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OpenvSwitch source code(3)

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這篇文章帶領大家透過閱讀原始碼的方來學習如何 OpenvSwitch 是如何處理封包的，當底層的 Kernel Switch(datapath) 沒有辦法轉發封包時，要如何將該封包送到上層的 User Space Table 進行 Openflow 規則的查詢。這部份牽扯到資料如何橫跨於 User-Space 以及 Kernel-Space.

Preface

這邊要探討的是當網卡收到封包後，在 KERNEL 中由下往上的流程

test.png

netdev_frame_hook

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static rx_handler_result_t netdev_frame_hook(struct sk_buff **pskb)
{
        struct sk_buff *skb = *pskb;
        struct vport *vport;

        if (unlikely(skb->pkt_type == PACKET_LOOPBACK))
                return RX_HANDLER_PASS;

        vport = ovs_netdev_get_vport(skb->dev);

        netdev_port_receive(vport, skb);

        return RX_HANDLER_CONSUMED;
}
  • net_device收到封包後，變變呼叫這個fucntion call來處理。
  • 先判斷是不是LOOPBACK的，是的話就不需要處理了。
  • 透過 ovs_netdev_get_vport取得該dev對應的vport
  • 呼叫 netdev_port_receive來處理這個封包

netdev_port_receive

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static void netdev_port_receive(struct vport *vport, struct sk_buff *skb)
{
        if (unlikely(!vport))
                goto error;
        if (unlikely(skb_warn_if_lro(skb)))
                goto error;
        /* Make our own copy of the packet.  Otherwise we will mangle the
         * packet for anyone who came before us (e.g. tcpdump via AF_PACKET).
         * (No one comes after us, since we tell handle_bridge() that we took
         * the packet.) */
        skb = skb_share_check(skb, GFP_ATOMIC);
        if (unlikely(!skb))
                return;

        skb_push(skb, ETH_HLEN);
        ovs_skb_postpush_rcsum(skb, skb->data, ETH_HLEN);

        ovs_vport_receive(vport, skb, NULL);
        return;

error:
        kfree_skb(skb);
}
  • skb_warn_if_lro判斷其LRO的設定有沒有問題
  • skb_push調整skb中的data指標
  • ovs_skb_postpush_rcsum 處理ip checksum。
  • 呼叫 ovs_vport_receive繼續處理

ovs_vport_receive

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void ovs_vport_receive(struct vport *vport, struct sk_buff *skb,
                       struct ovs_key_ipv4_tunnel *tun_key)
{
        struct pcpu_tstats *stats;

        stats = this_cpu_ptr(vport->percpu_stats);
        u64_stats_update_begin(&stats->syncp);
        stats->rx_packets++;
        stats->rx_bytes += skb->len;
        u64_stats_update_end(&stats->syncp);

        OVS_CB(skb)->tun_key = tun_key;
        ovs_dp_process_received_packet(vport, skb);
}
  • struct pcpu_tstats 紀錄當前cpu對於封包的一些計數 (rx,tx)
  • 由 this_cpu_ptr取得這個vport的(tx,rx) counter.
  • Update counters (packets,bytes)
  • 透過 ovs_dp_process_received_packet繼續處理

ovs_dp_process_received_packet

  • 這個function的目的就是
    1. 由skb內取出封包各欄位的資訊(L2,L3,L4)
      1. 去查詢該 datapath的flow table中否有符合的flow entry
      2. 有找到，就執行對應的action.
      3. 沒有找到，就執行 ovs_dp_upcall送到 user space
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void ovs_dp_process_received_packet(struct vport *p, struct sk_buff *skb)
{
        struct datapath *dp = p->dp;
        struct sw_flow *flow;
        struct dp_stats_percpu *stats;
        struct sw_flow_key key;
        u64 *stats_counter;
        int error;

        stats = this_cpu_ptr(dp->stats_percpu);

        /* Extract flow from 'skb' into 'key'. */
        error = ovs_flow_extract(skb, p->port_no, &key);
        if (unlikely(error)) {
                kfree_skb(skb);
                return;
        }

