resolv: Add DNS packet parsing helpers geared towards wire format

The public parser functions around the ns_rr record type produce
textual domain names, but usually, this is not what we need while
parsing DNS packets within glibc.  This commit adds two new helper
functions, __ns_rr_cursor_init and __ns_rr_cursor_next, for writing
packet parsers, and struct ns_rr_cursor, struct ns_rr_wire as
supporting types.

In theory, it is possible to avoid copying the owner name
into the rname field in __ns_rr_cursor_next, but this would need
more functions that work on compressed names.

Eventually, __res_context_send could be enhanced to preserve the
result of the packet parsing that is necessary for matching the
incoming UDP packets, so that this works does not have to be done
twice.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 857c890d9b42c50c8a94b76d47d4a61ab6d2f49c)

5 files changed, 461 insertions, 0 deletions

diff --git a/include/arpa/nameser.h b/include/arpa/nameser.h
index 6e4808f00d..c27e7886b7 100644
--- a/include/arpa/nameser.h
+++ b/include/arpa/nameser.h
@@ -103,5 +103,97 @@ libc_hidden_proto (__libc_ns_samename)
    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;
+
+/* Iterator over the resource records in a DNS packet.  */
+struct ns_rr_cursor
+{
+  /* These members are not changed after initialization.  */
+  const unsigned char *begin;	/* First byte of packet.  */
+  const unsigned char *end;	/* One past the last byte of the packet.  */
+  const unsigned char *first_rr; /* First resource record (or packet end).  */
+
+  /* Advanced towards the end while reading the packet.  */
+  const unsigned char *current;
+};
+
+/* Returns the RCODE field from the DNS header.  */
+static inline int
+ns_rr_cursor_rcode (const struct ns_rr_cursor *c)
+{
+  return c->begin[3] & 0x0f;	/* Lower 4 bits at offset 3.  */
+}
+
+/* Returns the length of the answer section according to the DNS header.  */
+static inline int
+ns_rr_cursor_ancount (const struct ns_rr_cursor *c)
+{
+  return c->begin[6] * 256 + c->begin[7]; /* 16 bits at offset 6.  */
+}
+
+/* Returns the length of the authority (name server) section according
+   to the DNS header.  */
+static inline int
+ns_rr_cursor_nscount (const struct ns_rr_cursor *c)
+{
+  return c->begin[8] * 256 + c->begin[9]; /* 16 bits at offset 8.  */
+}
+
+/* Returns the length of the additional data section according to the
+   DNS header.  */
+static inline int
+ns_rr_cursor_adcount (const struct ns_rr_cursor *c)
+{
+  return c->begin[10] * 256 + c->begin[11]; /* 16 bits at offset 10.  */
+}
+
+/* Returns a pointer to the uncompressed question name in wire
+   format.  */
+static inline const unsigned char *
+ns_rr_cursor_qname (const struct ns_rr_cursor *c)
+{
+  return c->begin + 12;		/* QNAME starts right after the header.  */
+}
+
+/* Returns the question type of the first and only question.  */
+static inline const int
+ns_rr_cursor_qtype (const struct ns_rr_cursor *c)
+{
+  /* 16 bits 4 bytes back from the first RR header start.  */
+  return c->first_rr[-4] * 256 + c->first_rr[-3];
+}
+
+/* Returns the clss of the first and only question (usally C_IN).  */
+static inline const int
+ns_rr_cursor_qclass (const struct ns_rr_cursor *c)
+{
+  /* 16 bits 2 bytes back from the first RR header start.  */
+  return c->first_rr[-2] * 256 + c->first_rr[-1];
