Linux Kernel 2.6.9源码分析 -- send/recieve 报文

Linux Kernel 2.6.9源码分析 – send/recieve 报文

可用户socket报文读写的函数有以下几对：

ssize_t read(int fd, void *buf, size_t count);
ssize_t write(int fd, const void *buf, size_t count);
ssize_t send(int sockfd, const void *buf, size_t len, int flags);
ssize_t recv(int sockfd, void *buf, size_t len, int flags);
ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);
ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,const struct sockaddr *dest_addr, socklen_t addrlen);
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,struct sockaddr *src_addr, socklen_t *addrlen);

区别：
通过“有链接”，“无连接”来区分：
上述1,2,3是面向“有链接”的，因为其参数中没有带上目标地址
上述4 是面向“无连接”的，其参数中有带上目标地址
共同点：
1,2,3,4对虽然对应的系统调用有所不同，但最终在内核中调用的函数是相同的：
发送：static inline int __sock_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg, size_t size)
—>sock->ops->sendmsg
接收：static inline int __sock_recvmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg, size_t size, int flags)
–> sock->ops->recvmsg
不同类型的socket，在创建的时候会注册不同的ops，对于TCP而言:
net/ipv4/tcp.c -->
int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,size_t size)
int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,size_t len, int nonblock, int flags, int *addr_len)

首先我们来看下报文的接收(待深入分析…)：

首先不是很明白TCP中的prequeue,receive queue,backlog queue三个的作用有什么不同，所以下面的这个代码难以理解.
当前能够理解的是:TCP的数据包是一个个struct sk_buff结构的list.而面向传入到用户层的数据结构是struct msghdr
将skb中的数据copy到struct msghdr.iovec

