send和recv背后数据的收发过程
send和recv是TCP常用的发送数据和接受数据函数，这两个函数具体在linux内核的代码实现上是如何实现的呢？
ssize_t recv(int sockfd, void *buf, size_t len, int flags)
ssize_t send(int sockfd, const void *buf, size_t len, int flags)
理论分析
对于send函数，比较容易理解，捋一下计算机网络的知识，可以大概的到实现的方法，首先TCP是面向连接的，会有三次握手，建立连接成功，即代表两个进程可以用send和recv通信，作为发送信息的一方，肯定是接收到了从用户程序发送数据的请求，即send函数的参数之一，接收到数据后，若数据的大小超过一定长度，肯定不可能直接发送除去，因此，首先要对数据分段，将数据分成一个个的代码段，其次，TCP协议位于传输层，有响应的头部字段，在传输时肯定要加在数据前，数据也就被准备好了。当然，TCP是没有能力直接通过物理链路发送出去的，要想数据正确传输，还需要一层一层的进行。所以，最后一步是将数据传递给网络层，网络层再封装，然后链路层、物理层，最后被发送除去。总结一下就是：
1.数据分段
2.封装头部
3.传递给下一层。
对于secv函数，有一个不太能理解的就是，作为接收方，我是否是一直在等待其他进程给我发送数据，如果是，那么就应该是不停的判断是否有数到了，如果有，就把数据保存起来，然后执行send的逆过程即可。若没有一直等，那就可能是进程被挂起了，如果有数据到达，内核通过中断唤醒进程，然后接收数据。至于具体是哪种，可以通过代码和调试得到结果。
源码分析
首先，当调用send()函数时，内核封装send()为sendto()，然后发起系统调用。其实也很好理解，send()就是sendto()的一种特殊情况，而sendto()在内核的系统调用服务程序为sys_sendto
int __sys_sendto(int fd, void __user *buff, size_t len, unsigned int flags,
struct sockaddr __user *addr, int addr_len)
{
struct socket *sock;
struct sockaddr_storage address;
int err;
struct msghdr msg;
struct iovec iov;
int fput_needed;
err = import_single_range(WRITE, buff, len, &iov, &msg.msg_iter);
if (unlikely(err))
return err;
sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;
msg.msg_name = NULL;
msg.msg_control = NULL;
msg.msg_controllen = 0;
msg.msg_namelen = 0;
if (addr) {
err = move_addr_to_kernel(addr, addr_len, &address);
if (err < 0)
goto out_put;
msg.msg_name = (struct sockaddr *)&address;
msg.msg_namelen = addr_len;
}
if (sock->file->f_flags & O_NONBLOCK)
flags |= MSG_DONTWAIT;
msg.msg_flags = flags;
err = sock_sendmsg(sock, &msg);
out_put:
fput_light(sock->file, fput_needed);
out:
return err;
}
这里定义了一个struct msghdr msg，他是用来表示要发送的数据的一些属性。
struct msghdr {
void *msg_name; /* 接收方的struct sockaddr结构体地址 （用于udp)*/
int msg_namelen; /* 接收方的struct sockaddr结构体地址（用于udp)*/
struct iov_iter msg_iter; /* io缓冲区的地址 */
void *msg_control; /* 辅助数据的地址 */
__kernel_size_t msg_controllen; /* 辅助数据的长度 */
unsigned int msg_flags; /*接受消息的表示 */
struct kiocb *msg_iocb; /* ptr to iocb for async requests */
};
还有一个struct iovec，他被称为io向量，故名思意，用来表示io数据的一些信息。
struct iovec
{
void __user *iov_base; /* 要传输数据的用户态下的地址*) */
__kernel_size_t iov_len; /*要传输数据的长度 */
};
所以，__sys_sendto函数其实做了3件事：1.通过fd获取了对应的struct socket2.创建了用来描述要发送的数据的结构体struct msghdr。3.调用了sock_sendmsg来执行实际的发送。继续追踪这个函数，会看到最终调用的是sock->ops->sendmsg(sock, msg, msg_data_left(msg));，即socet在初始化时赋值给结构体struct proto tcp_prot的函数tcp_sendmsg。
struct proto tcp_prot = {
.name = "TCP",
.owner = THIS_MODULE,
.close = tcp_close,
.pre_connect = tcp_v4_pre_connect,
.connect = tcp_v4_connect,
.disconnect = tcp_disconnect,
.accept = inet_csk_accept,
.ioctl = tcp_ioctl,
.init = tcp_v4_init_sock,
.destroy = tcp_v4_destroy_sock,
.shutdown = tcp_shutdown,
.setsockopt = tcp_setsockopt,
.getsockopt = tcp_getsockopt,
.keepalive = tcp_set_keepalive,
.recvmsg = tcp_recvmsg,
.sendmsg = tcp_sendmsg,
......
