Re: [Bugme-new] [Bug 15682] New: XFRM is not updating RTAX_ADVMSS metric

[Eduardo Panisset <eduardo.panisset@gmail.com>]

Hi,

My intention is only to report a problem that I have faced. The
solution proposed isn't (I know that) probably the best one to adopt
as I'm not a kernel specialist, it is more illustrative to allow you
guys understanding what I'm meaning and solve that on the better way
(hence I haven't submited a patch).

Regards,
Eduardo Panisset.

On Tue, Apr 6, 2010 at 10:40 AM, jamal <hadi@cyberus.ca> wrote:
>
> Herbert would give better answers. I dont think what Eduardo is
> doing is correct. You cant just start factoring in tcp headers
> at the xfrm level - and besides, the mtu calculation
> already takes care tunnel headers - so tcp should be able to
> compute correct MSS.
>
> cheers,
> jamal
>
> On Mon, 2010-04-05 at 12:50 -0700, Andrew Morton wrote:
>> (switched to email.  Please respond via emailed reply-to-all, not via the
>> bugzilla web interface).
>>
>> On Fri, 2 Apr 2010 17:34:35 GMT
>> bugzilla-daemon@bugzilla.kernel.org wrote:
>>
>> > https://bugzilla.kernel.org/show_bug.cgi?id=15682
>> >
>> >            Summary: XFRM is not updating RTAX_ADVMSS metric
>> >            Product: Networking
>> >            Version: 2.5
>> >     Kernel Version: 2.6.28-2
>> >           Platform: All
>> >         OS/Version: Linux
>> >               Tree: Mainline
>> >             Status: NEW
>> >           Severity: normal
>> >           Priority: P1
>> >          Component: Other
>> >         AssignedTo: acme@ghostprotocols.net
>> >         ReportedBy: eduardo.panisset@gmail.com
>> >         Regression: No
>> >
>> >
>> > I have been testing DSMIPv6 code which uses all kind of advanced
>> > features of XFRM framework and I believe I have found a bug related to
>> > update RTAX_ADVMSS route metric.
>> > The XFRM code on net/xfrm/xfrm_policy.c by its functions
>> > xfrm_init_pmtu and xfrm_bundle_ok updates RTAX_MTU route caching
>> > metric however I believe it must update RTAX_ADVMSS as this later is
>> > used by tcp connect function for adverting the MSS value on SYN
>> > messages.
>> >
>> > As MSS is not being updated by XFRM the TCP SYN messages (e.g.
>> > originated from a internet browser)  is erroneously informing its MSS
>> > (without taking into account the overhead added to IP packet size by
>> > XFRM transformations).  One result of that is the browser gets
>> > "frozen" after starts a TCP connection because TCP messages sent by
>> > TCP server will never get to it (TCP server is sending too large
>> > segments to browser).
>> >
>> > Below I describe the changes I have done (on xfrm_init_pmtu and
>> > xfrm_bundle_ok) and that seem to fix this problem:
>> >
>> > xfrm_init_pmtu:
>> >                  .
>> >                  .
>> >                  .
>> >
>> >         dst->metrics[RTAX_MTU-1] = pmtu; // original code, below my changes
>> >
>> >         if (dst->xfrm->props.mode == XFRM_MODE_TUNNEL)
>> >                  switch (dst->xfrm->props.family)
>> >                  {
>> >                  case AF_INET:
>> >                  dst->metrics[RTAX_ADVMSS-1] = max_t(unsigned int,
>> > pmtu - sizeof(struct iphdr) - sizeof(struct tcphdr), 256);
>> >                  break;
>> >
>> >                  case AF_INET6:
>> >                  dst->metrics[RTAX_ADVMSS-1] = max_t(unsigned int,
>> > pmtu - sizeof(struct ipv6hdr) - sizeof(struct tcphdr),
>> >                             dev_net(dst->dev)->ipv6.
>> > sysctl.ip6_rt_min_advmss);
>> >                  break;
>> >                  }
>> >
>> > xfrm_bundle_ok:
>> >
>> >                .
>> >                .
>> >                .
>> >
>> >         dst->metrics[RTAX_MTU-1] = mtu; // original code, below my changes
>> >
>> >         if (dst->xfrm->props.mode == XFRM_MODE_TUNNEL)
>> >                 switch (dst->xfrm->props.family)
>> >                 {
>> >                 case AF_INET:
>> >                         dst->metrics[RTAX_ADVMSS-1] = max_t(unsigned
>> > int, mtu - sizeof(struct iphdr) - sizeof(struct tcphdr), 256);
>> >                 break;
>> >
>> >                 case AF_INET6:
>> >                         dst->metrics[RTAX_ADVMSS-1] = max_t(unsigned
>> > int, mtu - sizeof(struct ipv6hdr) - sizeof(struct tcphdr),
>> >
>> > dev_net(dst->dev)->ipv6.sysctl.ip6_rt_min_advmss);
>> >                 break;
>> >                 }
>> >
>>
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