support_format_dns_packet: Fix CNAME and multiple RR handling

Before this change, the loop iterating over RRs in the answer
section stopped at the first CNAME record, never printing them.
The CNAME and PTR record contents was extracted from the wrong
buffer (whole packet instead RDATA).  This desynced the parsing
after the first CNAME or PTR record.

Also fix the AAAA record parsing by checking their sizes.

2017-03-15  Florian Weimer  <fweimer@redhat.com>

	* support/support_format_dns_packet.c (support_format_dns_packet):
	Handle CNAME records in the response.  Extract RDATA names from
	rdata, not the whole packet.  Check AAAA record length.
	* support/tst-support_format_dns_packet.c: New file.
	* support/Makefile (tests): Add tst-support_format_dns_packet.
	(tst-support_format_dns_packet): Link against libresolv.

diff --git a/support/Makefile b/support/Makefile
index 2ace559..db7bb13 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -111,6 +111,7 @@ endif
 tests = \
   README-testing \
   tst-support-namespace \
+  tst-support_format_dns_packet \
   tst-support_record_failure \
 
 ifeq ($(run-built-tests),yes)
@@ -125,4 +126,6 @@ $(objpfx)tst-support_record_failure-2.out: tst-support_record_failure-2.sh \
 	$(evaluate-test)
 endif
 
+$(objpfx)tst-support_format_dns_packet: $(common-objpfx)resolv/libresolv.so
+
 include ../Rules
diff --git a/support/support_format_dns_packet.c b/support/support_format_dns_packet.c
index 21fe7e5..2992c57 100644
--- a/support/support_format_dns_packet.c
+++ b/support/support_format_dns_packet.c
@@ -174,7 +174,7 @@ support_format_dns_packet (const unsigned char *buffer, size_t length)
           goto out;
         }
       /* Skip non-matching record types.  */
-      if (rtype != qtype || rclass != qclass)
+      if ((rtype != qtype && rtype != T_CNAME) || rclass != qclass)
         continue;
       switch (rtype)
         {
@@ -186,22 +186,29 @@ support_format_dns_packet (const unsigned char *buffer, size_t length)
                        rdata.data[2],
                        rdata.data[3]);
           else
-            fprintf (mem.out, "error: A record of size %d: %s\n", rdlen, rname.name);
+            fprintf (mem.out, "error: A record of size %d: %s\n",
+                     rdlen, rname.name);
           break;
         case T_AAAA:
           {
-            char buf[100];
-            if (inet_ntop (AF_INET6, rdata.data, buf, sizeof (buf)) == NULL)
-              fprintf (mem.out, "error: AAAA record decoding failed: %m\n");
+            if (rdlen == 16)
+              {
+                char buf[100];
+                if (inet_ntop (AF_INET6, rdata.data, buf, sizeof (buf)) == NULL)
+                  fprintf (mem.out, "error: AAAA record decoding failed: %m\n");
+                else
+                  fprintf (mem.out, "address: %s\n", buf);
+              }
             else
-              fprintf (mem.out, "address: %s\n", buf);
+              fprintf (mem.out, "error: AAAA record of size %d: %s\n",
+                       rdlen, rname.name);
           }
           break;
         case T_CNAME:
         case T_PTR:
           {
             struct dname name;
-            if (extract_name (full, &in, &name))
+            if (extract_name (full, &rdata, &name))
               fprintf (mem.out, "name: %s\n", name.name);
             else
               fprintf (mem.out, "error: malformed CNAME/PTR record\n");
diff --git a/support/tst-support_format_dns_packet.c b/support/tst-support_format_dns_packet.c
new file mode 100644
index 0000000..ecd7abf
--- /dev/null
+++ b/support/tst-support_format_dns_packet.c
@@ -0,0 +1,97 @@
+/* Tests for the support_format_dns_packet function.
+   Copyright (C) 2016-2017 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 <support/check.h>
+#include <support/format_nss.h>
+#include <support/run_diff.h>
+
+static void
+check_packet (const void *buffer, size_t length,
+              const char *name, const char *expected)
+{
+  char *actual = support_format_dns_packet (buffer, length);
+  if (strcmp (actual, expected) != 0)
+    {
+      support_record_failure ();
+      printf ("error: formatted packet does not match: %s\n", name);
+      support_run_diff ("expected", expected,
+                        "actual", actual);
+    }
+  free (actual);
+}
+
+static void
+test_aaaa_length (void)
+{
+  static const char packet[] =
+    /* Header: Response with two records.  */
+    "\x12\x34\x80\x00\x00\x01\x00\x02\x00\x00\x00\x00"
+    /* Question section.  www.example/IN/AAAA.  */
+    "\x03www\x07""example\x00\x00\x1c\x00\x01"
+    /* Answer section.  www.example AAAA [corrupted].  */
+    "\xc0\x0c"
+    "\x00\x1c\x00\x01\x00\x00\x00\x00\x00\x10"
+    "\x20\x01\x0d\xb8\x05\x06\x07\x08"
+    "\x11\x12\x13\x14\x15\x16\x17\x18"
+    /* www.example AAAA [corrupted].  */
+    "\xc0\x0c"
+    "\x00\x1c\x00\x01\x00\x00\x00\x00\x00\x11"
+    "\x01\x02\x03\x04\x05\x06\x07\x08"
+    "\x11\x12\x13\x14\x15\x16\x17\x18" "\xff";
+  check_packet (packet, sizeof (packet) - 1, __func__,
+                "name: www.example\n"
+                "address: 2001:db8:506:708:1112:1314:1516:1718\n"
+                "error: AAAA record of size 17: www.example\n");
+}
+
+static void
+test_multiple_cnames (void)
+{
+  static const char packet[] =
+    /* Header: Response with three records.  */
+    "\x12\x34\x80\x00\x00\x01\x00\x03\x00\x00\x00\x00"
+    /* Question section.  www.example/IN/A.  */
+    "\x03www\x07""example\x00\x00\x01\x00\x01"
+    /* Answer section.  www.example CNAME www1.example.  */
+    "\xc0\x0c"
+    "\x00\x05\x00\x01\x00\x00\x00\x00\x00\x07"
+    "\x04www1\xc0\x10"
+    /* www1 CNAME www2.  */
+    "\x04www1\xc0\x10"
+    "\x00\x05\x00\x01\x00\x00\x00\x00\x00\x07"
+    "\x04www2\xc0\x10"
+    /* www2 A 192.0.2.1.  */
+    "\x04www2\xc0\x10"
+    "\x00\x01\x00\x01\x00\x00\x00\x00\x00\x04"
+    "\xc0\x00\x02\x01";
+  check_packet (packet, sizeof (packet) - 1, __func__,
+                "name: www.example\n"
+                "name: www1.example\n"
+                "name: www2.example\n"
+                "address: 192.0.2.1\n");
+}
+
+static int
+do_test (void)
+{
+  test_aaaa_length ();
+  test_multiple_cnames ();
+  return 0;
+}
+
+#include <support/test-driver.c>
