Subject: Re: [RFC] Big TCP and ping support vs. max ICMP{,v6} packet size

From: Petr Vorel <[email-masked]>
Date: Wed, 21 Aug 2024 22:12:35 +0200

Hi Eric, Xin,

> On Tue, Aug 20, 2024 at 5:38 PM Petr Vorel <[email-masked]> wrote:

> > Hi Eric,

> > > On Mon, Aug 19, 2024 at 2:50 PM Petr Vorel <[email-masked]> wrote:

> > > > Hi Eric, Xin,

> > > > I see you both worked on Big TCP support for IPv4/IPv6. I wonder if anybody was
> > > > thinking about add Big TCP to raw socket or ICMP datagram socket. I'm not sure
> > > > what would be a real use case (due MTU limitation is Big TCP mostly used on
> > > > local networks anyway).

> > > I think you are mistaken.

> > > BIG TCP does not have any MTU restrictions and can be used on any network.

> > > Think about BIG TCP being GSO/TSO/GRO with bigger logical packet sizes.

> > First, thanks for a quick info. I need to study more BIG TCP. Because I was
> > wondering if this could be used for sending larger ICMP echo requests > 65k
> > as it's possible in FreeBSD, where it's done via Jumbograms [1]:

> >         ping -6 -b 70000 -s 68000 ::1

> I guess ip6_append_data() is a bit conservative and uses IPV6_MAXPLEN
> while it should not ;)

> Also ping needs to add the jumboheader if/when using RAW6 sockets

First I thought you mean to modify kernel net/ipv6/raw.c and net/ipv6/icmp.c
(+ net/ipv4/ping.c for ICMP datagram socket). I.e. to create "Big RAW" and "Big
UDP" (maybe the modification could be in just in net/ipv6/icmp.c for both types
of sockets).

But thinking it twice you may mean to modify userspace ping to add jumboheader.

> With the following patch, the following commands sends big packets just fine

> ifconfig lo mtu 90000
> ping -s 68000 ::1

Yes, it looks like with the above patch it's possible to send a bigger packet,
it goes from userspace to kernel, but here is broken.

>From what I observed for 65528 (the first value which exceeds the limit) on raw
socket (net/ipv6/raw.c, net/ipv6/ip6_output.c), rawv6_sendmsg() calls
ip6_append_data() and after that somewhere in 3rd pskb_pull() call skb->data_len
(unsigned int) changes from 65528 to 0, skb->len from 65576 to 40 (IP header).
Also checksum (likely due this) fails.

ICMP datagram socket starts with net/ipv[46]/ping.c but ping_v6_sendmsg() also
calls ip6_append_data() and suffers the same problem.
+ I obviously needed to commented out the check in ping_common_sendmsg()

	if (len > 0xFFFF)
		return -EMSGSIZE;

I'm obviously missing something.

Kind regards,
Petr

> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index ab504d31f0cdd8dec9ab01bf9d6e6517307609cd..6b1668e037dae3c88052c50f02f319355baf4304
> 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1473,7 +1473,7 @@ static int __ip6_append_data(struct sock *sk,
>         }

>         if (ip6_sk_ignore_df(sk))
> -               maxnonfragsize = sizeof(struct ipv6hdr) + IPV6_MAXPLEN;
> +               maxnonfragsize = max_t(u32, mtu, sizeof(struct
> ipv6hdr) + IPV6_MAXPLEN);
>         else
>                 maxnonfragsize = mtu;