        /* Look up flow. */
        flow = ovs_flow_lookup(rcu_dereference(dp->table), &key);
        if (unlikely(!flow)) {
                struct dp_upcall_info upcall;

                upcall.cmd = OVS_PACKET_CMD_MISS;
                upcall.key = &key;
                upcall.userdata = NULL;
                upcall.portid = p->upcall_portid;
                ovs_dp_upcall(dp, skb, &upcall);
                consume_skb(skb);
                stats_counter = &stats->n_missed;
                goto out;
        }

        OVS_CB(skb)->flow = flow;
        OVS_CB(skb)->pkt_key = &key;

        stats_counter = &stats->n_hit;
        ovs_flow_used(OVS_CB(skb)->flow, skb);
        ovs_execute_actions(dp, skb);

out:
        /* Update datapath statistics. */
        u64_stats_update_begin(&stats->sync);
        (*stats_counter)++;
        u64_stats_update_end(&stats->sync);
}
  • 先由 vport 取得 對應的 datapath
  • 由 this_cpu_ptr取得這個datapath的(tx,rx) counter.
  • 透過 ovs_flow_extract 把 skb, vport 的資訊都寫入 sw_flow_key key之中
  • 呼叫 ovs_flow_lookup 去查詢這個packet在table之中有沒有match的flow entry.
  • 如果沒有找到，那就透過 upcall的方式，把這個flow_miss告訴 ovs-vswitchd去處理
  • 如果有找到，先透過 ovs_flow_used更新該flow的一些資訊(usedtime,packet,byte,tcp_flag),接者在呼叫 ovs_execute_actions 執行這個flow 對應的actions

ovs_flow_extract

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int ovs_flow_extract(struct sk_buff *skb, u16 in_port, struct sw_flow_key *key)
{
        int error;
        struct ethhdr *eth;

        memset(key, 0, sizeof(*key));

        key->phy.priority = skb->priority;
        if (OVS_CB(skb)->tun_key)
                memcpy(&key->tun_key, OVS_CB(skb)->tun_key, sizeof(key->tun_key));
        key->phy.in_port = in_port;
        key->phy.skb_mark = skb->mark;

		skb_reset_mac_header(skb);
  • OVS_CB是一個marco,會把skbuff中的cb區域拿來使用，並且轉型成 ovs_skb_cb
    #define OVS_CB(skb) ((struct ovs_skb_cb *)(skb)->cb)
  • 如果該packet有使用 tunnel_key的話，就把該 tun_key給複製到 key。
  • 把收到封包的port number也記錄到key裡面( ingress port)
  • 使用 skb_reset_mac_header 得到 mac header (放在 skb->mac_header)
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/* Link layer.  We are guaranteed to have at least the 14 byte Ethernet
 * header in the linear data area.
 */
eth = eth_hdr(skb);
memcpy(key->eth.src, eth->h_source, ETH_ALEN);
memcpy(key->eth.dst, eth->h_dest, ETH_ALEN);

__skb_pull(skb, 2 * ETH_ALEN);
/* We are going to push all headers that we pull, so no need to
 * update skb->csum here. */

if (vlan_tx_tag_present(skb))
        key->eth.tci = htons(vlan_get_tci(skb));
else if (eth->h_proto == htons(ETH_P_8021Q))
        if (unlikely(parse_vlan(skb, key)))
                return -ENOMEM;

key->eth.type = parse_ethertype(skb);
if (unlikely(key->eth.type == htons(0)))
        return -ENOMEM;

skb_reset_network_header(skb);
__skb_push(skb, skb->data - skb_mac_header(skb));

....
  • 取得 ethernet header,並且把 sourcee/destinaion mac address給複製到key。
  • 透過 __skb_pull,把 skb-data給往下指 ETH_ALEN*2
  • 檢查有沒有用 vlan tag,有的話就把tci給抓近來
  • 使用 parse_ethertype 得到該封包的 ethernet type
  • 使用 skb_reset_network_header 得到 network header (放在 skb->network_header)
  • 透過 __skb_push 把skb中的data指標往上移(這樣可以取回mac header的一些資訊)，供未來使用
  • 後面就是針對 (IP,IPV6,ARP)等在做更細部的資料取得
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struct sw_flow *ovs_flow_lookup(struct flow_table *tbl,
                                const struct sw_flow_key *key)
{
        struct sw_flow *flow = NULL;
        struct sw_flow_mask *mask;