tcp_sendmsg实际上调用的是int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)。
int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
{
struct tcp_sock *tp = tcp_sk(sk);/*进行了强制类型转换*/
struct sk_buff *skb;
flags = msg->msg_flags;
......
if (copied)
tcp_push(sk, flags & ~MSG_MORE, mss_now,
TCP_NAGLE_PUSH, size_goal);
}
在tcp_sendmsg_locked中，完成的是将所有的数据组织成发送队列,这个发送队列是struct sock结构中的一个域sk_write_queue，这个队列的每一个元素是一个skb，里面存放的就是待发送的数据。然后调用了tcp_push()函数。
struct sock{
...
struct sk_buff_head sk_write_queue;/*指向skb队列的第一个元素*/
...
struct sk_buff *sk_send_head;/*指向队列第一个还没有发送的元素*/
}
在tcp协议的头部有几个标志字段：URG、ACK、RSH、RST、SYN、FIN，tcp_push中会判断这个skb的元素是否需要push，如果需要就将tcp头部字段的push置一，置一的过程如下：
static void tcp_push(struct sock *sk, int flags, int mss_now,
int nonagle, int size_goal)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
skb = tcp_write_queue_tail(sk);
if (!skb)
return;
if (!(flags & MSG_MORE) || forced_push(tp))
tcp_mark_push(tp, skb);
tcp_mark_urg(tp, flags);
if (tcp_should_autocork(sk, skb, size_goal)) {
/* avoid atomic op if TSQ_THROTTLED bit is already set */
if (!test_bit(TSQ_THROTTLED, &sk->sk_tsq_flags)) {
NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPAUTOCORKING);
set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
}
/* It is possible TX completion already happened
* before we set TSQ_THROTTLED.
*/
if (refcount_read(&sk->sk_wmem_alloc) > skb->truesize)
return;
}
if (flags & MSG_MORE)
nonagle = TCP_NAGLE_CORK;
__tcp_push_pending_frames(sk, mss_now, nonagle);
}
整个过程会有点绕，首先struct tcp_skb_cb结构体存放的就是tcp的头部，头部的控制位为tcp_flags，通过tcp_mark_push会将skb中的cb，也就是48个字节的数组，类型转换为struct tcp_skb_cb ，这样位于skb的cb就成了tcp的头部。
static inline void tcp_mark_push(struct tcp_sock *tp, struct sk_buff *skb)
{
TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
tp->pushed_seq = tp->write_seq;
}
...
#define TCP_SKB_CB(__skb) ((struct tcp_skb_cb *)&((__skb)->cb[0]))
...
struct sk_buff {
...
char cb[48] __aligned(8);
...
struct tcp_skb_cb {
__u32 seq; /* Starting sequence number */
__u32 end_seq; /* SEQ + FIN + SYN + datalen */
__u8 tcp_flags; /* tcp头部标志，位于第13个字节tcp[13]) */
......
};
然后,tcp_push调用了__tcp_push_pending_frames(sk, mss_now, nonagle);函数发送数据：
void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
int nonagle)
{
if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
sk_gfp_mask(sk, GFP_ATOMIC)))
tcp_check_probe_timer(sk);
}
随后又调用了tcp_write_xmit来发送数据：
static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
int push_one, gfp_t gfp)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
unsigned int tso_segs, sent_pkts;
int cwnd_quota;
int result;
bool is_cwnd_limited = false, is_rwnd_limited = false;
u32 max_segs;
/*统计已发送的报文总数*/
sent_pkts = 0;
......