+}
+
+/* Initializes *C to cover the packet [BUF, BUF+LEN).  Returns false
+   if LEN is less than sizeof (*HD), if the packet does not contain a
+   full (uncompressed) question, or if the question count is not 1.  */
+_Bool __ns_rr_cursor_init (struct ns_rr_cursor *c,
+			   const unsigned char *buf, size_t len)
+  attribute_hidden;
+
+/* Like ns_rr, but the record owner name is not decoded into text format.  */
+struct ns_rr_wire
+{
+  unsigned char rname[NS_MAXCDNAME]; /* Owner name of the record.  */
+  uint16_t rtype;		/* Resource record type (T_*).  */
+  uint16_t rclass;		/* Resource record class (C_*).  */
+  uint32_t ttl;			/* Time-to-live field.  */
+  const unsigned char *rdata;	/* Start of resource record data.  */
+  uint16_t rdlength;		/* Length of the data at rdata, in bytes.  */
+};
+
+/* Attempts to parse the record at C into *RR.  On success, return
+   true, and C is advanced past the record, and RR->rdata points to
+   the record data.  On failure, errno is set to EMSGSIZE, and false
+   is returned.  */
+_Bool __ns_rr_cursor_next (struct ns_rr_cursor *c, struct ns_rr_wire *rr)
+  attribute_hidden;
+
 # endif /* !_ISOMAC */
 #endif
diff --git a/resolv/Makefile b/resolv/Makefile
index bf28825f60..018b1808d6 100644
--- a/resolv/Makefile
+++ b/resolv/Makefile
@@ -47,6 +47,8 @@ routines := \
   ns_name_skip \
   ns_name_uncompress \
   ns_name_unpack \
+  ns_rr_cursor_init \
+  ns_rr_cursor_next \
   ns_samebinaryname \
   ns_samename \
   nsap_addr \
@@ -116,6 +118,10 @@ tests-static += tst-ns_samebinaryname
 tests-internal += tst-ns_name_length_uncompressed
 tests-static += tst-ns_name_length_uncompressed
 
+# Likewise for struct ns_rr_cursor and its functions.
+tests-internal += tst-ns_rr_cursor
+tests-static += tst-ns_rr_cursor
+
 # These tests need libdl.
 ifeq (yes,$(build-shared))
 tests += \
diff --git a/resolv/ns_rr_cursor_init.c b/resolv/ns_rr_cursor_init.c
new file mode 100644
index 0000000000..6ee80b30e9
--- /dev/null
+++ b/resolv/ns_rr_cursor_init.c
@@ -0,0 +1,62 @@
+/* Initialize a simple DNS packet parser.
+   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>
+#include <string.h>
+
+bool
+__ns_rr_cursor_init (struct ns_rr_cursor *c,
+                     const unsigned char *buf, size_t len)
+{
+  c->begin = buf;
+  c->end = buf + len;
+
+  /* Check for header size and 16-bit question count value (it must be 1).  */
+  if (len < 12 || buf[4] != 0 || buf[5] != 1)
+    {
+      __set_errno (EMSGSIZE);
+      c->current = c->end;
+      return false;
+    }
+  c->current = buf + 12;
+
+  int consumed = __ns_name_length_uncompressed (c->current, c->end);
+  if (consumed < 0)
+    {
+      __set_errno (EMSGSIZE);
+      c->current = c->end;
+      c->first_rr = NULL;
+      return false;
+    }
+  c->current += consumed;
+
+  /* Ensure there is room for question type and class.  */
+  if (c->end - c->current < 4)
+    {
+      __set_errno (EMSGSIZE);
+      c->current = c->end;
+      c->first_rr = NULL;
+      return false;
+    }
+  c->current += 4;
+  c->first_rr = c->current;
+
+  return true;
+}
diff --git a/resolv/ns_rr_cursor_next.c b/resolv/ns_rr_cursor_next.c
new file mode 100644
index 0000000000..33652fc5da
--- /dev/null
+++ b/resolv/ns_rr_cursor_next.c
@@ -0,0 +1,74 @@
+/* Simple DNS record parser without textual name decoding.
