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

On 2013年07月03日 21:23, Vlad Yasevich wrote:

On 07/02/2013 10:18 PM, Fan Du wrote:

quoted

On 2013年07月02日 22:29, Vlad Yasevich wrote:

quoted

On 07/02/2013 02:39 AM, Fan Du wrote:

quoted

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 <redacted>
---
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.

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.

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.

So please pronounce a final judgment.

-vlad

quoted

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 {

quoted

-vlad

quoted

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