[NET]: Use csum_start offset instead of skb_transport_header

The skb transport pointer is currently used to specify the start
of the checksum region for transmit checksum offload.  Unfortunately,
the same pointer is also used during receive side processing.

This creates a problem when we want to retransmit a received
packet with partial checksums since the skb transport pointer
would be overwritten.

This patch solves this problem by creating a new 16-bit csum_start
offset value to replace the skb transport header for the purpose
of checksums.  This offset is calculated from skb->head so that
it does not have to change when skb->data changes.

No extra space is required since csum_offset itself fits within
a 16-bit word so we can use the other 16 bits for csum_start.

For backwards compatibility, just before we push a packet with
partial checksums off into the device driver, we set the skb
transport header to what it would have been under the old scheme.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 3 insertions, 0 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index e11eaf4cc269..a091a99ad263 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -504,6 +504,7 @@ void tcp_v4_send_check(struct sock *sk, int len, struct sk_buff *skb)
         if (skb->ip_summed == CHECKSUM_PARTIAL) {
                 th->check = ~tcp_v4_check(len, inet->saddr,
                                           inet->daddr, 0);
+                skb->csum_start = skb_transport_header(skb) - skb->head;
                 skb->csum_offset = offsetof(struct tcphdr, check);
         } else {
                 th->check = tcp_v4_check(len, inet->saddr, inet->daddr,
@@ -526,6 +527,7 @@ int tcp_v4_gso_send_check(struct sk_buff *skb)
 
         th->check = 0;
         th->check = ~tcp_v4_check(skb->len, iph->saddr, iph->daddr, 0);
+        skb->csum_start = skb_transport_header(skb) - skb->head;
         skb->csum_offset = offsetof(struct tcphdr, check);
         skb->ip_summed = CHECKSUM_PARTIAL;
         return 0;
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 71b0b60ba538..5ad7a26e3091 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -427,6 +427,7 @@ static void udp4_hwcsum_outgoing(struct sock *sk, struct sk_buff *skb,
                 /*
                  * Only one fragment on the socket.
                  */
+                skb->csum_start = skb_transport_header(skb) - skb->head;
                 skb->csum_offset = offsetof(struct udphdr, check);
                 uh->check = ~csum_tcpudp_magic(src, dst, len, IPPROTO_UDP, 0);
         } else {