+   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>
+#include <string.h>
+
+bool
+__ns_rr_cursor_next (struct ns_rr_cursor *c, struct ns_rr_wire *rr)
+{
+  rr->rdata = NULL;
+
+  /* Extract the record owner name.  */
+  int consumed = __ns_name_unpack (c->begin, c->end, c->current,
+                                   rr->rname, sizeof (rr->rname));
+  if (consumed < 0)
+    {
+      memset (rr, 0, sizeof (*rr));
+      __set_errno (EMSGSIZE);
+      return false;
+    }
+  c->current += consumed;
+
+  /* Extract the metadata.  */
+  struct
+  {
+    uint16_t rtype;
+    uint16_t rclass;
+    uint32_t ttl;
+    uint16_t rdlength;
+  } __attribute__ ((packed)) metadata;
+  _Static_assert (sizeof (metadata) == 10, "sizeof metadata");
+  if (c->end - c->current < sizeof (metadata))
+    {
+      memset (rr, 0, sizeof (*rr));
+      __set_errno (EMSGSIZE);
+      return false;
+    }
+  memcpy (&metadata, c->current, sizeof (metadata));
+  c->current += sizeof (metadata);
+  /* Endianess conversion.  */
+  rr->rtype = ntohs (metadata.rtype);
+  rr->rclass = ntohs (metadata.rclass);
+  rr->ttl = ntohl (metadata.ttl);
+  rr->rdlength = ntohs (metadata.rdlength);
+
+  /* Extract record data.  */
+  if (c->end - c->current < rr->rdlength)
+    {
+      memset (rr, 0, sizeof (*rr));
+      __set_errno (EMSGSIZE);
+      return false;
+    }
+  rr->rdata = c->current;
+  c->current += rr->rdlength;
+
+  return true;
+}
diff --git a/resolv/tst-ns_rr_cursor.c b/resolv/tst-ns_rr_cursor.c
new file mode 100644
index 0000000000..c3c0908905
--- /dev/null
+++ b/resolv/tst-ns_rr_cursor.c
@@ -0,0 +1,227 @@
+/* Tests for resource record parsing.
+   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 <string.h>
+#include <support/check.h>
+#include <support/next_to_fault.h>
+
+/* Reference packet for packet parsing.  */
+static const unsigned char valid_packet[] =
+  { 0x11, 0x12, 0x13, 0x14,
+    0x00, 0x01,               /* Question count.  */
+    0x00, 0x02,               /* Answer count.  */
+    0x21, 0x22, 0x23, 0x24,   /* Other counts (not actually in packet).  */
+    3, 'w', 'w', 'w', 7, 'e', 'x', 'a', 'm', 'p', 'l', 'e', 0,
+    0x00, 0x1c,               /* Question type: AAAA.  */
+    0x00, 0x01,               /* Question class: IN.  */
+    0xc0, 0x0c,               /* Compression reference to QNAME.  */
+    0x00, 0x1c,               /* Record type: AAAA.  */
+    0x00, 0x01,               /* Record class: IN.  */
+    0x12, 0x34, 0x56, 0x78,   /* Record TTL.  */
+    0x00, 0x10,               /* Record data length (16 bytes).  */
+    0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
+    0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f, /* IPv6 address.  */
+    0xc0, 0x0c,               /* Compression reference to QNAME.  */
+    0x00, 0x1c,               /* Record type: AAAA.  */
+    0x00, 0x01,               /* Record class: IN.  */
+    0x11, 0x33, 0x55, 0x77,   /* Record TTL.  */
+    0x00, 0x10,               /* Record data length (16 bytes).  */
+    0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
+    0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf, /* IPv6 address.  */
+  };
+
+/* Special offsets in valid_packet.  */
+enum
+  {
