Subject: Re: [RFC PATCH] sctp: Don't lookup dst if transport dst is still valid

From: Vlad Yasevich <[email-masked]>
Date: Fri, 05 Jul 2013 10:09:36 -0400

On 07/03/2013 10:33 PM, Fan Du wrote:
>
>
> On 2013年07月03日 21:23, Vlad Yasevich wrote:
>> On 07/02/2013 10:18 PM, Fan Du wrote:
>>>
>>>
>>> On 2013年07月02日 22:29, Vlad Yasevich wrote:
>>>> On 07/02/2013 02:39 AM, Fan Du wrote:
>>>>> When sctp sits on IPv6, sctp_transport_dst_check pass cookie as ZERO,
>>>>> as a result ip6_dst_check always fail out. This behaviour makes
>>>>> transport->dst useless, because every sctp_packet_transmit must look
>>>>> for valid dst(Is this what supposed to be?)
>>>>>
>>>>> One aggressive way is to call rt_genid_bump which invalid all dst to
>>>>> make new dst for transport, apparently it also hurts others.
>>>>> I'm sure this may not be the best for all, so any commnets?
>>>>>
>>>>> Signed-off-by: Fan Du <[email-masked]>
>>>>> ---
>>>>> include/net/sctp/sctp.h | 18 ++++++++++++------
>>>>> net/sctp/ipv6.c | 2 ++
>>>>> 2 files changed, 14 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
>>>>> index cd89510..f05af01 100644
>>>>> --- a/include/net/sctp/sctp.h
>>>>> +++ b/include/net/sctp/sctp.h
>>>>> @@ -719,14 +719,20 @@ static inline void sctp_v4_map_v6(union
>>>>> sctp_addr *addr)
>>>>> addr->v6.sin6_addr.s6_addr32[2] = htonl(0x0000ffff);
>>>>> }
>>>>>
>>>>> -/* The cookie is always 0 since this is how it's used in the
>>>>> - * pmtu code.
>>>>> - */
>>>>> +/* Set cookie with the right one for IPv6 and zero for others */
>>>>> static inline struct dst_entry *sctp_transport_dst_check(struct
>>>>> sctp_transport *t)
>>>>> {
>>>>> - if (t->dst && !dst_check(t->dst, 0)) {
>>>>> - dst_release(t->dst);
>>>>> - t->dst = NULL;
>>>>> +
>>>>> + if (t->dst) {
>>>>> + struct rt6_info *rt = (struct rt6_info *)t->dst;
>>>>> + u32 cookie = 0;
>>>>> +
>>>>> + if ((t->af_specific->sa_family == AF_INET6) && rt->rt6i_node)
>>>>> + cookie = rt->rt6i_node->fn_sernum;
>>>>> + if (!dst_check(t->dst, cookie)) {
>>>>> + dst_release(t->dst);
>>>>> + t->dst = NULL;
>>>>> + }
>>>>> }
>>>>
>>>> I think it would be better if we stored the dst_cookie in the
>>>> transport structure and initialized it at lookup time. If you do that,
>>>> then if the route table changes, we'd correctly detect it without
>>>> artificially bumping rt_genid (and hurting ipv4).
>>>
>>> Hi Vlad/Neil
>>>
>>> Is this what you mean?
>>
>> Yes, exactly.
>>
>
> Hi Vlad
>
> I thinks twice about below patch, this is actually a chicken-egg issue.
> Look below scenario:
>   (1) The first time we push packet through a transport, dst_cookie is 0,
>       so sctp_transport_dst_check also pass cookie as 0, then return dst
> as NULL.
>       Then we lookup dst by sctp_transport_route, and in there we
> initiate dst_cookie
>       with rt->rt6i_node->fn_sernum
>
>   (2) Then the next time we push packet through this transport again,
>       we pass dst_cookie(rt->rt6i_node->fn_sernum) to ip6_dst_check, and
>       return valid dst without bothering to lookup dst again.

No, if the route was removed rt6i_node will be NULL, and NULL will be 
returned from ip6_dst_check().  If the route still exists then we'll 
compare the serial number with a cookie.