int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,size_t len, int nonblock, int flags, int *addr_len)
{struct tcp_opt *tp = tcp_sk(sk);int copied = 0;u32 peek_seq;u32 *seq;unsigned long used;int err;int target;      /* Read at least this many bytes */long timeo;struct task_struct *user_recv = NULL;lock_sock(sk);TCP_CHECK_TIMER(sk);err = -ENOTCONN;if (sk->sk_state == TCP_LISTEN)goto out;timeo = sock_rcvtimeo(sk, nonblock);/* Urgent data needs to be handled specially. */if (flags & MSG_OOB)goto recv_urg;seq = &tp->copied_seq;if (flags & MSG_PEEK) {peek_seq = tp->copied_seq;seq = &peek_seq;}target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);do {struct sk_buff *skb;u32 offset;/* Are we at urgent data? Stop if we have read anything or have SIGURG pending. */if (tp->urg_data && tp->urg_seq == *seq) {if (copied)break;if (signal_pending(current)) {copied = timeo ? sock_intr_errno(timeo) : -EAGAIN;break;}}/* Next get a buffer. */skb = skb_peek(&sk->sk_receive_queue);do {if (!skb)break;/* Now that we have two receive queues this* shouldn't happen.*/if (before(*seq, TCP_SKB_CB(skb)->seq)) {printk(KERN_INFO "recvmsg bug: copied %X ""seq %X\n", *seq, TCP_SKB_CB(skb)->seq);break;}offset = *seq - TCP_SKB_CB(skb)->seq;if (skb->h.th->syn)offset--;if (offset < skb->len)goto found_ok_skb;if (skb->h.th->fin)goto found_fin_ok;BUG_TRAP(flags & MSG_PEEK);skb = skb->next;} while (skb != (struct sk_buff *)&sk->sk_receive_queue);/* Well, if we have backlog, try to process it now yet. */if (copied >= target && !sk->sk_backlog.tail)break;if (copied) {if (sk->sk_err ||sk->sk_state == TCP_CLOSE ||(sk->sk_shutdown & RCV_SHUTDOWN) ||!timeo ||signal_pending(current) ||(flags & MSG_PEEK))break;} else {if (sock_flag(sk, SOCK_DONE))break;if (sk->sk_err) {copied = sock_error(sk);break;}if (sk->sk_shutdown & RCV_SHUTDOWN)break;if (sk->sk_state == TCP_CLOSE) {if (!sock_flag(sk, SOCK_DONE)) {/* This occurs when user tries to read* from never connected socket.*/copied = -ENOTCONN;break;}break;}if (!timeo) {copied = -EAGAIN;break;}if (signal_pending(current)) {copied = sock_intr_errno(timeo);break;}}cleanup_rbuf(sk, copied);if (tp->ucopy.task == user_recv) {/* Install new reader */if (!user_recv && !(flags & (MSG_TRUNC | MSG_PEEK))) {user_recv = current;tp->ucopy.task = user_recv;tp->ucopy.iov = msg->msg_iov;}tp->ucopy.len = len;BUG_TRAP(tp->copied_seq == tp->rcv_nxt ||(flags & (MSG_PEEK | MSG_TRUNC)));/* Ugly... If prequeue is not empty, we have to* process it before releasing socket, otherwise* order will be broken at second iteration.* More elegant solution is required!!!** Look: we have the following (pseudo)queues:** 1. packets in flight* 2. backlog* 3. prequeue* 4. receive_queue** Each queue can be processed only if the next ones* are empty. At this point we have empty receive_queue.* But prequeue _can_ be not empty after 2nd iteration,* when we jumped to start of loop because backlog* processing added something to receive_queue.* We cannot release_sock(), because backlog contains* packets arrived _after_ prequeued ones.** Shortly, algorithm is clear --- to process all* the queues in order. We could make it more directly,* requeueing packets from backlog to prequeue, if* is not empty. It is more elegant, but eats cycles,* unfortunately.*/if (skb_queue_len(&tp->ucopy.prequeue))goto do_prequeue;/* __ Set realtime policy in scheduler __ */}if (copied >= target) {/* Do not sleep, just process backlog. */release_sock(sk);lock_sock(sk);} elsesk_wait_data(sk, &timeo);if (user_recv) {int chunk;/* __ Restore normal policy in scheduler __ */if ((chunk = len - tp->ucopy.len) != 0) {NET_ADD_STATS_USER(LINUX_MIB_TCPDIRECTCOPYFROMBACKLOG, chunk);len -= chunk;copied += chunk;}if (tp->rcv_nxt == tp->copied_seq &&skb_queue_len(&tp->ucopy.prequeue)) {do_prequeue:tcp_prequeue_process(sk);if ((chunk = len - tp->ucopy.len) != 0) {NET_ADD_STATS_USER(LINUX_MIB_TCPDIRECTCOPYFROMPREQUEUE, chunk);len -= chunk;copied += chunk;}}}if ((flags & MSG_PEEK) && peek_seq != tp->copied_seq) {if (net_ratelimit())printk(KERN_DEBUG "TCP(%s:%d): Application bug, race in MSG_PEEK.\n",current->comm, current->pid);peek_seq = tp->copied_seq;}continue;found_ok_skb:/* Ok so how much can we use? */used = skb->len - offset;if (len < used)used = len;/* Do we have urgent data here? */if (tp->urg_data) {u32 urg_offset = tp->urg_seq - *seq;if (urg_offset < used) {if (!urg_offset) {if (!sock_flag(sk, SOCK_URGINLINE)) {++*seq;offset++;used--;if (!used)goto skip_copy;}} elseused = urg_offset;}}if (!(flags & MSG_TRUNC)) {err = skb_copy_datagram_iovec(skb, offset,msg->msg_iov, used);if (err) {/* Exception. Bailout! */if (!copied)copied = -EFAULT;break;}}*seq += used;copied += used;len -= used;tcp_rcv_space_adjust(sk);skip_copy:if (tp->urg_data && after(tp->copied_seq, tp->urg_seq)) {tp->urg_data = 0;tcp_fast_path_check(sk, tp);}if (used + offset < skb->len)continue;if (skb->h.th->fin)goto found_fin_ok;if (!(flags & MSG_PEEK))sk_eat_skb(sk, skb);continue;found_fin_ok:/* Process the FIN. */++*seq;if (!(flags & MSG_PEEK))sk_eat_skb(sk, skb);break;} while (len > 0);if (user_recv) {if (skb_queue_len(&tp->ucopy.prequeue)) {int chunk;tp->ucopy.len = copied > 0 ? len : 0;tcp_prequeue_process(sk);if (copied > 0 && (chunk = len - tp->ucopy.len) != 0) {NET_ADD_STATS_USER(LINUX_MIB_TCPDIRECTCOPYFROMPREQUEUE, chunk);len -= chunk;copied += chunk;}}tp->ucopy.task = NULL;tp->ucopy.len = 0;}/* According to UNIX98, msg_name/msg_namelen are ignored* on connected socket. I was just happy when found this 8) --ANK*//* Clean up data we have read: This will do ACK frames. */cleanup_rbuf(sk, copied);TCP_CHECK_TIMER(sk);release_sock(sk);return copied;out:TCP_CHECK_TIMER(sk);release_sock(sk);return err;recv_urg:err = tcp_recv_urg(sk, timeo, msg, len, flags, addr_len);goto out;
}