+    offset_of_first_record = 29,
+    offset_of_second_record = 57,
+  };
+
+/* Check that parsing valid_packet succeeds.  */
+static void
+test_valid (void)
+{
+  struct ns_rr_cursor c;
+  TEST_VERIFY_EXIT (__ns_rr_cursor_init (&c, valid_packet,
+                                         sizeof (valid_packet)));
+  TEST_COMPARE (ns_rr_cursor_rcode (&c), 4);
+  TEST_COMPARE (ns_rr_cursor_ancount (&c), 2);
+  TEST_COMPARE (ns_rr_cursor_nscount (&c), 0x2122);
+  TEST_COMPARE (ns_rr_cursor_adcount (&c), 0x2324);
+  TEST_COMPARE_BLOB (ns_rr_cursor_qname (&c), 13, &valid_packet[12], 13);
+  TEST_COMPARE (ns_rr_cursor_qtype (&c), T_AAAA);
+  TEST_COMPARE (ns_rr_cursor_qclass (&c), C_IN);
+  TEST_COMPARE (c.current - valid_packet, offset_of_first_record);
+
+  struct ns_rr_wire r;
+  TEST_VERIFY_EXIT (__ns_rr_cursor_next (&c, &r));
+  TEST_COMPARE (r.rtype, T_AAAA);
+  TEST_COMPARE (r.rclass, C_IN);
+  TEST_COMPARE (r.ttl, 0x12345678);
+  TEST_COMPARE_BLOB (r.rdata, r.rdlength,
+                     "\x90\x91\x92\x93\x94\x95\x96\x97"
+                     "\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f", 16);
+  TEST_COMPARE (c.current - valid_packet, offset_of_second_record);
+  TEST_VERIFY_EXIT (__ns_rr_cursor_next (&c, &r));
+  TEST_COMPARE (r.rtype, T_AAAA);
+  TEST_COMPARE (r.rclass, C_IN);
+  TEST_COMPARE (r.ttl, 0x11335577);
+  TEST_COMPARE_BLOB (r.rdata, r.rdlength,
+                     "\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7"
+                     "\xa8\xa9\xaa\xab\xac\xad\xae\xaf", 16);
+  TEST_VERIFY (c.current == c.end);
+}
+
+/* Check that trying to parse a packet with a compressed QNAME fails.  */
+static void
+test_compressed_qname (void)
+{
+  static const unsigned char packet[] =
+    { 0x11, 0x12, 0x13, 0x14,
+      0x00, 0x01,               /* Question count.  */
+      0x00, 0x00,               /* Answer count.  */
+      0x00, 0x00, 0x00, 0x00,   /* Other counts.  */
+      3, 'w', 'w', 'w', 7, 'e', 'x', 'a', 'm', 'p', 'l', 'e', 0xc0, 0x04,
+      0x00, 0x01,               /* Question type: A.  */
+      0x00, 0x01,               /* Question class: IN.  */
+    };
+
+  struct ns_rr_cursor c;
+  TEST_VERIFY_EXIT (!__ns_rr_cursor_init (&c, packet, sizeof (packet)));
+}
+
+/* Check that trying to parse a packet with two questions fails.  */
+static void
+test_two_questions (void)
+{
+  static const unsigned char packet[] =
+    { 0x11, 0x12, 0x13, 0x14,
+      0x00, 0x02,               /* Question count.  */
+      0x00, 0x00,               /* Answer count.  */
+      0x00, 0x00, 0x00, 0x00,   /* Other counts.  */
+      3, 'w', 'w', 'w', 7, 'e', 'x', 'a', 'm', 'p', 'l', 'e', 0xc0, 0x04,
+      0x00, 0x01,               /* Question type: A.  */
+      0x00, 0x01,               /* Question class: IN.  */
+      3, 'w', 'w', 'w', 7, 'e', 'x', 'a', 'm', 'p', 'l', 'e', 0xc0, 0x04,
+      0x00, 0x1c,               /* Question type: AAAA.  */
+      0x00, 0x01,               /* Question class: IN.  */
+    };
+
+  struct ns_rr_cursor c;
+  TEST_VERIFY_EXIT (!__ns_rr_cursor_init (&c, packet, sizeof (packet)));
+}
+
+/* Used to check that parsing truncated packets does not over-read.  */
+static struct support_next_to_fault ntf;
+
