Linux内核--网络栈实现分析(六)--应用层获取数据包(上)
本文分析基于内核Linux 1.2.13
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作者:闫明
注:标题中的”(上)“,”(下)“表示分析过程基于数据包的传递方向:”(上)“表示分析是从底层向上分析、”(下)“表示分析是从上向下分析。
上篇博文分析了传输层从网络层获取数据包后将数据包缓存结构sk_buff挂载到特定的sock结构的接收队列中。
这里接着分析应用程序是如何从传输层获取网络数据包的。应用层要得到传输层的数据包有两种主要的方式:系统调用和文件操作。
系统调用:
Linux下用户程序是通过系统调用来从用户态到内核态,调用内核功能来完成相应的服务。
网络栈的一些功能是通过系统调用sys_socketcall来完成的
具体的代码在net/socket.c中,该文件中的函数就相当于一个桥梁,在系统调用和内核网络栈之间。
- /*
- * System call vectors. Since I (RIB) want to rewrite sockets as streams,
- * we have this level of indirection. Not a lot of overhead, since more of
- * the work is done via read/write/select directly.
- *
- * I‘m now expanding this up to a higher level to separate the assorted
- * kernel/user space manipulations and global assumptions from the protocol
- * layers proper - AC.
- */
- asmlinkage int sys_socketcall(int call, unsigned long *args)
- {
- int er;
- switch(call)
- {
- case SYS_SOCKET:
- er=verify_area(VERIFY_READ, args, 3 * sizeof(long));
- if(er)
- return er;
- return(sock_socket(get_fs_long(args+0),
- get_fs_long(args+1),
- get_fs_long(args+2)));
- case SYS_BIND:
- er=verify_area(VERIFY_READ, args, 3 * sizeof(long));
- if(er)
- return er;
- return(sock_bind(get_fs_long(args+0),
- (struct sockaddr *)get_fs_long(args+1),
- get_fs_long(args+2)));
- case SYS_CONNECT:
- er=verify_area(VERIFY_READ, args, 3 * sizeof(long));
- if(er)
- return er;
- return(sock_connect(get_fs_long(args+0),
- (struct sockaddr *)get_fs_long(args+1),
- get_fs_long(args+2)));
- case SYS_LISTEN:
- er=verify_area(VERIFY_READ, args, 2 * sizeof(long));
- if(er)
- return er;
- return(sock_listen(get_fs_long(args+0),
- get_fs_long(args+1)));
- case SYS_ACCEPT:
- er=verify_area(VERIFY_READ, args, 3 * sizeof(long));
- if(er)
- return er;
- return(sock_accept(get_fs_long(args+0),
- (struct sockaddr *)get_fs_long(args+1),
- (int *)get_fs_long(args+2)));
- case SYS_GETSOCKNAME:
- er=verify_area(VERIFY_READ, args, 3 * sizeof(long));
- if(er)
- return er;
- return(sock_getsockname(get_fs_long(args+0),
- (struct sockaddr *)get_fs_long(args+1),
- (int *)get_fs_long(args+2)));
- case SYS_GETPEERNAME:
- er=verify_area(VERIFY_READ, args, 3 * sizeof(long));
- if(er)
- return er;
- return(sock_getpeername(get_fs_long(args+0),
- (struct sockaddr *)get_fs_long(args+1),
- (int *)get_fs_long(args+2)));
- case SYS_SOCKETPAIR:
- er=verify_area(VERIFY_READ, args, 4 * sizeof(long));
- if(er)
- return er;
- return(sock_socketpair(get_fs_long(args+0),
- get_fs_long(args+1),
- get_fs_long(args+2),
- (unsigned long *)get_fs_long(args+3)));
- case SYS_SEND:
- er=verify_area(VERIFY_READ, args, 4 * sizeof(unsigned long));
- if(er)
- return er;
- return(sock_send(get_fs_long(args+0),
- (void *)get_fs_long(args+1),
- get_fs_long(args+2),
- get_fs_long(args+3)));
- case SYS_SENDTO:
- er=verify_area(VERIFY_READ, args, 6 * sizeof(unsigned long));
- if(er)
- return er;
- return(sock_sendto(get_fs_long(args+0),
- (void *)get_fs_long(args+1),
- get_fs_long(args+2),
- get_fs_long(args+3),
- (struct sockaddr *)get_fs_long(args+4),
