[DCCP]: Support for partial checksums (RFC 4340, sec. 9.2)

This patch does the following:
  a) introduces variable-length checksums as specified in [RFC 4340, sec. 9.2]
  b) provides necessary socket options and documentation as to how to use them
  c) basic support and infrastructure for the Minimum Checksum Coverage feature
     [RFC 4340, sec. 9.2.1]: acceptability tests, user notification and user
     interface

In addition, it

 (1) fixes two bugs in the DCCPv4 checksum computation:
 	* pseudo-header used checksum_len instead of skb->len
	* incorrect checksum coverage calculation based on dccph_x
 (2) removes dccp_v4_verify_checksum() since it reduplicates code of the
     checksum computation; code calling this function is updated accordingly.
 (3) now uses skb_checksum(), which is safer than checksum_partial() if the
     sk_buff has is a non-linear buffer (has pages attached to it).
 (4) fixes an outstanding TODO item:
        * If P.CsCov is too large for the packet size, drop packet and return.

The code has been tested with applications, the latest version of tcpdump now
comes with support for partial DCCP checksums.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 24 insertions, 5 deletions

diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index 2990bfb12587..d5c414bf7819 100644
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -129,6 +129,30 @@ DECLARE_SNMP_STAT(struct dccp_mib, dccp_statistics);
 #define DCCP_ADD_STATS_USER(field, val) \
                         SNMP_ADD_STATS_USER(dccp_statistics, field, val)
 
+/*
+ *      Checksumming routines
+ */
+static inline int dccp_csum_coverage(const struct sk_buff *skb)
+{
+        const struct dccp_hdr* dh = dccp_hdr(skb);
+
+        if (dh->dccph_cscov == 0)
+                return skb->len;
+        return (dh->dccph_doff + dh->dccph_cscov - 1) * sizeof(u32);
+}
+
+static inline void dccp_csum_outgoing(struct sk_buff *skb)
+{
+        int cov = dccp_csum_coverage(skb);
+
+        if (cov >= skb->len)
+                dccp_hdr(skb)->dccph_cscov = 0;
+
+        skb->csum = skb_checksum(skb, 0, (cov > skb->len)? skb->len : cov, 0);
+}
+
+extern void dccp_v4_send_check(struct sock *sk, int len, struct sk_buff *skb);
+
 extern int  dccp_retransmit_skb(struct sock *sk, struct sk_buff *skb);
 
 extern void dccp_send_ack(struct sock *sk);
@@ -214,14 +238,9 @@ extern void	   dccp_shutdown(struct sock *sk, int how);
 extern int         inet_dccp_listen(struct socket *sock, int backlog);
 extern unsigned int dccp_poll(struct file *file, struct socket *sock,
                              poll_table *wait);
-extern void        dccp_v4_send_check(struct sock *sk, int len,
-                                      struct sk_buff *skb);
 extern int         dccp_v4_connect(struct sock *sk, struct sockaddr *uaddr,
                                    int addr_len);
 
-extern int         dccp_v4_checksum(const struct sk_buff *skb,
-                                    const __be32 saddr, const __be32 daddr);
-
 extern int         dccp_send_reset(struct sock *sk, enum dccp_reset_codes code);
 extern void        dccp_send_close(struct sock *sk, const int active);
 extern int         dccp_invalid_packet(struct sk_buff *skb);