/*若发送队列未满，则准备发送报文*/
while ((skb = tcp_send_head(sk))) {
unsigned int limit;
if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
/* "skb_mstamp_ns" is used as a start point for the retransmit timer */
skb->skb_mstamp_ns = tp->tcp_wstamp_ns = tp->tcp_clock_cache;
list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
tcp_init_tso_segs(skb, mss_now);
goto repair; /* Skip network transmission */
}
if (tcp_pacing_check(sk))
break;
tso_segs = tcp_init_tso_segs(skb, mss_now);
BUG_ON(!tso_segs);
/*检查发送窗口的大小*/
cwnd_quota = tcp_cwnd_test(tp, skb);
if (!cwnd_quota) {
if (push_one == 2)
/* Force out a loss probe pkt. */
cwnd_quota = 1;
else
break;
}
if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
is_rwnd_limited = true;
break;
......
limit = mss_now;
if (tso_segs > 1 && !tcp_urg_mode(tp))
limit = tcp_mss_split_point(sk, skb, mss_now,
min_t(unsigned int,
cwnd_quota,
max_segs),
nonagle);
if (skb->len > limit &&
unlikely(tso_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
skb, limit, mss_now, gfp)))
break;
if (tcp_small_queue_check(sk, skb, 0))
break;
if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
break;
......
tcp_write_xmit位于tcpoutput.c中，它实现了tcp的拥塞控制，然后调用了tcp_transmit_skb(sk, skb, 1, gfp)传输数据，实际上调用的是__tcp_transmit_skb
static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
int clone_it, gfp_t gfp_mask, u32 rcv_nxt)
{
skb_push(skb, tcp_header_size);
skb_reset_transport_header(skb);
......
/* 构建TCP头部和校验和 */
th = (struct tcphdr *)skb->data;
th->source = inet->inet_sport;
th->dest = inet->inet_dport;
th->seq = htonl(tcb->seq);
th->ack_seq = htonl(rcv_nxt);
tcp_options_write((__be32 *)(th + 1), tp, &opts);
skb_shinfo(skb)->gso_type = sk->sk_gso_type;
if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
th->window = htons(tcp_select_window(sk));
tcp_ecn_send(sk, skb, th, tcp_header_size);
} else {
/* RFC1323: The window in SYN & SYN/ACK segments
* is never scaled.
*/
th->window = htons(min(tp->rcv_wnd, 65535U));
}
......
icsk->icsk_af_ops->send_check(sk, skb);
if (likely(tcb->tcp_flags & TCPHDR_ACK))
tcp_event_ack_sent(sk, tcp_skb_pcount(skb), rcv_nxt);
if (skb->len != tcp_header_size) {
tcp_event_data_sent(tp, sk);
tp->data_segs_out += tcp_skb_pcount(skb);
tp->bytes_sent += skb->len - tcp_header_size;
}
if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
tcp_skb_pcount(skb));
tp->segs_out += tcp_skb_pcount(skb);
/* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
/* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */
/* Cleanup our debris for IP stacks */
memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
sizeof(struct inet6_skb_parm)));
err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);
......
}
tcp_transmit_skb是tcp发送数据位于传输层的最后一步，这里首先对TCP数据段的头部进行了处理，然后调用了网络层提供的发送接口icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);实现了数据的发送，自此，数据离开了传输层，传输层的任务也就结束了。
对于recv函数，与send类似，自然也是recvfrom的特殊情况，调用的也就是__sys_recvfrom，整个函数的调用路径与send非常类似：
int __sys_recvfrom(int fd, void __user *ubuf, size_t size, unsigned int flags,
struct sockaddr __user *addr, int __user *addr_len)
{
......
err = import_single_range(READ, ubuf, size, &iov, &msg.msg_iter);
if (unlikely(err))
return err;
sock = sockfd_lookup_light(fd, &err, &fput_needed);
.....
