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

[jamal <hadi@cyberus.ca>]

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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