On Wed, Feb 7, 2018 at 12:23 PM, Yves-Alexis Perez corsac@debian.org wrote:

On Wed, 2018-02-07 at 18:05 +0100, Yves-Alexis Perez wrote:

...

I'll try to printk the mtu before returning EINVAL to see why it's lower than 1280, but maybe the IP encapsulation is not correctly handled?

I did:

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c index 3763dc01e374..d3c651158d35 100644 --- a/net/ipv6/ip6_output.c +++ b/net/ipv6/ip6_output.c @@ -1215,7 +1215,7 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork, mtu = np->frag_size; } if (mtu < IPV6_MIN_MTU)

•           return -EINVAL;
  

•           printk("mtu: %d\n", mtu);
    cork->base.fragsize = mtu;
    if (dst_allfrag(rt->dst.path))
            cork->base.flags |= IPCORK_ALLFRAG;
  
  

and I get:

févr. 07 18:19:50 scapa kernel: mtu: 1218

and it doesn't depend on the original packet size (same thing happens with ping -s 100). It also happens with UDP (DNS) traffic, but apparently not with TCP.

Regards,

Yves-Alexis

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.

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.

Thanks. -- Mike Maloney