[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, 52 insertions, 53 deletions

diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index 91bffaa..4961120 100644
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -349,13 +349,19 @@ out:
 	sock_put(sk);
 }
 
-/* This routine computes an IPv4 DCCP checksum. */
-void dccp_v4_send_check(struct sock *sk, int len, struct sk_buff *skb)
+static inline u16 dccp_v4_csum_finish(struct sk_buff *skb,
+				      __be32 src, __be32 dst)
+{
+	return csum_tcpudp_magic(src, dst, skb->len, IPPROTO_DCCP, skb->csum);
+}
+
+void dccp_v4_send_check(struct sock *sk, int unused, struct sk_buff *skb)
 {
 	const struct inet_sock *inet = inet_sk(sk);
 	struct dccp_hdr *dh = dccp_hdr(skb);
 
-	dh->dccph_checksum = dccp_v4_checksum(skb, inet->saddr, inet->daddr);
+	dccp_csum_outgoing(skb);
+	dh->dccph_checksum = dccp_v4_csum_finish(skb, inet->saddr, inet->daddr);
 }
 
 EXPORT_SYMBOL_GPL(dccp_v4_send_check);
@@ -454,47 +460,6 @@ static struct sock *dccp_v4_hnd_req(struct sock *sk, struct sk_buff *skb)
 	return sk;
 }
 
-int dccp_v4_checksum(const struct sk_buff *skb, const __be32 saddr,
-		     const __be32 daddr)
-{
-	const struct dccp_hdr* dh = dccp_hdr(skb);
-	int checksum_len;
-	u32 tmp;
-
-	if (dh->dccph_cscov == 0)
-		checksum_len = skb->len;
-	else {
-		checksum_len = (dh->dccph_cscov + dh->dccph_x) * sizeof(u32);
-		checksum_len = checksum_len < skb->len ? checksum_len :
-							 skb->len;
-	}
-
-	tmp = csum_partial((unsigned char *)dh, checksum_len, 0);
-	return csum_tcpudp_magic(saddr, daddr, checksum_len,
-				 IPPROTO_DCCP, tmp);
-}
-
-EXPORT_SYMBOL_GPL(dccp_v4_checksum);
-
-static int dccp_v4_verify_checksum(struct sk_buff *skb,
-				   const __be32 saddr, const __be32 daddr)
-{
-	struct dccp_hdr *dh = dccp_hdr(skb);
-	int checksum_len;
-	u32 tmp;
-
-	if (dh->dccph_cscov == 0)
-		checksum_len = skb->len;
-	else {
-		checksum_len = (dh->dccph_cscov + dh->dccph_x) * sizeof(u32);
-		checksum_len = checksum_len < skb->len ? checksum_len :
-							 skb->len;
-	}
-	tmp = csum_partial((unsigned char *)dh, checksum_len, 0);
-	return csum_tcpudp_magic(saddr, daddr, checksum_len,
-				 IPPROTO_DCCP, tmp) == 0 ? 0 : -1;
-}
-
 static struct dst_entry* dccp_v4_route_skb(struct sock *sk,
 					   struct sk_buff *skb)
 {
@@ -536,8 +501,8 @@ static int dccp_v4_send_response(struct sock *sk, struct request_sock *req,
 		const struct inet_request_sock *ireq = inet_rsk(req);
 		struct dccp_hdr *dh = dccp_hdr(skb);
 
-		dh->dccph_checksum = dccp_v4_checksum(skb, ireq->loc_addr,
-						      ireq->rmt_addr);
+		dh->dccph_checksum = dccp_v4_csum_finish(skb, ireq->loc_addr,
+							      ireq->rmt_addr);
 		memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
 		err = ip_build_and_send_pkt(skb, sk, ireq->loc_addr,
 					    ireq->rmt_addr,
@@ -602,8 +567,9 @@ static void dccp_v4_ctl_send_reset(struct sk_buff *rxskb)
 	dccp_hdr_set_ack(dccp_hdr_ack_bits(skb),
 			 DCCP_SKB_CB(rxskb)->dccpd_seq);
 
