netdev - SCTP over GRE (w csum)

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Message-ID: <20191223174535.GF6274@localhost.localdomain>
Date:   Mon, 23 Dec 2019 18:45:35 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     netdev@...r.kernel.org
Cc:     dcaratti@...hat.com, lucien.xin@...il.com,
        marcelo.leitner@...il.com
Subject: SCTP over GRE (w csum)

Hi all,

I am currently working on the following scenario:

sctp --> ipv4 --> gretap --> ipv6 --> eth

If NETIF_F_SCTP_CRC is not supported by the network device (it is the
case for gre), sctp will fallback computing the crc32 in sw with
sctp_gso_make_checksum(), where SKB_GSO_CB(skb)->csum is set to ~0 by
gso_reset_checksum(). After the gso segmentation, gre_gso_segment()
will try to compute gre csum with gso_make_checksum() even if skb->ip_summed
is set to CHECKSUM_NONE (and so using ~0 as partial).
One possible (trivial and not tested) solution would be to recompute the
gre checksum, doing in gre_gso_segment() something like:

	if (skb->ip_summed == CHECKSUM_NONE) {
		...
		err = skb_checksum_help(skb);
		if (err < 0)
			return return ERR_PTR(err);
	} else {
		*pcsum = gso_make_checksum(skb, 0);
	}

One possible improvement would be offload the GRE checksum computation if the
hw exports this capability in netdev_features and fall back to the sw
implementation if not.
Am I missing something? Is there a better approach?

Regards,
Lorenzo

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