>
> BUT, suppose when deleting the source address of this dst after
> transport->dst_cookie
> has been well initialized. transport->dst_cookie still holds
> rt->rt6i_node->fn_sernum,
> meaning ip6_dst_check will return valid dst, which it shouldn't in this
> case, the
> result will be association ABORT.

No, removing the address cause the route for that prefix to be removed 
as well.  This will set rt6i_node to NULL.

>
> Other way is invalid all transport->dst which using the deleting address
> as source address
> without bumping gen_id, problem is the traverse times depends heavily on
> transport number,
> and also need to take account locking issue it will introduce.
>
>  >
>  > No, you are not missing anything. IPv4 doesn't use the cookie and
> always seems to pass it as 0.
>  >
>  > Yes, ipv4 will bump the gen_id thus invalidating all routes (there
> has been disagreement about it).
>  > IPv6 doesn't do that. In ipv6, when the addresses are added or
> removed, routes are also added or removed and
>  > any time the route is added it will have a new serial number. So, you
> don't have to disturb ipv4 cache when ipv6 routing info changes.
>
> Thank you very much for your explanation!
>
> IPv6 don't bump gen_id, when adding/deleting address, and tag an serial
> number with each route.
> Doing this way loose the semantic of dst_check, because SCTP depends no
> dst_check fulfill its
> duty to actually check whether the holding dst is still valid, well most
> other Layer 4 protocol
> simply rely on ip6_route_output/ip6_dst_lookup_flow to grab dst every
> time sending data out.

Look at how other protocols (tcp, dccp) do this.  It is sufficient to 
cache the route serial number into the dst_cookie at the time the route
was lookup-up and cached.  Then the cookie is passed to dst_check to 
validate the route.

-vlad
>
> So please pronounce a final judgment.
>
>> -vlad
>>
>>>
>>> diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
>>> index cd89510..0a646a5 100644
>>> --- a/include/net/sctp/sctp.h
>>> +++ b/include/net/sctp/sctp.h
>>> @@ -724,7 +724,7 @@ static inline void sctp_v4_map_v6(union sctp_addr
>>> *addr)
>>> */
>>> static inline struct dst_entry *sctp_transport_dst_check(struct
>>> sctp_transport *t)
>>> {
>>> - if (t->dst && !dst_check(t->dst, 0)) {
>>> + if (t->dst && !dst_check(t->dst, t->dst_cookie)) {
>>> dst_release(t->dst);
>>> t->dst = NULL;
>>> }
>>> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
>>> index 1bd4c41..cafdd19 100644
>>> --- a/include/net/sctp/structs.h
>>> +++ b/include/net/sctp/structs.h
>>> @@ -946,6 +946,8 @@ struct sctp_transport {
>>> __u64 hb_nonce;
>>>
>>> struct rcu_head rcu;
>>> +
>>> + u32 dst_cookie;
>>> };
>>>
>>> struct sctp_transport *sctp_transport_new(struct net *, const union
>>> sctp_addr *,
>>> diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
>>> index 8ee553b..82a420f 100644
>>> --- a/net/sctp/ipv6.c
>>> +++ b/net/sctp/ipv6.c
>>> @@ -350,6 +350,7 @@ out:
>>> struct rt6_info *rt;
>>> rt = (struct rt6_info *)dst;
>>> t->dst = dst;
>>> + t->dst_cookie = rt->rt6i_node ? rt->rt6i_node->fn_sernum
>>> : 0;
>>> SCTP_DEBUG_PRINTK("rt6_dst:%pI6 rt6_src:%pI6\n",
>>> &rt->rt6i_dst.addr, &fl6->saddr);
>>> } else {
>>>
>>>
>>>
>>>
>>>
>>>> -vlad
>>>>
>>>>>
>>>>> return t->dst;
>>>>> diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
>>>>> index 8ee553b..cfae77e 100644
>>>>> --- a/net/sctp/ipv6.c
>>>>> +++ b/net/sctp/ipv6.c
>>>>> @@ -137,6 +137,8 @@ static int sctp_inet6addr_event(struct
>>>>> notifier_block *this, unsigned long ev,
>>>>> break;
>>>>> }
>>>>>
>>>>> + /* invalid all transport dst forcing to look up new dst */
>>>>> + rt_genid_bump(net);
>>>>> return NOTIFY_DONE;
>>>>> }
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>>
>