        list_for_each_entry_rcu(mask, tbl->mask_list, list) {
                flow = ovs_masked_flow_lookup(tbl, key, mask);
                if (flow)  /* Found */
                        break;
        }

        return flow;
}
  • 從 datapath的flow table中先取得所有的 mask_list
  • 使用 ovs_masked_flow_lookup去搜尋進來的封包是否有match
  • 最後回傳 flow.
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static struct sw_flow *ovs_masked_flow_lookup(struct flow_table *table,
                                    const struct sw_flow_key *unmasked,
                                    struct sw_flow_mask *mask)
{
        struct sw_flow *flow;
        struct hlist_head *head;
        int key_start = mask->range.start;
        int key_end = mask->range.end;
        u32 hash;
        struct sw_flow_key masked_key;

        ovs_flow_key_mask(&masked_key, unmasked, mask);
        hash = ovs_flow_hash(&masked_key, key_start, key_end);
        head = find_bucket(table, hash);
        hlist_for_each_entry_rcu(flow, head, hash_node[table->node_ver]) {
                if (flow->mask == mask &&
                    __flow_cmp_masked_key(flow, &masked_key,
                                          key_start, key_end))
                        return flow;
        }
        return NULL;
}
  • OVS 2.0後增加對megaflow的支持，所以在kernel端也可以支援wildcard的flow matching.
  • sw_flow_key實際上就是個wildcard，每個進來的封包都先跟wildcard做 ovs_flow_key_mask,然後在用mask後的結果去table中尋找有沒有可以match的
  • 使用mask過後的結果來做hash,並且透過 find_bucket找到那個hash值所在的bucket
  • 針對那個bucket中所有的flow去做比對，如果 mask相同且 flow_cmpmasked結果為真，就代表match.
  • key的start & end 還不是很明瞭其目的以及用途，待釐清
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void ovs_flow_key_mask(struct sw_flow_key *dst, const struct sw_flow_key *src,
                       const struct sw_flow_mask *mask)
{
        const long *m = (long *)((u8 *)&mask->key + mask->range.start);
        const long *s = (long *)((u8 *)src + mask->range.start);
        long *d = (long *)((u8 *)dst + mask->range.start);
        int i;

        /* The memory outside of the 'mask->range' are not set since
         * further operations on 'dst' only uses contents within
         * 'mask->range'.
         */
        for (i = 0; i < range_n_bytes(&mask->range); i += sizeof(long))
                *d++ = *s++ & *m++;
}
  • 把 src用 mask去處理，結果放到 dst上
  • 這邊可以看到做mask的方法就是不停地用 &來取結果而已。

Found

尋找到flow後

  1. 更新該flow的一些統計資訊 ( ovs_flow_used)
  2. 執行該flow entry上的actions (ovs_execute_actions)

ovs_flow_used

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void ovs_flow_used(struct sw_flow *flow, struct sk_buff *skb)
{
        u8 tcp_flags = 0;

        if ((flow->key.eth.type == htons(ETH_P_IP) ||
             flow->key.eth.type == htons(ETH_P_IPV6)) &&
            flow->key.ip.proto == IPPROTO_TCP &&
            likely(skb->len >= skb_transport_offset(skb) + sizeof(struct tcphdr))) {
                u8 *tcp = (u8 *)tcp_hdr(skb);
                tcp_flags = *(tcp + TCP_FLAGS_OFFSET) & TCP_FLAG_MASK;
        }

        spin_lock(&flow->lock);
        flow->used = jiffies;
        flow->packet_count++;
        flow->byte_count += skb->len;
        flow->tcp_flags |= tcp_flags;
        spin_unlock(&flow->lock);
}
  • 如果該封包滿足(IP/IPV6,TCP)且TCP有額外的flag的話，就更新其TCP_FLAGS
  • 更新該flow的相關資訊
  • used(上次使用時間),單位是 jiffies
  • counter.

ovs_execute_actions

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/* Execute a list of actions against 'skb'. */
int ovs_execute_actions(struct datapath *dp, struct sk_buff *skb)
{
        struct sw_flow_actions *acts = rcu_dereference(OVS_CB(skb)->flow->sf_acts);
        struct loop_counter *loop;
        int error;

