On Wed, 2018-02-07 at 13:50 -0500, Mike Maloney wrote: > On Wed, Feb 7, 2018 at 12:23 PM, Yves-Alexis Perez <corsac@xxxxxxxxxx> > > Hi Yves-Alexis - > > I apologize for the problem. It seems to me that tunneling with an > outer MTU that causes the inner MTU to be smaller than the min, is > potentially problematic in other ways as well. Maybe. I tried with removing the MTU setting, and I get (on ping again) févr. 07 20:44:01 scapa kernel: mtu: 1266 which means I would get -EINVAL on standards kernels, which is not really good either. > But also it could seem unfortunate that the code with my fix does not > look at actual packet size, but instead only looks at the MTU and then > fails, even if no packet was going to be so large. The intention of > my patch was to prevent a negative number while calculating the > maxfraglen in __ip6_append_data(). An alternative fix maybe to > instead return an error only if the mtu is less than or equal to the > fragheaderlen. Something like: > > diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c > index 3763dc01e374..5d912a289b95 100644 > --- a/net/ipv6/ip6_output.c > +++ b/net/ipv6/ip6_output.c > @@ -1214,8 +1214,6 @@ static int ip6_setup_cork(struct sock *sk, > struct inet_cork_full *cork, > if (np->frag_size) > mtu = np->frag_size; > } > - if (mtu < IPV6_MIN_MTU) > - return -EINVAL; > cork->base.fragsize = mtu; > if (dst_allfrag(rt->dst.path)) > cork->base.flags |= IPCORK_ALLFRAG; > @@ -1264,6 +1262,8 @@ static int __ip6_append_data(struct sock *sk, > > fragheaderlen = sizeof(struct ipv6hdr) + rt->rt6i_nfheader_len + > (opt ? opt->opt_nflen : 0); > + if (mtu < fragheaderlen + 8) > + return -EINVAL; > maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen - > sizeof(struct frag_hdr); > (opt ? opt->opt_nflen : 0); > > But then we also have to convince ourselves that maxfraglen can never > be <= 0. I'd have to think about that. > > I am not sure if others have thoughts on supporting MTUs configured > below the min in the spec. > Here, the MTU is not below, so I'm not sure what happens. Regards, -- Yves-Alexis
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