net: Support for multiple checksums with gso

When creating a GSO packet segment we may need to set more than
one checksum in the packet (for instance a TCP checksum and
UDP checksum for VXLAN encapsulation). To be efficient, we want
to do checksum calculation for any part of the packet at most once.

This patch adds csum_start offset to skb_gso_cb. This tracks the
starting offset for skb->csum which is initially set in skb_segment.
When a protocol needs to compute a transport checksum it calls
gso_make_checksum which computes the checksum value from the start
of transport header to csum_start and then adds in skb->csum to get
the full checksum. skb->csum and csum_start are then updated to reflect
the checksum of the resultant packet starting from the transport header.

This patch also adds a flag to skbuff, encap_hdr_csum, which is set
in *gso_segment fucntions to indicate that a tunnel protocol needs
checksum calculation

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 1 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 3f6c7e8be8a4..05f4bef2ce12 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2885,7 +2885,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
         if (unlikely(!proto))
                 return ERR_PTR(-EINVAL);
 
-        csum = !!can_checksum_protocol(features, proto);
+        csum = !head_skb->encap_hdr_csum &&
+            !!can_checksum_protocol(features, proto);
+
         __skb_push(head_skb, doffset);
         headroom = skb_headroom(head_skb);
         pos = skb_headlen(head_skb);
@@ -2983,6 +2985,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
                         nskb->csum = skb_copy_and_csum_bits(head_skb, offset,
                                                             skb_put(nskb, len),
                                                             len, 0);
+                        SKB_GSO_CB(nskb)->csum_start =
+                            skb_headroom(nskb) + offset;
                         continue;
                 }
 
@@ -3052,6 +3056,8 @@ perform_csum_check:
                         nskb->csum = skb_checksum(nskb, doffset,
                                                   nskb->len - doffset, 0);
                         nskb->ip_summed = CHECKSUM_NONE;
+                        SKB_GSO_CB(nskb)->csum_start =
+                            skb_headroom(nskb) + doffset;
                 }
         } while ((offset += len) < head_skb->len);