += tst-ns_samebinaryname
 
+# Likewise for __ns_name_length_uncompressed.
+tests-internal += tst-ns_name_length_uncompressed
+tests-static += tst-ns_name_length_uncompressed
+
 # These tests need libdl.
 ifeq (yes,$(build-shared))
 tests += \

resolv/ns_name_length_uncompressed.c

@@ -0,0 +1,72 @@
+/* Skip over an uncompressed name in wire format.
+   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 <errno.h>
+#include <stdbool.h>
+
+int
+__ns_name_length_uncompressed (const unsigned char *p,
+                                const unsigned char *eom)
+{
+  const unsigned char *start = p;
+
+  while (true)
+    {
+      if (p == eom)
+        {
+          /* Truncated packet: no room for label length.  */
+          __set_errno (EMSGSIZE);
+          return -1;
+        }
+
+      unsigned char b = *p;
+      ++p;
+      if (b == 0)
+        {
+          /* Root label.  */
+          size_t length = p - start;
+          if (length > NS_MAXCDNAME)
+            {
+              /* Domain name too long.  */
+              __set_errno (EMSGSIZE);
+              return -1;
+            }
+          return length;
+        }
+
+      if (b <= 63)
+        {
+          /* Regular label.  */
+          if (b <= eom - p)
+            p += b;
+          else
+            {
+              /* Truncated packet: label incomplete.  */
+              __set_errno (EMSGSIZE);
+              return -1;
+            }
+        }
+      else
+        {
+          /* Compression reference or corrupted label length.  */
+          __set_errno (EMSGSIZE);
+          return -1;
+        }
+    }
+}

resolv/tst-ns_name_length_uncompressed.c

@@ -0,0 +1,135 @@
+/* Test __ns_name_length_uncompressed.
+   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 <errno.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/next_to_fault.h>
+
+/* Reference implementation based on other building blocks.  */
+static int
+reference_length (const unsigned char *p, const unsigned char *eom)
+{
+  unsigned char buf[NS_MAXCDNAME];
+  int n = __ns_name_unpack (p, eom, p, buf, sizeof (buf));
+  if (n < 0)
+    return n;
+  const unsigned char *q = buf;
+  if (__ns_name_skip (&q, array_end (buf)) < 0)
+    return -1;
+  if (q - buf != n)
+    /* Compressed name.  */
+    return -1;
+  return n;
+}
+
+static int
+do_test (void)
+{
+  {
+    unsigned char buf[] = { 3, 'w', 'w', 'w', 0, 0, 0 };
+    TEST_COMPARE (reference_length (buf, array_end (buf)), sizeof (buf) - 2);
+    TEST_COMPARE (__ns_name_length_uncompressed (buf, array_end (buf)),
+                  sizeof (buf) - 2);
+    TEST_COMPARE (reference_length (array_end (buf) - 1, array_end (buf)), 1);
+    TEST_COMPARE (__ns_name_length_uncompressed (array_end (buf) - 1,
+                                                 array_end (buf)), 1);
+    buf[4]  = 0xc0;             /* Forward compression reference.  */
+    buf[5]  = 0x06;
+    TEST_COMPARE (reference_length (buf, array_end (buf)), -1);
+    TEST_COMPARE (__ns_name_length_uncompressed (buf, array_end (buf)), -1);
+  }
+
+  struct support_next_to_fault ntf = support_next_to_fault_allocate (300);
+
+  /* Buffer region with all possible bytes at start and end.  */
+  for (int length = 1; length <= 300; ++length)
+    {
+      unsigned char *end = (unsigned char *) ntf.buffer + ntf.length;
+      unsigned char *start = end - length;
+      memset (start, 'X', length);
+      for (int first = 0; first <= 255; ++first)
+        {
+          *start = first;
+          for (int last = 0; last <= 255; ++last)
+            {
+              start[length - 1] = last;
+              TEST_COMPARE (reference_length (start, end),
+                            __ns_name_length_uncompressed (start, end));
+            }
+        }
+    }
+
+  /* Poor man's fuzz testing: patch two bytes.   */
+  {
+    unsigned char ref[] =
+      {
+        7, 'e', 'x', 'a', 'm', 'p', 'l', 'e', 3, 'n', 'e', 't', 0, 0, 0
+      };
+    TEST_COMPARE (reference_length (ref, array_end (ref)), 13);
+    TEST_COMPARE (__ns_name_length_uncompressed (ref, array_end (ref)), 13);
+
+    int good = 0;
+    int bad = 0;
+    for (int length = 1; length <= sizeof (ref); ++length)
+      {
+        unsigned char *end = (unsigned char *) ntf.buffer + ntf.length;
+        unsigned char *start = end - length;
+        memcpy (start, ref, length);
+
+        for (int patch1_pos = 0; patch1_pos < length; ++patch1_pos)
+          {
+            for (int patch1_value = 0; patch1_value <= 255; ++patch1_value)
+              {
+                start[patch1_pos] = patch1_value;
+                for (int patch2_pos = 0; patch2_pos < length; ++patch2_pos)
+                  {
+                    for (int patch2_value = 0; patch2_value <= 255;
+                         ++patch2_value)
+                      {
+                        start[patch2_pos] = patch2_value;
+                        int expected = reference_length (start, end);
+                        errno = EINVAL;
+                        int actual
+                          =  __ns_name_length_uncompressed (start, end);
+                        if (actual > 0)
+                          ++good;
+                        else
+                          {
+                            TEST_COMPARE (errno, EMSGSIZE);
+                            ++bad;
+                          }
+                        TEST_COMPARE (expected, actual);
+                      }
+                    start[patch2_pos] = ref[patch2_pos];
+                  }
+              }
+            start[patch1_pos] = ref[patch1_pos];
+          }
+      }
+    printf ("info: patched inputs with success: %d\n", good);
+    printf ("info: patched inputs with failure: %d\n", bad);
+  }
+
+  support_next_to_fault_free (&ntf);
+  return 0;
+}
+
+#include <support/test-driver.c>