【Linux4.1.12源码分析】二层报文发送之报文GSO分段（UDP）

UDP报文GSO分段的入口函数是udp4_ufo_fragment，由udpv4_offload常量中定义。

1、udp4_ufo_fragment函数

 

static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
					 netdev_features_t features)
{
	struct sk_buff *segs = ERR_PTR(-EINVAL);
	unsigned int mss;
	__wsum csum;
	struct udphdr *uh;
	struct iphdr *iph;

	if (skb->encapsulation &&
	    (skb_shinfo(skb)->gso_type &
	     (SKB_GSO_UDP_TUNNEL|SKB_GSO_UDP_TUNNEL_CSUM))) {	
		segs = skb_udp_tunnel_segment(skb, features, false);	//封装报文的GSO分段，可以基于内层报文进行GSO分段
		goto out;
	}

	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
		goto out;

	mss = skb_shinfo(skb)->gso_size;
	if (unlikely(skb->len <= mss))
		goto out;

	if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {		
		/* Packet is from an untrusted source, reset gso_segs. */
		int type = skb_shinfo(skb)->gso_type;

		if (unlikely(type & ~(SKB_GSO_UDP | SKB_GSO_DODGY |
				      SKB_GSO_UDP_TUNNEL |
				      SKB_GSO_UDP_TUNNEL_CSUM |
				      SKB_GSO_TUNNEL_REMCSUM |
				      SKB_GSO_IPIP |
				      SKB_GSO_GRE | SKB_GSO_GRE_CSUM) ||
			     !(type & (SKB_GSO_UDP))))
			goto out;

		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);	//如果报文来源不可信，则重新计算segs，返回

		segs = NULL;
		goto out;
	}

	/* Do software UFO. Complete and fill in the UDP checksum as
	 * HW cannot do checksum of UDP packets sent as multiple
	 * IP fragments.
	 */

	uh = udp_hdr(skb);
	iph = ip_hdr(skb);

	uh->check = 0;
	csum = skb_checksum(skb, 0, skb->len, 0);	//计算csum值
	uh->check = udp_v4_check(skb->len, iph->saddr, iph->daddr, csum);	//计算udp头的check值
	if (uh->check == 0)
		uh->check = CSUM_MANGLED_0;

	skb->ip_summed = CHECKSUM_NONE;

	/* Fragment the skb. IP headers of the fragments are updated in
	 * inet_gso_segment()
	 */
	segs = skb_segment(skb, features);	//报文根据mss进行分段，因为包含UDP头，所以分段的结果是IP分片报文
out:
	return segs;
}

2、skb_udp_tunnel_segment函数

 

 

struct sk_buff *skb_udp_tunnel_segment(struct sk_buff *skb,
				       netdev_features_t features,
				       bool is_ipv6)
{
	__be16 protocol = skb->protocol;
	const struct net_offload **offloads;
	const struct net_offload *ops;
	struct sk_buff *segs = ERR_PTR(-EINVAL);
	struct sk_buff *(*gso_inner_segment)(struct sk_buff *skb,
					     netdev_features_t features);

	rcu_read_lock();

	switch (skb->inner_protocol_type) {	//vxlan封装时，该值为ENCAP_TYPE_ETHER
	case ENCAP_TYPE_ETHER:
		protocol = skb->inner_protocol;
		gso_inner_segment = skb_mac_gso_segment;  //vxlan封装，内层报文为完整的报文（二层、三层、四层），继续从mac开始分段
		break;
	case ENCAP_TYPE_IPPROTO:
		offloads = is_ipv6 ? inet6_offloads : inet_offloads;
		ops = rcu_dereference(offloads[skb->inner_ipproto]);
		if (!ops || !ops->callbacks.gso_segment)
			goto out_unlock;
		gso_inner_segment = ops->callbacks.gso_segment;		//调用4层协议的GSO分段能力，GRE/IPIP等等
		break;
	default:
		goto out_unlock;
	}

	segs = __skb_udp_tunnel_segment(skb, features, gso_inner_segment,	//upd封装报文GSO分段
					protocol, is_ipv6);

out_unlock:
	rcu_read_unlock();

	return segs;
}

3、__skb_udp_tunnel_segment函数

 

