TCP報文GSO分段的入口函數是tcp4_gso_segment,在tcpv4_offload對象中定義。
1、tcp4_gso_segment函數
static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
netdev_features_t features)
{
if (!pskb_may_pull(skb, sizeof(struct tcphdr)))
return ERR_PTR(-EINVAL);
if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
const struct iphdr *iph = ip_hdr(skb);
struct tcphdr *th = tcp_hdr(skb); //ip層報文保證了transport header值
/* Set up checksum pseudo header, usually expect stack to
* have done this already.
*/
th->check = 0;
skb->ip_summed = CHECKSUM_PARTIAL;
__tcp_v4_send_check(skb, iph->saddr, iph->daddr); //計算僞頭check值
}
return tcp_gso_segment(skb, features); //TCP GSO分段
}
2、tcp_gso_segment函數
struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
netdev_features_t features)
{
struct sk_buff *segs = ERR_PTR(-EINVAL);
unsigned int sum_truesize = 0;
struct tcphdr *th;
unsigned int thlen;
unsigned int seq;
__be32 delta;
unsigned int oldlen;
unsigned int mss;
struct sk_buff *gso_skb = skb;
__sum16 newcheck;
bool ooo_okay, copy_destructor;
th = tcp_hdr(skb);
thlen = th->doff * 4; //得到tcp頭的長度
if (thlen < sizeof(*th))
goto out;
if (!pskb_may_pull(skb, thlen))
goto out;
oldlen = (u16)~skb->len;
__skb_pull(skb, thlen); //skb移動到用戶數據區(payload)
mss = tcp_skb_mss(skb); //得到mss值
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_TCPV4 |
SKB_GSO_DODGY |
SKB_GSO_TCP_ECN |
SKB_GSO_TCPV6 |
SKB_GSO_GRE |
SKB_GSO_GRE_CSUM |
SKB_GSO_IPIP |
SKB_GSO_SIT |
SKB_GSO_UDP_TUNNEL |
SKB_GSO_UDP_TUNNEL_CSUM |
SKB_GSO_TUNNEL_REMCSUM |
0) ||
!(type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))))
goto out;
skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss); //如果報文來源不可信,則重新計算segs,返回
segs = NULL;
goto out;
}
copy_destructor = gso_skb->destructor == tcp_wfree;
ooo_okay = gso_skb->ooo_okay;
/* All segments but the first should have ooo_okay cleared */
skb->ooo_okay = 0;
segs = skb_segment(skb, features); //調用payload根據mss值分段
if (IS_ERR(segs))
goto out;
/* Only first segment might have ooo_okay set */
segs->ooo_okay = ooo_okay;
delta = htonl(oldlen + (thlen + mss)); //TCP頭+mss - 原始報文,該值爲負值
skb = segs;
th = tcp_hdr(skb); //skb_segment分段後,可以直接從skb中獲取tcp頭, skb_segment或udp4_ufo_fragment保證
seq = ntohl(th->seq);
if (unlikely(skb_shinfo(gso_skb)->tx_flags & SKBTX_SW_TSTAMP))
tcp_gso_tstamp(segs, skb_shinfo(gso_skb)->tskey, seq, mss);
newcheck = ~csum_fold((__force __wsum)((__force u32)th->check + //第一個報文基於原先值,根據delta快速計算
(__force u32)delta));
do { //刷新分段後報文的TCP頭設置
th->fin = th->psh = 0;
th->check = newcheck;
if (skb->ip_summed != CHECKSUM_PARTIAL)
th->check = gso_make_checksum(skb, ~th->check); //重新計算check值
seq += mss;
if (copy_destructor) {
skb->destructor = gso_skb->destructor;
skb->sk = gso_skb->sk;
sum_truesize += skb->truesize;
}
skb = skb->next;
th = tcp_hdr(skb);
th->seq = htonl(seq);
th->cwr = 0;
} while (skb->next);
/* Following permits TCP Small Queues to work well with GSO :
* The callback to TCP stack will be called at the time last frag
* is freed at TX completion, and not right now when gso_skb
* is freed by GSO engine
*/
if (copy_destructor) {
swap(gso_skb->sk, skb->sk);
swap(gso_skb->destructor, skb->destructor);
sum_truesize += skb->truesize;
atomic_add(sum_truesize - gso_skb->truesize,
&skb->sk->sk_wmem_alloc);
}
delta = htonl(oldlen + (skb_tail_pointer(skb) -
skb_transport_header(skb)) + //最後一個報文的delta值不同
skb->data_len);
th->check = ~csum_fold((__force __wsum)((__force u32)th->check +
(__force u32)delta));
if (skb->ip_summed != CHECKSUM_PARTIAL)
th->check = gso_make_checksum(skb, ~th->check); //重新計算check值
out:
return segs;
}