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>

3 files changed, 40 insertions, 2 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 7a9beeb1c458..d8d397acb52c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -567,7 +567,8 @@ struct sk_buff {
          * headers if needed
          */
         __u8                    encapsulation:1;
-        /* 6/8 bit hole (depending on ndisc_nodetype presence) */
+        __u8                    encap_hdr_csum:1;
+        /* 5/7 bit hole (depending on ndisc_nodetype presence) */
         kmemcheck_bitfield_end(flags2);
 
 #if defined CONFIG_NET_DMA || defined CONFIG_NET_RX_BUSY_POLL
@@ -2988,6 +2989,7 @@ static inline struct sec_path *skb_sec_path(struct sk_buff *skb)
 struct skb_gso_cb {
         int     mac_offset;
         int     encap_level;
+        __u16   csum_start;
 };
 #define SKB_GSO_CB(skb) ((struct skb_gso_cb *)(skb)->cb)
 
@@ -3012,6 +3014,28 @@ static inline int gso_pskb_expand_head(struct sk_buff *skb, int extra)
         return 0;
 }
 
+/* Compute the checksum for a gso segment. First compute the checksum value
+ * from the start of transport header to SKB_GSO_CB(skb)->csum_start, and
+ * then add in skb->csum (checksum from csum_start to end of packet).
+ * skb->csum and csum_start are then updated to reflect the checksum of the
+ * resultant packet starting from the transport header-- the resultant checksum
+ * is in the res argument (i.e. normally zero or ~ of checksum of a pseudo
+ * header.
+ */
+static inline __sum16 gso_make_checksum(struct sk_buff *skb, __wsum res)
+{
+        int plen = SKB_GSO_CB(skb)->csum_start - skb_headroom(skb) -
+            skb_transport_offset(skb);
+        __u16 csum;
+
+        csum = csum_fold(csum_partial(skb_transport_header(skb),
+                                      plen, skb->csum));
+        skb->csum = res;
+        SKB_GSO_CB(skb)->csum_start -= plen;
+
+        return csum;
+}
+
 static inline bool skb_is_gso(const struct sk_buff *skb)
 {
         return skb_shinfo(skb)->gso_size;
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);
 
diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 847e69cbff7e..f4c987bb7e94 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -135,6 +135,14 @@ struct sk_buff *iptunnel_handle_offloads(struct sk_buff *skb,
                 return skb;
         }
 
+        /* If packet is not gso and we are resolving any partial checksum,
+         * clear encapsulation flag. This allows setting CHECKSUM_PARTIAL
+         * on the outer header without confusing devices that implement
+         * NETIF_F_IP_CSUM with encapsulation.
+         */
+        if (csum_help)
+                skb->encapsulation = 0;
+
         if (skb->ip_summed == CHECKSUM_PARTIAL && csum_help) {
                 err = skb_checksum_help(skb);
                 if (unlikely(err))