msg.msg_control = NULL;
msg.msg_controllen = 0;
/* Save some cycles and don't copy the address if not needed */
msg.msg_name = addr ? (struct sockaddr *)&address : NULL;
/* We assume all kernel code knows the size of sockaddr_storage */
msg.msg_namelen = 0;
msg.msg_iocb = NULL;
msg.msg_flags = 0;
if (sock->file->f_flags & O_NONBLOCK)
flags |= MSG_DONTWAIT;
err = sock_recvmsg(sock, &msg, flags);
if (err >= 0 && addr != NULL) {
err2 = move_addr_to_user(&address,
msg.msg_namelen, addr, addr_len);
.....
}
__sys_recvfrom调用了sock_recvmsg来接收数据，整个函数实际调用的是sock->ops->recvmsg(sock, msg, msg_data_left(msg), flags);，同样，根据tcp_prot结构的初始化，调用的其实是tcp_rcvmsg
.接受函数比发送函数要复杂得多，因为数据接收不仅仅只是接收，tcp的三次握手也是在接收函数实现的，所以收到数据后要判断当前的状态，是否正在建立连接等，根据发来的信息考虑状态是否要改变，在这里，我们仅仅考虑在连接建立后数据的接收。
int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int nonblock,
int flags, int *addr_len)
{
......
if (sk_can_busy_loop(sk) && skb_queue_empty(&sk->sk_receive_queue) &&
(sk->sk_state == TCP_ESTABLISHED))
sk_busy_loop(sk, nonblock);
lock_sock(sk);
.....
if (unlikely(tp->repair)) {
err = -EPERM;
if (!(flags & MSG_PEEK))
goto out;
if (tp->repair_queue == TCP_SEND_QUEUE)
goto recv_sndq;
err = -EINVAL;
if (tp->repair_queue == TCP_NO_QUEUE)
goto out;
......
last = skb_peek_tail(&sk->sk_receive_queue);
skb_queue_walk(&sk->sk_receive_queue, skb) {
last = skb;
......
if (!(flags & MSG_TRUNC)) {
err = skb_copy_datagram_msg(skb, offset, msg, used);
if (err) {
/* Exception. Bailout! */
if (!copied)
copied = -EFAULT;
break;
}
}
*seq += used;
copied += used;
len -= used;
tcp_rcv_space_adjust(sk);
这里共维护了三个队列：prequeue、backlog、receive_queue，分别为预处理队列，后备队列和接收队列，在连接建立后，若没有数据到来，接收队列为空，进程会在sk_busy_loop函数内循环等待，知道接收队列不为空，并调用函数数skb_copy_datagram_msg将接收到的数据拷贝到用户态，实际调用的是__skb_datagram_iter,这里同样用了struct msghdr *msg来实现。
int __skb_datagram_iter(const struct sk_buff *skb, int offset,
struct iov_iter *to, int len, bool fault_short,
size_t (*cb)(const void *, size_t, void *, struct iov_iter *),
void *data)
{
int start = skb_headlen(skb);
int i, copy = start - offset, start_off = offset, n;
struct sk_buff *frag_iter;
/* 拷贝tcp头部 */
if (copy > 0) {
if (copy > len)
copy = len;
n = cb(skb->data + offset, copy, data, to);
offset += n;
if (n != copy)
goto short_copy;
if ((len -= copy) == 0)
return 0;
}
/* 拷贝数据部分 */
for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
int end;
const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
WARN_ON(start > offset + len);
end = start + skb_frag_size(frag);
if ((copy = end - offset) > 0) {
struct page *page = skb_frag_page(frag);
u8 *vaddr = kmap(page);
if (copy > len)
copy = len;
n = cb(vaddr + frag->page_offset +
offset - start, copy, data, to);
kunmap(page);
offset += n;
if (n != copy)
goto short_copy;
if (!(len -= copy))
return 0;
}
start = end;
}
拷贝完成后，函数返回，整个接收的过程也就完成了。
整体来讲与我们的分析并不大，用一张函数间的相互调用图可以表示
gdb调试分析
接下来用gdb调试验证上面的分析，调试环境为linux5.0.1+menuos(64位)，测试程序为hello/hi网络聊天程序，将其植入到menuos，命令为client。
首先看send的调用关系，分别将断点打在__sys_sendto、tcp_sendmsg_locked、tcp_push、__tcp_push_pending_frames、tcp_write_xmit、__tcp_transmit_skb。观察函数的调用顺序，与我们的分析是否一致。
(gdb) file vmlinux
Reading symbols from vmlinux...done.
