[PATCH] ipv6: remove min MTU check for tunnel destinations

[PATCH] ipv6: remove min MTU check for tunnel destinations

[Ashwanth Goli]

With 749439bfac "fix udpv6 sendmsg crash caused by too small MTU"
tunnel dst's that report a MTU less than IPV6_MIN_MTU are broken
even for packets that are smaller than IPV6_MIN_MTU.

According to rfc2473#section-7.1

    if the original IPv6 packet is equal or  smaller  than  the
    IPv6 minimum link MTU, the tunnel entry-point node
    encapsulates the original packet, and subsequently
    fragments the resulting IPv6 tunnel packet into IPv6
    fragments that do not exceed the Path MTU to the tunnel
    exit-point.

This patch drops the MTU check for tunnel destinations.

Signed-off-by: Ashwanth Goli <ashwanth@codeaurora.org>
---
 net/ipv6/ip6_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 2e891d2..c4c3313 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1235,7 +1235,7 @@ 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)
+	if (!(rt->dst.flags & DST_XFRM_TUNNEL) && mtu < IPV6_MIN_MTU)
 		return -EINVAL;
 	cork->base.fragsize = mtu;
 	if (dst_allfrag(xfrm_dst_path(&rt->dst)))
-- 
1.9.1

Re: [PATCH] ipv6: remove min MTU check for tunnel destinations

[David Miller]

From: Ashwanth Goli <ashwanth@codeaurora.org>
Date: Mon, 30 Apr 2018 00:36:47 +0530

> With 749439bfac "fix udpv6 sendmsg crash caused by too small MTU"
> tunnel dst's that report a MTU less than IPV6_MIN_MTU are broken
> even for packets that are smaller than IPV6_MIN_MTU.
> 
> According to rfc2473#section-7.1
> 
>     if the original IPv6 packet is equal or  smaller  than  the
>     IPv6 minimum link MTU, the tunnel entry-point node
>     encapsulates the original packet, and subsequently
>     fragments the resulting IPv6 tunnel packet into IPv6
>     fragments that do not exceed the Path MTU to the tunnel
>     exit-point.
> 
> This patch drops the MTU check for tunnel destinations.
> 
> Signed-off-by: Ashwanth Goli <ashwanth@codeaurora.org>

RFC 2473 is generally about ipv6 tunnels....

> -	if (mtu < IPV6_MIN_MTU)
> +	if (!(rt->dst.flags & DST_XFRM_TUNNEL) && mtu < IPV6_MIN_MTU)
>  		return -EINVAL;

But the check you are adding is specifically checking only IPSEC
tunnels.

If what you say is true in your commit message, this test must
more generally trigger for all ipv6 tunnel types, not just IPSEC
ones.

If IPSEC tunnels are being targetting in this patch intentionally,
that needs to be explained in the commit message.

Re: [PATCH] ipv6: remove min MTU check for tunnel destinations

[Ashwanth Goli]

On 2018-05-01 21:52, David Miller wrote:
> From: Ashwanth Goli <ashwanth@codeaurora.org>
> Date: Mon, 30 Apr 2018 00:36:47 +0530
> 
>> With 749439bfac "fix udpv6 sendmsg crash caused by too small MTU"
>> tunnel dst's that report a MTU less than IPV6_MIN_MTU are broken
>> even for packets that are smaller than IPV6_MIN_MTU.
>> 
>> According to rfc2473#section-7.1
>> 
>>     if the original IPv6 packet is equal or  smaller  than  the
>>     IPv6 minimum link MTU, the tunnel entry-point node
>>     encapsulates the original packet, and subsequently
>>     fragments the resulting IPv6 tunnel packet into IPv6
>>     fragments that do not exceed the Path MTU to the tunnel
>>     exit-point.
>> 
>> This patch drops the MTU check for tunnel destinations.
>> 
>> Signed-off-by: Ashwanth Goli <ashwanth@codeaurora.org>
> 
> RFC 2473 is generally about ipv6 tunnels....
> 
>> -	if (mtu < IPV6_MIN_MTU)
>> +	if (!(rt->dst.flags & DST_XFRM_TUNNEL) && mtu < IPV6_MIN_MTU)
>>  		return -EINVAL;
> 
> But the check you are adding is specifically checking only IPSEC
> tunnels.
> 
> If what you say is true in your commit message, this test must
> more generally trigger for all ipv6 tunnel types, not just IPSEC
> ones.
> 
> If IPSEC tunnels are being targetting in this patch intentionally,
> that needs to be explained in the commit message.

My intention is to fix the issue for IPSEC tunnels. Will resend by 
changing the commit text.