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

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Date:	Fri, 5 Jul 2013 09:03:53 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Fan Du <fan.du@...driver.com>
Cc:	Vlad Yasevich <vyasevich@...il.com>, nicolas.dichtel@...nd.com,
	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [RFC PATCH] sctp: Don't lookup dst if transport dst is still
 valid

On Thu, Jul 04, 2013 at 10:33:35AM +0800, 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 <fan.du@...driver.com>
> >>>>---
> >>>>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.
> 
Have you tried this?  It seems to me in the situation you describe, deleting a
source address will result in fib_inetaddr_event getting called, which will call
rt_cache_flush, bumping the rt_genid to get bumped for that network namespace.
That will cause any subsequent calls to dst_check->ip6_dst_check to return NULL
rather than the cached dst_entry for that transport.

Neil

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