 

static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb,
	netdev_features_t features,
	struct sk_buff *(*gso_inner_segment)(struct sk_buff *skb,
					     netdev_features_t features),
	__be16 new_protocol, bool is_ipv6)
{
	struct sk_buff *segs = ERR_PTR(-EINVAL);
	u16 mac_offset = skb->mac_header;
	int mac_len = skb->mac_len;
	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);	//vxlan头长度 UDP + vxlan，
	__be16 protocol = skb->protocol;
	netdev_features_t enc_features;
	int udp_offset, outer_hlen;
	unsigned int oldlen;
	bool need_csum = !!(skb_shinfo(skb)->gso_type &
			    SKB_GSO_UDP_TUNNEL_CSUM);
	bool remcsum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TUNNEL_REMCSUM);
	bool offload_csum = false, dont_encap = (need_csum || remcsum);

	oldlen = (u16)~skb->len;

	if (unlikely(!pskb_may_pull(skb, tnl_hlen)))
		goto out;

	skb->encapsulation = 0;
	__skb_pull(skb, tnl_hlen);	//报文移动到内层报文的MAC头
	skb_reset_mac_header(skb);	//设置skb的mac header
	skb_set_network_header(skb, skb_inner_network_offset(skb));  //设置skb的 ip header
	skb->mac_len = skb_inner_network_offset(skb);	//设置skb mac len
	skb->protocol = new_protocol;		//设置skb protocol，至此skb已经切换到内层，可以继续进行GSO分段
	skb->encap_hdr_csum = need_csum;
	skb->remcsum_offload = remcsum;		

	/* Try to offload checksum if possible */
	offload_csum = !!(need_csum &&
			  (skb->dev->features &
			   (is_ipv6 ? NETIF_F_V6_CSUM : NETIF_F_V4_CSUM)));

	/* segment inner packet. */
	enc_features = skb->dev->hw_enc_features & features;
	segs = gso_inner_segment(skb, enc_features);	//如果是vxlan报文，则重新开始mac层的GSO分段
	if (IS_ERR_OR_NULL(segs)) {
		skb_gso_error_unwind(skb, protocol, tnl_hlen, mac_offset,
				     mac_len);
		goto out;
	}

	outer_hlen = skb_tnl_header_len(skb);	//计算外层报文的长度
	udp_offset = outer_hlen - tnl_hlen;	//外层UDP头的偏移
	skb = segs;				//此时skb指向内层报文的mac头位置
	do {
		struct udphdr *uh;
		int len;
		__be32 delta;

		if (dont_encap) {
			skb->encapsulation = 0;
			skb->ip_summed = CHECKSUM_NONE;
		} else {
			/* Only set up inner headers if we might be offloading
			 * inner checksum.
			 */
			skb_reset_inner_headers(skb);	//此时skb指向内层报文，可以建立inner header值
			skb->encapsulation = 1;
		}

		skb->mac_len = mac_len;
		skb->protocol = protocol;

		skb_push(skb, outer_hlen);	//skb移到外层报文的mac头
		skb_reset_mac_header(skb);	//设置mac header
		skb_set_network_header(skb, mac_len);	//设置network header，ip层需要
		skb_set_transport_header(skb, udp_offset);	//设置transport header
		len = skb->len - udp_offset;
		uh = udp_hdr(skb);	//找到UDP头很重要，GSO分段后，有些数据需要刷新，包括长度等
		uh->len = htons(len);

		if (!need_csum)
			continue;

		delta = htonl(oldlen + len);

		uh->check = ~csum_fold((__force __wsum)
				       ((__force u32)uh->check +
					(__force u32)delta));
		if (offload_csum) {
			skb->ip_summed = CHECKSUM_PARTIAL;	
			skb->csum_start = skb_transport_header(skb) - skb->head; //重新计算csum值，位置更新了
			skb->csum_offset = offsetof(struct udphdr, check);
		} else if (remcsum) {
			/* Need to calculate checksum from scratch,
			 * inner checksums are never when doing
			 * remote_checksum_offload.
			 */

			skb->csum = skb_checksum(skb, udp_offset,	//软件计算csum值
						 skb->len - udp_offset,
						 0);
			uh->check = csum_fold(skb->csum);
			if (uh->check == 0)
				uh->check = CSUM_MANGLED_0;
		} else {
			uh->check = gso_make_checksum(skb, ~uh->check); //计算伪头check值

			if (uh->check == 0)
				uh->check = CSUM_MANGLED_0;
		}
	} while ((skb = skb->next));
out:
	return segs;
}

udp4_ufo_fragment提升了对vxlan等封装报文的支持，对GSO报文封装后报文的能够正确GSO分段，而不会产生IP分片报文。 实现这个功能，离不开skb_segment函数的支持，能够复制外层报文头而不管有多长，所以只要少量修改，内核能够支持多层封装的GSO分段。