Re: [PATCH] dns: realloc address buffer after each packet

[Andrei Borzenkov]

On Wed, Feb 24, 2016 at 10:11 PM, Josef Bacik <address@hidden> wrote:
> Sometimes DNS responses come in slower than we poll for them which can lead us
> to process multiple DNS packets which overflows the addresses array.  So 
> instead
> realloc the array each time to make sure we are accounting for any answers we
> currently have in the address array.  We also move the caching of the 
> addresses
> outside of the recv hook so we can be sure to cache all the responses at once
> instead of one packet at a time.  Thanks,
>

This still does not address the problem that we stop waiting for
further packets as soon as we get any response, so we still depend on
delivery order to get correct record.

What about following

- send both A and AAAA query concurrently if requested
- keep track of both requests (i.e. have data.id[2] and data.addresses[2])
- reset request ID (or otherwise mark it as "received") as soon as we
got reply. Note that reply may contain no addresses - NXDOMAIN is
prerfectly valid - so condition should not be "got any record of type
A or AAAA" as it is now but rather simply "got reply to request with
id XX". This also allows us to implement negative caching at some
point :)
- return both A and AAAA results separately to grub_net_dns_lookup()
- combine them in grub_net_dns_lookup() depending on preference - i.e.
put either A or AAAA first in final result.

This seems to cover all issues so far - we do not wait too long, we
are guaranteed to get both A and AAAA if we request them and we return
them in proper order for further processing.

Do I miss something?

> Signed-off-by: Josef Bacik <address@hidden>
> ---
>  grub-core/net/dns.c | 65 
> ++++++++++++++++++++++++++++-------------------------
>  1 file changed, 34 insertions(+), 31 deletions(-)
>
> diff --git a/grub-core/net/dns.c b/grub-core/net/dns.c
> index 89741dd..d8258c6 100644
> --- a/grub-core/net/dns.c
> +++ b/grub-core/net/dns.c
> @@ -109,11 +109,10 @@ struct recv_data
>  {
>    grub_size_t *naddresses;
>    struct grub_net_network_level_address **addresses;
> -  int cache;
>    grub_uint16_t id;
> +  grub_uint32_t ttl;
>    int dns_err;
>    char *name;
> -  const char *oname;
>    int stop;
>  };
>
> @@ -230,6 +229,7 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
> ((unused)),
>            struct grub_net_buff *nb,
>            void *data_)
>  {
> +  struct grub_net_network_level_address *addresses;
>    struct dns_header *head;
>    struct recv_data *data = data_;
>    int i, j;
> @@ -276,14 +276,17 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
> ((unused)),
>        ptr++;
>        ptr += 4;
>      }
> -  *data->addresses = grub_malloc (sizeof ((*data->addresses)[0])
> -                                * grub_be_to_cpu16 (head->ancount));
> -  if (!*data->addresses)
> +  addresses = grub_realloc (*data->addresses,
> +                           sizeof ((*data->addresses)[0])
> +                           * (grub_be_to_cpu16 (head->ancount)
> +                              + *data->naddresses));
> +  if (!addresses)
>      {
>        grub_errno = GRUB_ERR_NONE;
>        grub_netbuff_free (nb);
>        return GRUB_ERR_NONE;
>      }
> +  *data->addresses = addresses;
>    reparse_ptr = ptr;
>   reparse:
>    for (i = 0, ptr = reparse_ptr; i < grub_be_to_cpu16 (head->ancount); i++)
> @@ -386,31 +389,6 @@ recv_hook (grub_net_udp_socket_t sock __attribute__ 
> ((unused)),
>         }
>        ptr += length;
>      }
> -  if (ttl_all && *data->naddresses && data->cache)
> -    {
> -      int h;
> -      grub_dprintf ("dns", "caching for %d seconds\n", ttl_all);
> -      h = hash (data->oname);
> -      grub_free (dns_cache[h].name);
> -      dns_cache[h].name = 0;
> -      grub_free (dns_cache[h].addresses);
> -      dns_cache[h].addresses = 0;
> -      dns_cache[h].name = grub_strdup (data->oname);
> -      dns_cache[h].naddresses = *data->naddresses;
> -      dns_cache[h].addresses = grub_malloc (*data->naddresses
> -                                           * sizeof 
> (dns_cache[h].addresses[0]));
> -      dns_cache[h].limit_time = grub_get_time_ms () + 1000 * ttl_all;
> -      if (!dns_cache[h].addresses || !dns_cache[h].name)
> -       {
> -         grub_free (dns_cache[h].name);
> -         dns_cache[h].name = 0;
> -         grub_free (dns_cache[h].addresses);
> -         dns_cache[h].addresses = 0;
> -       }
> -      grub_memcpy (dns_cache[h].addresses, *data->addresses,
> -                  *data->naddresses
> -                  * sizeof (dns_cache[h].addresses[0]));
> -    }
>    grub_netbuff_free (nb);
>    grub_free (redirect_save);
>    return GRUB_ERR_NONE;
> @@ -435,7 +413,7 @@ grub_net_dns_lookup (const char *name,
>    grub_uint8_t *qtypeptr;
>    grub_err_t err = GRUB_ERR_NONE;
>    struct recv_data data = {naddresses, addresses, cache,
> -                          grub_cpu_to_be16 (id++), 0, 0, name, 0};
> +                          grub_cpu_to_be16 (id++), ~0U, 0, 0};
>    grub_uint8_t *nbd;
>    grub_size_t try_server = 0;
>
> @@ -602,6 +580,31 @@ grub_net_dns_lookup (const char *name,
>
>    grub_free (sockets);
>
> +  if (data.ttl && *data.naddresses && cache)
> +    {
> +      int h;
> +      grub_dprintf ("dns", "caching for %d seconds\n", data.ttl);
> +      h = hash (name);
> +      grub_free (dns_cache[h].name);
> +      dns_cache[h].name = 0;
> +      grub_free (dns_cache[h].addresses);
> +      dns_cache[h].addresses = 0;
> +      dns_cache[h].name = grub_strdup (name);
> +      dns_cache[h].naddresses = *data.naddresses;
> +      dns_cache[h].addresses = grub_malloc (*data.naddresses
> +                                           * sizeof 
> (dns_cache[h].addresses[0]));
> +      dns_cache[h].limit_time = grub_get_time_ms () + 1000 * data.ttl;
> +      if (!dns_cache[h].addresses || !dns_cache[h].name)
> +       {
> +         grub_free (dns_cache[h].name);
> +         dns_cache[h].name = 0;
> +         grub_free (dns_cache[h].addresses);
> +         dns_cache[h].addresses = 0;
> +       }
> +      grub_memcpy (dns_cache[h].addresses, *data.addresses,
> +                  *data.naddresses
> +                  * sizeof (dns_cache[h].addresses[0]));
> +    }
>    if (*data.naddresses)
>      return GRUB_ERR_NONE;
>    if (data.dns_err)
> --
> 2.5.0
>
>
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