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

On 07/05/2013 10:09 AM, Vlad Yasevich wrote:

On 07/03/2013 10:33 PM, Fan Du wrote:

quoted

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

quoted

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.

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.

quoted

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.

quoted

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

Thanks
-vlad

-vlad

quoted

So please pronounce a final judgment.

quoted

-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;
}