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

From: Fan Du <[email-masked]>
Date: Wed, 10 Jul 2013 13:26:51 +0800

On 2013年07月09日 23:11, Vlad Yasevich wrote:
> On 07/05/2013 10:09 AM, Vlad Yasevich wrote:
>> 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.
>
>
> Hi Fan
>
> Have you tried the updated patch you sent? Based on what the tcp/udp code is doing, the updated patch should work correctly. If it does, can you re-post with attribution/sign-off
>

Hello Vlad and Neil

I don't know whether my test procedure has drawbacks or something else.
It seems the updated patch does not works well, but my first patch is ok under below configuration.

Host A:
	ifconfig eth1 inet6 add 2001::803/64
	ifconfig eth1 inet6 add 2001::804/64
	sctp_darn -H 2001::803 -B 2001::804 -P 5001 -l

Host B:
	ifconfig eth1 inet6 add 2001::805/64
	ifconfig eth1 inet6 add 2001::806/64
         sctp_darn -H 2001::805 -B 2001::806 -P 5002 -h 2001::803 -p 5001 -s

hbinterval set to 2 seconds, after setup the association, primary address: 2001::804 <--> 2001::806

Step 1:

After adding in IPv6 address to an interface, we populate
struct inet6_ifaddr *ifp->rt->rt6i_node, this rt6i_node is stored in a radix tree.

addrconf_add_ifaddr
  ->inet6_addr_add
     ->ipv6_add_addr  <-- Do DAD checking afterward.
     ->addrconf_prefix_route
       ->ip6_route_add
         ->__ip6_ins_rt
           ->fib6_add
             ->fib6_add_1  <-- Create struct fib6_node *fn
               ->fib6_add_rt2node
                 ->rt->rt6i_node = fn; (*1*)

Step 2:

In host A ,for the packet destinate for 2001::806 using source address 2001::804,
In sctp_v6_get_dst, let's print the rt, rt->rt6i_node, and rt->rt6i_node->fn_sernum
after ip6_dst_lookup_flow got valid dst. The problem is the transport->dst->rt6i_node
we have looked for is *not* the rt6i_node in (*1*), I'm not drunk here.....
So in Step 3, let's be naughty: ifconfig eth1 inet6 del 2001::804/64

Step 3:

Then we delete the IPv6 address, certainly this struct inet6_ifaddr *ifp->rt
will be delete as well.

addrconf_del_ifaddr
   ->inet6_addr_del
     ->ipv6_del_addr
       ->ipv6_ifa_notify /RTM_DELADDR
         ->ip6_del_rt
           ->__ip6_del_rt
             ->fib6_del
               ->fib6_del_route
                -> rt->rt6i_node = NULL; (*2*) here we undo the operation in (*1*),
                                               but rt6i_node in Step2 is untouched!

Step 4:
Then we do the dst checking, for sctp_tranport_dst_check it looks like nothing in Step3
has consequence on it. So transport->dst is still valid, let's ship the packet out
using 2001::804. The real world test result is ASSOCIATION ABORT after a while.
As far as I can tell, rt in Step 2 got deleted *after* the ASSOCIATION ABORT.

    sctp_tranport_dst_check
       ->dst_check
         -> ip6_dst_check

Root node
    *
   / \
      * <-- fn node we are using.
     / \
        *
       / \
      *
     / \
        * <--- Add a new fn node, on the path from the root node down to here, each fn
               now has new fn_sernum, which force dst_check return NULL for lookup again.
               Then previously save fn_sernum will take effect now. But when delete address
               only the ifp->rt->rt6i_node is set to NULL.

> Thanks
> -vlad
>
>>
>> -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;
>>>>>>> }
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>
>

-- 
浮沉随浪只记今朝笑

--fan