-	dh->dccph_checksum = dccp_v4_checksum(skb, rxskb->nh.iph->saddr,
-					      rxskb->nh.iph->daddr);
+	dccp_csum_outgoing(skb);
+	dh->dccph_checksum = dccp_v4_csum_finish(skb, rxskb->nh.iph->saddr,
+						      rxskb->nh.iph->daddr);
 
 	bh_lock_sock(dccp_v4_ctl_socket->sk);
 	err = ip_build_and_send_pkt(skb, dccp_v4_ctl_socket->sk,
@@ -779,6 +745,7 @@ EXPORT_SYMBOL_GPL(dccp_v4_do_rcv);
 int dccp_invalid_packet(struct sk_buff *skb)
 {
 	const struct dccp_hdr *dh;
+	unsigned int cscov;
 
 	if (skb->pkt_type != PACKET_HOST)
 		return 1;
@@ -830,6 +797,22 @@ int dccp_invalid_packet(struct sk_buff *skb)
 		return 1;
 	}
 
+	/*
+	 * If P.CsCov is too large for the packet size, drop packet and return.
+	 * This must come _before_ checksumming (not as RFC 4340 suggests).
+	 */
+	cscov = dccp_csum_coverage(skb);
+	if (cscov > skb->len) {
+		LIMIT_NETDEBUG(KERN_WARNING
+			       "DCCP: P.CsCov %u exceeds packet length %d\n",
+			       dh->dccph_cscov, skb->len);
+		return 1;
+	}
+
+	/* If header checksum is incorrect, drop packet and return.
+	 * (This step is completed in the AF-dependent functions.) */
+	skb->csum = skb_checksum(skb, 0, cscov, 0);
+
 	return 0;
 }
 
@@ -840,16 +823,17 @@ static int dccp_v4_rcv(struct sk_buff *skb)
 {
 	const struct dccp_hdr *dh;
 	struct sock *sk;
+	int min_cov;
 
-	/* Step 1: Check header basics: */
+	/* Step 1: Check header basics */
 
 	if (dccp_invalid_packet(skb))
 		goto discard_it;
 
-	/* If the header checksum is incorrect, drop packet and return */
-	if (dccp_v4_verify_checksum(skb, skb->nh.iph->saddr,
-				    skb->nh.iph->daddr) < 0) {
-		LIMIT_NETDEBUG(KERN_WARNING "%s: incorrect header checksum\n",
+	/* Step 1: If header checksum is incorrect, drop packet and return */
+	if (dccp_v4_csum_finish(skb, skb->nh.iph->saddr, skb->nh.iph->daddr)) {
+		LIMIT_NETDEBUG(KERN_WARNING
+			       "%s: dropped packet with invalid checksum\n",
 			       __FUNCTION__);
 		goto discard_it;
 	}
@@ -905,6 +889,21 @@ static int dccp_v4_rcv(struct sk_buff *skb)
 		goto no_dccp_socket;
 	}
 
+	/*
+	 * RFC 4340, sec. 9.2.1: Minimum Checksum Coverage
+	 * 	o if MinCsCov = 0, only packets with CsCov = 0 are accepted
+	 * 	o if MinCsCov > 0, also accept packets with CsCov >= MinCsCov
+	 */
+	min_cov = dccp_sk(sk)->dccps_pcrlen;
+	if (dh->dccph_cscov && (min_cov == 0 || dh->dccph_cscov < min_cov))  {
+		dccp_pr_debug("Packet CsCov %d does not satisfy MinCsCov %d\n",
+			      dh->dccph_cscov, min_cov);
+		/* FIXME: "Such packets SHOULD be reported using Data Dropped
+		 *         options (Section 11.7) with Drop Code 0, Protocol
+		 *         Constraints."                                     */
+		goto discard_and_relse;
+	}
+
 	if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_and_relse;
 	nf_reset(skb);