        /* Check whether we've looped too much. */
        loop = &__get_cpu_var(loop_counters);
        if (unlikely(++loop->count > MAX_LOOPS))
                loop->looping = true;
        if (unlikely(loop->looping)) {
                error = loop_suppress(dp, acts);
                kfree_skb(skb);
                goto out_loop;
        }

        OVS_CB(skb)->tun_key = NULL;
        error = do_execute_actions(dp, skb, acts->actions,
                                         acts->actions_len, false);

        /* Check whether sub-actions looped too much. */
        if (unlikely(loop->looping))
                error = loop_suppress(dp, acts);

out_loop:
        /* Decrement loop counter. */
        if (!--loop->count)
                loop->looping = false;

        return error;
}
  • 先從flow_sf_acts中取出對應的actions(sw_flow_actions)
  • 這邊會限制執行 do_execute_actions的次數，設計理念尚未明瞭。
  • 呼叫 do_execute_actions來做後續的處理

do_execute_actions

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static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
                        const struct nlattr *attr, int len, bool keep_skb)
{
        /* Every output action needs a separate clone of 'skb', but the common
         * case is just a single output action, so that doing a clone and
         * then freeing the original skbuff is wasteful.  So the following code
         * is slightly obscure just to avoid that. */
        int prev_port = -1;
        const struct nlattr *a;
        int rem;

        for (a = attr, rem = len; rem > 0;
             a = nla_next(a, &rem)) {
                int err = 0;

                if (prev_port != -1) {
                        do_output(dp, skb_clone(skb, GFP_ATOMIC), prev_port);
                        prev_port = -1;
                }

                switch (nla_type(a)) {
                case OVS_ACTION_ATTR_OUTPUT:
                        prev_port = nla_get_u32(a);
                        break;

                case OVS_ACTION_ATTR_USERSPACE:
                        output_userspace(dp, skb, a);
                        break;

                case OVS_ACTION_ATTR_PUSH_VLAN:
                        err = push_vlan(skb, nla_data(a));
                        if (unlikely(err)) /* skb already freed. */
                                return err;
                        break;

                case OVS_ACTION_ATTR_POP_VLAN:
                        err = pop_vlan(skb);
                        break;

                case OVS_ACTION_ATTR_SET:
                        err = execute_set_action(skb, nla_data(a));
                        break;

                case OVS_ACTION_ATTR_SAMPLE:
                        err = sample(dp, skb, a);
                        break;
                }

                if (unlikely(err)) {
                        kfree_skb(skb);
                        return err;
                }
        }

        if (prev_port != -1) {
                if (keep_skb)
                        skb = skb_clone(skb, GFP_ATOMIC);

                do_output(dp, skb, prev_port);
        } else if (!keep_skb)
                consume_skb(skb);

        return 0;
  • flow的action都是透過nlattr來儲存，這是netlink相關的資料結構，因為 user space也會透過netlink的方式要求kernel直接處理封包，所以action都用 nlattr來處理
  • 因為可以 output到多個port去，每次都會需要複製 skb，所以這邊使用 prev_port來處理只有一次 output的情況(不用複製)

Not found

如果沒有找到該flow，kernel就會透過netlink的方式，把這個封包送到 ovs-vswitched去處理。

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struct dp_upcall_info upcall;
upcall.cmd = OVS_PACKET_CMD_MISS;
upcall.key = &key;
upcall.userdata = NULL;
upcall.portid = p->upcall_portid;
ovs_dp_upcall(dp, skb, &upcall);
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struct dp_upcall_info {
        u8 cmd;
        const struct sw_flow_key *key;
        const struct nlattr *userdata;
        u32 portid;
};
  • 每個 dp_upcall_info都是透過 netlink的方式把資料送到 userspace,這邊要記錄
  • 資料設定完畢後， 呼叫 ovs_dp_upcall來處理
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int ovs_dp_upcall(struct datapath *dp, struct sk_buff *skb,
                  const struct dp_upcall_info *upcall_info)
{
        struct dp_stats_percpu *stats;
        int dp_ifindex;
        int err;

