[PATCH] More ECN Fixes: make writable before writing

Patrick McHardy spotted this, on top of previous fix.  I neatened it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

net/ipv4/netfilter/ipt_ECN.c

@@ -52,44 +52,41 @@ set_ect_ip(struct sk_buff **pskb, const struct ipt_ECN_info *einfo)
 static inline int
 set_ect_tcp(struct sk_buff **pskb, const struct ipt_ECN_info *einfo, int inward)
 {
-	struct tcphdr _tcph, *th;
+	struct tcphdr _tcph, *tcph;
 	u_int16_t diffs[2];
 
 	/* Not enought header? */
-	th = skb_header_pointer(*pskb, (*pskb)->nh.iph->ihl*4,
-				sizeof(_tcph), &_tcph);
-	if (th == NULL)
+	tcph = skb_header_pointer(*pskb, (*pskb)->nh.iph->ihl*4,
+				  sizeof(_tcph), &_tcph);
+	if (!tcph)
 		return 0;
 
-	diffs[0] = ((u_int16_t *)th)[6];
-	if (einfo->operation & IPT_ECN_OP_SET_ECE)
-		th->ece = einfo->proto.tcp.ece;
+	if (!(einfo->operation & IPT_ECN_OP_SET_ECE
+	      || tcph->ece == einfo->proto.tcp.ece)
+	    && (!(einfo->operation & IPT_ECN_OP_SET_CWR
+		  || tcph->cwr == einfo->proto.tcp.cwr)))
+		return 1;
+
+	if (!skb_ip_make_writable(pskb, (*pskb)->nh.iph->ihl*4+sizeof(*tcph)))
+		return 0;
+	tcph = (void *)(*pskb)->nh.iph + (*pskb)->nh.iph->ihl*4;
 
+	diffs[0] = ((u_int16_t *)tcph)[6];
+	if (einfo->operation & IPT_ECN_OP_SET_ECE)
+		tcph->ece = einfo->proto.tcp.ece;
 	if (einfo->operation & IPT_ECN_OP_SET_CWR)
-		th->cwr = einfo->proto.tcp.cwr;
-	diffs[1] = ((u_int16_t *)th)[6];
-
-	/* Only mangle if it's changed. */
-	if (diffs[0] != diffs[1]) {
-		diffs[0] = diffs[0] ^ 0xFFFF;
-		if (!skb_ip_make_writable(pskb,
-					  (*pskb)->nh.iph->ihl*4+sizeof(_tcph)))
+		tcph->cwr = einfo->proto.tcp.cwr;
+	diffs[1] = ((u_int16_t *)tcph)[6];
+	diffs[0] = diffs[0] ^ 0xFFFF;
+
+	if ((*pskb)->ip_summed != CHECKSUM_HW)
+		tcph->check = csum_fold(csum_partial((char *)diffs,
+						     sizeof(diffs),
+						     tcph->check^0xFFFF));
+	else
+		if (skb_checksum_help(*pskb, inward))
 			return 0;
-
-		if (th != &_tcph)
-			memcpy(&_tcph, th, sizeof(_tcph));
-
-		if ((*pskb)->ip_summed != CHECKSUM_HW)
-			_tcph.check = csum_fold(csum_partial((char *)diffs,
-							     sizeof(diffs),
-							     _tcph.check^0xFFFF));
-		memcpy((*pskb)->data + (*pskb)->nh.iph->ihl*4,
-		       &_tcph, sizeof(_tcph));
-		if ((*pskb)->ip_summed == CHECKSUM_HW)
-			if (skb_checksum_help(*pskb, inward))
-				return 0;
-		(*pskb)->nfcache |= NFC_ALTERED;
-	}
+	(*pskb)->nfcache |= NFC_ALTERED;
 	return 1;
 }