warning: File "/home/netlab/netlab/linux-5.2.7/scripts/gdb/vmlinux-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
To enable execution of this file add
add-auto-load-safe-path /home/netlab/netlab/linux-5.2.7/scripts/gdb/vmlinux-gdb.py
line to your configuration file "/home/netlab/.gdbinit".
To completely disable this security protection add
set auto-load safe-path /
line to your configuration file "/home/netlab/.gdbinit".
For more information about this security protection see the
"Auto-loading safe path" section in the GDB manual. E.g., run from the shell:
info "(gdb)Auto-loading safe path"
(gdb) target remote: 1234
Remote debugging using : 1234
0x0000000000000000 in fixed_percpu_data ()
(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
default_idle () at arch/x86/kernel/process.c:581
581 trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
(gdb) b __sys_sendto
Breakpoint 1 at 0xffffffff817ef560: file net/socket.c, line 1929.
(gdb) b tcp_sendmsg_locked
Breakpoint 2 at 0xffffffff81895000: file net/ipv4/tcp.c, line 1158.
(gdb) b tcp_push
Breakpoint 3 at 0xffffffff818907c0: file ./include/linux/skbuff.h, line 1766.
(gdb) b __tcp_push_pending_frames
Breakpoint 4 at 0xffffffff818a44a0: file net/ipv4/tcp_output.c, line 2584.
(gdb) b tcp_wrtie_xmit
Function "tcp_wrtie_xmit" not defined.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) b tcp_write_xmit
Breakpoint 5 at 0xffffffff818a32d0: file net/ipv4/tcp_output.c, line 2330.
(gdb) b __tcp_transmit_skb
Breakpoint 6 at 0xffffffff818a2830: file net/ipv4/tcp_output.c, line 1015.
(gdb)
执行client命令，观察程序暂停的位置：
Breakpoint 6, __tcp_transmit_skb (sk=0xffff888006478880,
skb=0xffff888006871400, clone_it=1, gfp_mask=3264, rcv_nxt=0)
at net/ipv4/tcp_output.c:1015
1015 {
(gdb)
并非我们预想的那样，程序停在了__tcp_transmit_skb，但仔细分析，这应该是三次握手的过程，继续调试
Breakpoint 6, __tcp_transmit_skb (sk=0xffff888006478880,
skb=0xffff888006871400, clone_it=1, gfp_mask=3264, rcv_nxt=0)
at net/ipv4/tcp_output.c:1015
1015 {
(gdb) c
Continuing.
Breakpoint 6, __tcp_transmit_skb (sk=0xffff888006478880,
skb=0xffff88800757a100, clone_it=0, gfp_mask=0, rcv_nxt=1155786088)
at net/ipv4/tcp_output.c:1015
1015 {
(gdb) c
Continuing.
又有两次停在了这里，恰恰验证了猜想，因为这个程序的服务端和客户端都在同一台主机上，共用了同一个TCP协议栈，在TCP三次握手时，客户端发送两次，服务端发送一次，恰好三次。下面我们用客户端向服务器端发送，分析程序的调用过程：
Breakpoint 1, __sys_sendto (fd=5, buff=0x7ffc33c54bc0, len=2, flags=0,
addr=0x0 <fixed_percpu_data>, addr_len=0) at net/socket.c:1929
1929 {
(gdb) c
Continuing.
Breakpoint 2, tcp_sendmsg_locked (sk=0xffff888006479100,
msg=0xffffc900001f7e28, size=2) at net/ipv4/tcp.c:1158
1158 {
(gdb) c
Continuing.
