[ovs-dev,v6,3/3] conntrack: limit port clash resolution attempts

From: wenxu <wenxu@ucloud.cn>

In case almost or all available ports are taken, clash resolution can
take a very long time, resulting in pmd hang in conntrack.

This can happen when many to-be-natted hosts connect to same
destination:port (e.g. a proxy) and all connections pass the same SNAT.

Pick a random offset in the acceptable range, then try ever smaller
number of adjacent port numbers, until either the limit is reached or a
useable port was found.  This results in at most 248 attempts
(128 + 64 + 32 + 16 + 8, i.e. 4 restarts with new search offset)
instead of 64000+.

And if thenumber of ip address will limit the max attempts and which
will lead the total attempts under 248.

Signed-off-by: wenxu <wenxu@ucloud.cn>
---
 lib/conntrack.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 46 insertions(+), 4 deletions(-)

Didn't go through the whole patch, as the discussion on #2 may require
some changes to #3 as well. There's one nit I noticed, though

wenxu@ucloud.cn writes:

> From: wenxu <wenxu@ucloud.cn>
>
> In case almost or all available ports are taken, clash resolution can
> take a very long time, resulting in pmd hang in conntrack.
>
> This can happen when many to-be-natted hosts connect to same
> destination:port (e.g. a proxy) and all connections pass the same SNAT.
>
> Pick a random offset in the acceptable range, then try ever smaller
> number of adjacent port numbers, until either the limit is reached or a
> useable port was found.  This results in at most 248 attempts
> (128 + 64 + 32 + 16 + 8, i.e. 4 restarts with new search offset)
> instead of 64000+.
>
> And if thenumber of ip address will limit the max attempts and which
> will lead the total attempts under 248.
>
> Signed-off-by: wenxu <wenxu@ucloud.cn>
> ---
>  lib/conntrack.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 46 insertions(+), 4 deletions(-)
>
> diff --git a/lib/conntrack.c b/lib/conntrack.c
> index a6784ba..1b5d6c2 100644
> --- a/lib/conntrack.c
> +++ b/lib/conntrack.c
> @@ -2426,7 +2426,11 @@ nat_get_unique_tuple(struct conntrack *ct, const struct conn *conn,
>      uint32_t hash = nat_range_hash(conn, ct->hash_basis, nat_info);
>      bool pat_proto = conn->key.nw_proto == IPPROTO_TCP ||
>                       conn->key.nw_proto == IPPROTO_UDP;
> -    uint16_t min_port, max_port, curr_port;
> +    uint16_t min_port, max_port, curr_port, orig_port;
> +    unsigned int attempts, max_attempts, min_attempts;
> +    uint16_t range_port;
> +    uint32_t range_addr;
> +    unsigned int i;
>  
>      min_addr = nat_info->min_addr;
>      max_addr = nat_info->max_addr;
> @@ -2438,9 +2442,25 @@ nat_get_unique_tuple(struct conntrack *ct, const struct conn *conn,
>       * we can stop once we reach it. */
>      guard_addr = curr_addr;
>  
> -    set_port_range(nat_info, &conn->key, hash, &curr_port,
> +    set_port_range(nat_info, &conn->key, hash, &orig_port,
>                     &min_port, &max_port);
>  
> +    range_port = max_port - min_port + 1;
> +    if (conn->key.dl_type == htons(ETH_TYPE_IP)) {
> +        range_addr = ntohl(max_addr.ipv4) - ntohl(min_addr.ipv4) + 1;
> +    } else {
> +        range_addr = nat_ipv6_addrs_delta(&nat_info->min_addr.ipv6,
> +                                          &nat_info->max_addr.ipv6) + 1;
> +    }
> +    max_attempts = 128 / range_addr;
> +    if (max_attempts < 1) {
> +        max_attempts = 1;
> +    }
> +    min_attempts = 16 / range_addr;
> +    if (min_attempts < 2) {
> +        min_attempts = 2;
> +    }
> +
>  another_round:
>      store_addr_to_key(&curr_addr, &nat_conn->rev_key,
>                        nat_info->nat_action);
> @@ -2454,17 +2474,39 @@ another_round:
>          goto next_addr;
>      }
>  
> +    curr_port = orig_port;
> +
> +    attempts = range_port;
> +    if (attempts > max_attempts) {
> +        attempts = max_attempts;
> +    }
> +
> +another_port_round:
> +    i = 0;
>      FOR_EACH_PORT_IN_RANGE(curr_port, min_port, max_port) {
>          if (nat_info->nat_action & NAT_ACTION_SRC) {
>              nat_conn->rev_key.dst.port = htons(curr_port);
>          } else {
>              nat_conn->rev_key.src.port = htons(curr_port);
>          }
> -        if (!conn_lookup(ct, &nat_conn->rev_key, time_msec(), NULL, NULL)) {
> -            return true;
> +        if (i++ < attempts) {
> +            if (!conn_lookup(ct, &nat_conn->rev_key,
> +                time_msec(), NULL, NULL)) {
> +                return true;
> +            }
> +        } else {
> +            goto next_attempts;

if you use a break here you could get rid of next_attempts, right?

>          }
>      }
>  
> +next_attempts:
> +    if (attempts < range_port && attempts >= min_attempts) {
> +        attempts /= 2;
> +        curr_port = min_port + (random_uint32() % range_port);
> +
> +        goto another_port_round;
> +    }
> +
>      /* Check if next IP is in range and respin. Otherwise, notify
>       * exhaustion to the caller. */
>  next_addr:
> -- 
> 1.8.3.1