[PATCH v2 3/3] dns: fix lookup error when no IPv6 is returned

[Renaud Métrich]

When trying to resolve DNS names into IP addresses, the DNS code fails
from time to time with the following error:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
error: ../../grub-core/net/dns.c:688:no DNS record found.
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

This happens when both IPv4 and IPv6 queries are performed against the
DNS server (e.g. 8.8.8.8) but there is no IP returned for IPv6 query, as
shown below:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
grub> net_del_dns 192.168.122.1
grub> net_add_dns 8.8.8.8
grub> net_nslookup ipv4.test-ipv6.com
error: ../../grub-core/net/dns.c:688:no DNS record found.
grub> net_nslookup ipv4.test-ipv6.com
216.218.228.115
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

The root cause is the code exiting prematurely when the data->addresses
buffer has been allocated in recv_hook(), even if there was no address
returned last time recv_hook() executed.

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>
---
 grub-core/net/dns.c | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/grub-core/net/dns.c b/grub-core/net/dns.c
index 6993a4aba..2dfb98d98 100644
--- a/grub-core/net/dns.c
+++ b/grub-core/net/dns.c
@@ -261,7 +261,7 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
((unused)),
   /* Code apparently assumed that only one packet is received as response.
      We may get multiple responses due to network condition, so check here
      and quit early. */
-  if (*data->addresses)
+  if (*data->naddresses)
     goto out;
 
   head = (struct dns_header *) nb->data;
@@ -305,11 +305,7 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
((unused)),
       grub_uint32_t ttl = 0;
       grub_uint16_t length;
       if (ptr >= nb->tail)
-       {
-         if (!*data->naddresses)
-           grub_free (*data->addresses);
-         goto out;
-       }
+       goto out;
       ignored = !check_name (ptr, nb->data, nb->tail, data->name);
       while (ptr < nb->tail && !((*ptr & 0xc0) || *ptr == 0))
        ptr += *ptr + 1;
@@ -317,11 +313,7 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
((unused)),
        ptr++;
       ptr++;
       if (ptr + 10 >= nb->tail)
-       {
-         if (!*data->naddresses)
-           grub_free (*data->addresses);
-         goto out;
-       }
+       goto out;
       if (*ptr++ != 0)
        ignored = 1;
       class = *ptr++;
@@ -337,11 +329,7 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
((unused)),
       length = *ptr++ << 8;
       length |= *ptr++;
       if (ptr + length > nb->tail)
-       {
-         if (!*data->naddresses)
-           grub_free (*data->addresses);
-         goto out;
-       }
+       goto out;
       if (!ignored)
        {
          if (ttl_all > ttl)
@@ -429,6 +417,8 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
((unused)),
  out:
   grub_netbuff_free (nb);
   grub_free (redirect_save);
+  if (!*data->naddresses)
+    grub_free (*data->addresses);
   return GRUB_ERR_NONE;
 }
 
-- 
2.40.1