        if (upcall_info->portid == 0) {
                err = -ENOTCONN;
                goto err;
        }

        dp_ifindex = get_dpifindex(dp);
        if (!dp_ifindex) {
                err = -ENODEV;
                goto err;
        }

        if (!skb_is_gso(skb))
                err = queue_userspace_packet(ovs_dp_get_net(dp), dp_ifindex, skb, upcall_info);
        else
                err = queue_gso_packets(ovs_dp_get_net(dp), dp_ifindex, skb, upcall_info);
        if (err)
                goto err;

        return 0;

err:
        stats = this_cpu_ptr(dp->stats_percpu);

        u64_stats_update_begin(&stats->sync);
        stats->n_lost++;
        u64_stats_update_end(&stats->sync);

        return err;
}
  • 檢查 porrid(port number)是否正常
  • 取得該 datapath的index
  • 根據有使用 gso的話，就會特別處理，因為封包的長度比較大，會透過多次的 queue_userspace_packet來處理支援 gso的封包。
  • 如果發生error，意味者這個封包就不會有人處理了，因此把lost的值增加

queue_userspace_packet

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 static int queue_userspace_packet(struct net *net, int dp_ifindex,
                                  struct sk_buff *skb,
                                  const struct dp_upcall_info *upcall_info)
{
        struct ovs_header *upcall;
        struct sk_buff *nskb = NULL;
        struct sk_buff *user_skb; /* to be queued to userspace */
        struct nlattr *nla;
        int err;
        if (vlan_tx_tag_present(skb)) {
                nskb = skb_clone(skb, GFP_ATOMIC);
                if (!nskb)
                        return -ENOMEM;

                nskb = __vlan_put_tag(nskb, nskb->vlan_proto, vlan_tx_tag_get(nskb));
                if (!nskb)
                        return -ENOMEM;

                vlan_set_tci(nskb, 0);

                skb = nskb;
        }

        if (nla_attr_size(skb->len) > USHRT_MAX) {
                err = -EFBIG;
                goto out;
        }

        user_skb = genlmsg_new(upcall_msg_size(skb, upcall_info->userdata), GFP_ATOMIC);
        if (!user_skb) {
                err = -ENOMEM;
                goto out;
        }

        upcall = genlmsg_put(user_skb, 0, 0, &dp_packet_genl_family,
                             0, upcall_info->cmd);
        upcall->dp_ifindex = dp_ifindex;

        nla = nla_nest_start(user_skb, OVS_PACKET_ATTR_KEY);
        ovs_flow_to_nlattrs(upcall_info->key, upcall_info->key, user_skb);
        nla_nest_end(user_skb, nla);

        if (upcall_info->userdata)
                __nla_put(user_skb, OVS_PACKET_ATTR_USERDATA,
                          nla_len(upcall_info->userdata),
                          nla_data(upcall_info->userdata));

        nla = __nla_reserve(user_skb, OVS_PACKET_ATTR_PACKET, skb->len);

        skb_copy_and_csum_dev(skb, nla_data(nla));
        err = genlmsg_unicast(net, user_skb, upcall_info->portid);

out:
        kfree_skb(nskb);
        return err;
}
  • 這邊產生 generic netlink 然後把資料設定完畢後，就送出到 userspace
  • genlmsg_系列尚未完全瞭解，待補充

個人資訊

我目前於 Hiskio 平台上面有開設 Kubernetes 相關課程，歡迎有興趣的人參考並分享，裡面有我從底層到實戰中對於 Kubernetes 的各種想法

組合包
https://hiskio.com/packages/7ey2vdnyN

疑難雜症除錯篇
https://hiskio.com/courses/440/about?promo_code=LG28Q5G

單堂(CI/CD)
https://hiskio.com/courses/385?promo_code=13K49YE&p=blog1

基礎概念
https://hiskio.com/courses/349?promo_code=13LY5RE

另外，歡迎按讚加入我個人的粉絲專頁，裡面會定期分享各式各樣的文章，有的是翻譯文章，也有部分是原創文章，主要會聚焦於 CNCF 領域
https://www.facebook.com/technologynoteniu

如果有使用 Telegram 的也可以訂閱下列頻道來，裡面我會定期推播通知各類文章
https://t.me/technologynote

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