Breakpoint 3, tcp_push (sk=0xffff888006479100, flags=0, mss_now=32752,
nonagle=0, size_goal=32752) at net/ipv4/tcp.c:699
699 skb = tcp_write_queue_tail(sk);
(gdb) c
Continuing.
Breakpoint 4, __tcp_push_pending_frames (sk=0xffff888006479100, cur_mss=32752,
nonagle=0) at net/ipv4/tcp_output.c:2584
2584 if (unlikely(sk->sk_state == TCP_CLOSE))
(gdb) c
Continuing.
Breakpoint 5, tcp_write_xmit (sk=0xffff888006479100, mss_now=32752, nonagle=0,
push_one=0, gfp=2592) at net/ipv4/tcp_output.c:2330
2330 {
(gdb) c
Continuing.
Breakpoint 6, __tcp_transmit_skb (sk=0xffff888006479100,
skb=0xffff888006871400, clone_it=1, gfp_mask=2592, rcv_nxt=405537035)
at net/ipv4/tcp_output.c:1015
1015 {
(gdb) c
Continuing.
Breakpoint 6, __tcp_transmit_skb (sk=0xffff888006478880,
skb=0xffff88800757a100, clone_it=0, gfp_mask=0, rcv_nxt=1155786090)
at net/ipv4/tcp_output.c:1015
1015 {
(gdb) c
可以看到，与我们分析的顺序是一致的，但是最后__tcp_transmit_skb调用了两次，经过仔细分析，终于找到原因——这是接收方接收到数据后发送ACK使用的。
验证完send，来验证一下recv
将断点分别设在：
__sys_recvfrom、sock_recvmsg、tcp_rcvmsg、__skb_datagram_iter处，以同样的方式观察：
Breakpoint 1, __sys_recvfrom (fd=5, ubuf=0x7ffd9428d960, size=1024, flags=0,
addr=0x0 <fixed_percpu_data>, addr_len=0x0 <fixed_percpu_data>)
at net/socket.c:1990
1990 {
(gdb) c
Continuing.
Breakpoint 2, sock_recvmsg (sock=0xffff888006df1900, msg=0xffffc900001f7e28,
flags=0) at net/socket.c:891
891 {
(gdb) c
Continuing.
Breakpoint 3, tcp_recvmsg (sk=0xffff888006479100, msg=0xffffc900001f7e28,
len=1024, nonblock=0, flags=0, addr_len=0xffffc900001f7df4)
at net/ipv4/tcp.c:1933
1933 {
(gdb) c
在未发送之前，程序也会暂停在断点处，根据之前的分析，这也是三次握手的过程，但是为什么没有__skb_datagram_iter呢？，因为三次握手时，并没有发送数据过来，所以并没有数据被拷贝到用户空间。
同样，尝试发送数据观察调用过程。
Breakpoint 1, __sys_recvfrom (fd=5, ubuf=0x7ffd9428d960, size=1024, flags=0,
addr=0x0 <fixed_percpu_data>, addr_len=0x0 <fixed_percpu_data>)
at net/socket.c:1990
1990 {
(gdb) c
Continuing.
Breakpoint 2, sock_recvmsg (sock=0xffff888006df1900, msg=0xffffc900001f7e28,
flags=0) at net/socket.c:891
891 {
(gdb) c
Continuing.
Breakpoint 3, tcp_recvmsg (sk=0xffff888006479100, msg=0xffffc900001f7e28,
len=1024, nonblock=0, flags=0, addr_len=0xffffc900001f7df4)
at net/ipv4/tcp.c:1933
1933 {
(gdb) c
Continuing.
Breakpoint 4, __skb_datagram_iter (skb=0xffff8880068714e0, offset=0,
to=0xffffc900001efe38, len=2, fault_short=false,
cb=0xffffffff817ff860 <simple_copy_to_iter>, data=0x0 <fixed_percpu_data>)
at net/core/datagram.c:414
414 {
验证完毕。