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>

1 files changed, 74 insertions, 0 deletions

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;
+}