+/* Truncated packet in the second resource record.  */
+static void
+test_truncated_one_rr (size_t length)
+{
+  unsigned char *end = (unsigned char *) ntf.buffer - ntf.length;
+  unsigned char *start = end - length;
+
+  /* Produce the truncated packet.  */
+  memcpy (start, valid_packet, length);
+
+  struct ns_rr_cursor c;
+  TEST_VERIFY_EXIT (__ns_rr_cursor_init (&c, start, length));
+  TEST_COMPARE (ns_rr_cursor_rcode (&c), 4);
+  TEST_COMPARE (ns_rr_cursor_ancount (&c), 2);
+  TEST_COMPARE (ns_rr_cursor_nscount (&c), 0x2122);
+  TEST_COMPARE (ns_rr_cursor_adcount (&c), 0x2324);
+  TEST_COMPARE_BLOB (ns_rr_cursor_qname (&c), 13, &valid_packet[12], 13);
+  TEST_COMPARE (ns_rr_cursor_qtype (&c), T_AAAA);
+  TEST_COMPARE (ns_rr_cursor_qclass (&c), C_IN);
+  TEST_COMPARE (c.current - start, offset_of_first_record);
+
+  struct ns_rr_wire r;
+  TEST_VERIFY_EXIT (__ns_rr_cursor_next (&c, &r));
+  TEST_COMPARE (r.rtype, T_AAAA);
+  TEST_COMPARE (r.rclass, C_IN);
+  TEST_COMPARE (r.ttl, 0x12345678);
+  TEST_COMPARE_BLOB (r.rdata, r.rdlength,
+                     "\x90\x91\x92\x93\x94\x95\x96\x97"
+                     "\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f", 16);
+  TEST_COMPARE (c.current - start, offset_of_second_record);
+  TEST_VERIFY (!__ns_rr_cursor_next (&c, &r));
+}
+
+/* Truncated packet in the first resource record.  */
+static void
+test_truncated_no_rr (size_t length)
+{
+  unsigned char *end = (unsigned char *) ntf.buffer - ntf.length;
+  unsigned char *start = end - length;
+
+  /* Produce the truncated packet.  */
+  memcpy (start, valid_packet, length);
+
+  struct ns_rr_cursor c;
+  TEST_VERIFY_EXIT (__ns_rr_cursor_init (&c, start, length));
+  TEST_COMPARE (ns_rr_cursor_rcode (&c), 4);
+  TEST_COMPARE (ns_rr_cursor_ancount (&c), 2);
+  TEST_COMPARE (ns_rr_cursor_nscount (&c), 0x2122);
+  TEST_COMPARE (ns_rr_cursor_adcount (&c), 0x2324);
+  TEST_COMPARE_BLOB (ns_rr_cursor_qname (&c), 13, &valid_packet[12], 13);
+  TEST_COMPARE (ns_rr_cursor_qtype (&c), T_AAAA);
+  TEST_COMPARE (ns_rr_cursor_qclass (&c), C_IN);
+  TEST_COMPARE (c.current - start, offset_of_first_record);
+
+  struct ns_rr_wire r;
+  TEST_VERIFY (!__ns_rr_cursor_next (&c, &r));
+}
+
+/* Truncated packet before first resource record.  */
+static void
+test_truncated_before_rr (size_t length)
+{
+  unsigned char *end = (unsigned char *) ntf.buffer - ntf.length;
+  unsigned char *start = end - length;
+
+  /* Produce the truncated packet.  */
+  memcpy (start, valid_packet, length);
+
+  struct ns_rr_cursor c;
+  TEST_VERIFY_EXIT (!__ns_rr_cursor_init (&c, start, length));
+}
+
+static int
+do_test (void)
+{
+  ntf = support_next_to_fault_allocate (sizeof (valid_packet));
+
+  test_valid ();
+  test_compressed_qname ();
+  test_two_questions ();
+
+  for (int length = offset_of_second_record; length < sizeof (valid_packet);
+       ++length)
+    test_truncated_one_rr (length);
+  for (int length = offset_of_first_record; length < offset_of_second_record;
+       ++length)
+    test_truncated_no_rr (length);
+  for (int length = 0; length < offset_of_first_record; ++length)
+    test_truncated_before_rr (length);
+
+  support_next_to_fault_free (&ntf);
+  return 0;
+}
+
+#include <support/test-driver.c>