[PATCH net-next v2] net: Loosen constraints for recalculating checksum in skb_segment()

[Simon Horman <horms@verge.net.au>]

In the case where the ability to offload checksums changes
then it may be necessary for software checksumming of an skb to occur.

An example of such a case is where a non-GRE packet is received but
is to be encapsulated and transmitted as GRE.

Another example relates to my proposed support for for GSO packets
that are non-MPLS when received but MPLS when transmitted.

The cost of this change is that csum may be checked when it previously
was not. In the case where csum is true this is pure overhead. In the
case where csum is false it leads to software checksumming, which
I believe also leads to correct checksums in transmitted packets for
the cases described above.

This patch relies on the return value of can_checksum_protocol()
being correct and in turn the return value of skb_network_protocol(),
used to provide the protocol parameter of can_checksum_protocol(),
being correct. It also relies on the features passed to skb_segment()
and in turn to can_checksum_protocol() being correct.

I believe that this problem does has not been observed for VLANs because it
appears that all drivers except xgbe set vlan_features such that that the
checksum offload support for VLAN packets is greater than or equal to that
of non-VLAN packets.

I wonder if the code in xgbe may be an oversight and the hardware does
support checksumming of VLAN packets.  If so it may be worth updating the
vlan_features of the driver as this patch will force such checksums to be
performed in software rather than hardware.

Signed-off-by: Simon Horman <horms@verge.net.au>

---

Tested using the bnx2 driver.

v2
Rewrite changlog after further analysis
---
 net/core/skbuff.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index af9185d..3a43be4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2853,7 +2853,7 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
 						 doffset + tnl_hlen);
 
 		if (fskb != skb_shinfo(skb)->frag_list)
-			continue;
+			goto csum;
 
 		if (!sg) {
 			nskb->ip_summed = CHECKSUM_NONE;
@@ -2917,6 +2917,7 @@ skip_fraglist:
 		nskb->len += nskb->data_len;
 		nskb->truesize += nskb->data_len;
 
+csum:
 		if (!csum) {
 			nskb->csum = skb_checksum(nskb, doffset,
 						  nskb->len - doffset, 0);
-- 
1.8.2.1