TCP 数据发送:（后续分析…）

int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,size_t size)
{struct iovec *iov;struct tcp_opt *tp = tcp_sk(sk);struct sk_buff *skb;int iovlen, flags;int mss_now;int err, copied;long timeo;lock_sock(sk);TCP_CHECK_TIMER(sk);flags = msg->msg_flags;timeo = sock_sndtimeo(sk, flags & MSG_DONTWAIT);/* Wait for a connection to finish. */if ((1 << sk->sk_state) & ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))if ((err = sk_stream_wait_connect(sk, &timeo)) != 0)goto out_err;/* This should be in poll */clear_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags);mss_now = tcp_current_mss(sk, !(flags&MSG_OOB));/* Ok commence sending. */iovlen = msg->msg_iovlen;iov = msg->msg_iov;copied = 0;err = -EPIPE;if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))goto do_error;while (--iovlen >= 0) {int seglen = iov->iov_len;unsigned char __user *from = iov->iov_base;iov++;while (seglen > 0) {int copy;skb = sk->sk_write_queue.prev;if (!sk->sk_send_head ||(copy = mss_now - skb->len) <= 0) {new_segment:/* Allocate new segment. If the interface is SG,* allocate skb fitting to single page.*/if (!sk_stream_memory_free(sk))goto wait_for_sndbuf;skb = sk_stream_alloc_pskb(sk, select_size(sk, tp),0, sk->sk_allocation);if (!skb)goto wait_for_memory;/** Check whether we can use HW checksum.*/if (sk->sk_route_caps &(NETIF_F_IP_CSUM | NETIF_F_NO_CSUM |NETIF_F_HW_CSUM))skb->ip_summed = CHECKSUM_HW;skb_entail(sk, tp, skb);copy = mss_now;}/* Try to append data to the end of skb. */if (copy > seglen)copy = seglen;/* Where to copy to? */if (skb_tailroom(skb) > 0) {/* We have some space in skb head. Superb! */if (copy > skb_tailroom(skb))copy = skb_tailroom(skb);if ((err = skb_add_data(skb, from, copy)) != 0)goto do_fault;} else {int merge = 0;int i = skb_shinfo(skb)->nr_frags;struct page *page = TCP_PAGE(sk);int off = TCP_OFF(sk);if (skb_can_coalesce(skb, i, page, off) &&off != PAGE_SIZE) {/* We can extend the last page* fragment. */merge = 1;} else if (i == MAX_SKB_FRAGS ||(!i &&!(sk->sk_route_caps & NETIF_F_SG))) {/* Need to add new fragment and cannot* do this because interface is non-SG,* or because all the page slots are* busy. */tcp_mark_push(tp, skb);goto new_segment;} else if (page) {/* If page is cached, align* offset to L1 cache boundary*/off = (off + L1_CACHE_BYTES - 1) &~(L1_CACHE_BYTES - 1);if (off == PAGE_SIZE) {put_page(page);TCP_PAGE(sk) = page = NULL;}}if (!page) {/* Allocate new cache page. */if (!(page = sk_stream_alloc_page(sk)))goto wait_for_memory;off = 0;}if (copy > PAGE_SIZE - off)copy = PAGE_SIZE - off;/* Time to copy data. We are close to* the end! */err = skb_copy_to_page(sk, from, skb, page,off, copy);if (err) {/* If this page was new, give it to the* socket so it does not get leaked.*/if (!TCP_PAGE(sk)) {TCP_PAGE(sk) = page;TCP_OFF(sk) = 0;}goto do_error;}/* Update the skb. */if (merge) {skb_shinfo(skb)->frags[i - 1].size +=copy;} else {skb_fill_page_desc(skb, i, page, off, copy);if (TCP_PAGE(sk)) {get_page(page);} else if (off + copy < PAGE_SIZE) {get_page(page);TCP_PAGE(sk) = page;}}TCP_OFF(sk) = off + copy;}if (!copied)TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_PSH;tp->write_seq += copy;TCP_SKB_CB(skb)->end_seq += copy;skb_shinfo(skb)->tso_segs = 0;from += copy;copied += copy;if ((seglen -= copy) == 0 && iovlen == 0)goto out;if (skb->len != mss_now || (flags & MSG_OOB))continue;if (forced_push(tp)) {tcp_mark_push(tp, skb);__tcp_push_pending_frames(sk, tp, mss_now, TCP_NAGLE_PUSH);} else if (skb == sk->sk_send_head)tcp_push_one(sk, mss_now);continue;wait_for_sndbuf:set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
wait_for_memory:if (copied)tcp_push(sk, tp, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH);if ((err = sk_stream_wait_memory(sk, &timeo)) != 0)goto do_error;mss_now = tcp_current_mss(sk, !(flags&MSG_OOB));}}out:if (copied)tcp_push(sk, tp, flags, mss_now, tp->nonagle);TCP_CHECK_TIMER(sk);release_sock(sk);return copied;do_fault:if (!skb->len) {if (sk->sk_send_head == skb)sk->sk_send_head = NULL;__skb_unlink(skb, skb->list);sk_stream_free_skb(sk, skb);}do_error:if (copied)goto out;
out_err:err = sk_stream_error(sk, flags, err);TCP_CHECK_TIMER(sk);release_sock(sk);return err;
}

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Linux Kernel 2.6.9源码分析 -- send/recieve 报文

Linux Kernel 2.6.9源码分析 – send/recieve 报文

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