- get_fs_long(args+5)));
- case SYS_RECV:
- er=verify_area(VERIFY_READ, args, 4 * sizeof(unsigned long));
- if(er)
- return er;
- return(sock_recv(get_fs_long(args+0),
- (void *)get_fs_long(args+1),
- get_fs_long(args+2),
- get_fs_long(args+3)));
- case SYS_RECVFROM:
- er=verify_area(VERIFY_READ, args, 6 * sizeof(unsigned long));
- if(er)
- return er;
- return(sock_recvfrom(get_fs_long(args+0),
- (void *)get_fs_long(args+1),
- get_fs_long(args+2),
- get_fs_long(args+3),
- (struct sockaddr *)get_fs_long(args+4),
- (int *)get_fs_long(args+5)));
- case SYS_SHUTDOWN:
- er=verify_area(VERIFY_READ, args, 2* sizeof(unsigned long));
- if(er)
- return er;
- return(sock_shutdown(get_fs_long(args+0),
- get_fs_long(args+1)));
- case SYS_SETSOCKOPT:
- er=verify_area(VERIFY_READ, args, 5*sizeof(unsigned long));
- if(er)
- return er;
- return(sock_setsockopt(get_fs_long(args+0),
- get_fs_long(args+1),
- get_fs_long(args+2),
- (char *)get_fs_long(args+3),
- get_fs_long(args+4)));
- case SYS_GETSOCKOPT:
- er=verify_area(VERIFY_READ, args, 5*sizeof(unsigned long));
- if(er)
- return er;
- return(sock_getsockopt(get_fs_long(args+0),
- get_fs_long(args+1),
- get_fs_long(args+2),
- (char *)get_fs_long(args+3),
- (int *)get_fs_long(args+4)));
- default:
- return(-EINVAL);
- }
- }
上面系统调用的宏定义如下:
- #define SYS_SOCKET 1 /* sys_socket(2) */
- #define SYS_BIND 2 /* sys_bind(2) */
- #define SYS_CONNECT 3 /* sys_connect(2) */
- #define SYS_LISTEN 4 /* sys_listen(2) */
- #define SYS_ACCEPT 5 /* sys_accept(2) */
- #define SYS_GETSOCKNAME 6 /* sys_getsockname(2) */
- #define SYS_GETPEERNAME 7 /* sys_getpeername(2) */
- #define SYS_SOCKETPAIR 8 /* sys_socketpair(2) */
- #define SYS_SEND 9 /* sys_send(2) */
- #define SYS_RECV 10 /* sys_recv(2) */
- #define SYS_SENDTO 11 /* sys_sendto(2) */
- #define SYS_RECVFROM 12 /* sys_recvfrom(2) */
- #define SYS_SHUTDOWN 13 /* sys_shutdown(2) */
- #define SYS_SETSOCKOPT 14 /* sys_setsockopt(2) */
- #define SYS_GETSOCKOPT 15 /* sys_getsockopt(2) */
应用层在一系列操作后就可以通过参数SYS_RECV或SYS_RECVFROM来获取数据包。由于UDP是无连接的,所以如果需要回复,必须使用recvfrom才能得知是谁发送的数据包。当然UDP也可以用recv类函数,只是它不能回复,只能接收。
这里还是以INET中UDP来举例说明。
如果系统调用参数是SYS_RECVFROM,则会进行内存校验后执行函数socket_recvform()函数。
- /*
- * Receive a frame from the socket and optionally record the address of the
- * sender. We verify the buffers are writable and if needed move the
- * sender address from kernel to user space.
- */
- static int sock_recvfrom(int fd, void * buff, int len, unsigned flags,
- struct sockaddr *addr, int *addr_len)
- {
- struct socket *sock;
- struct file *file;
- char address[MAX_SOCK_ADDR];
- int err;
- int alen;
- if (fd < 0 || fd >= NR_OPEN || ((file = current->files->fd[fd]) == NULL))
- return(-EBADF);
- if (!(sock = sockfd_lookup(fd, NULL)))
- return(-ENOTSOCK);
- if(len<0)
- return -EINVAL;
- if(len==0)
- return 0;
- err=verify_area(VERIFY_WRITE,buff,len);
- if(err)
- return err;
- //进行相应检查后调用下层函数,INET域则为inet_recvfrom()函数
- len=sock->ops->recvfrom(sock, buff, len, (file->f_flags & O_NONBLOCK),
- flags, (struct sockaddr *)address, &alen);
- if(len<0)
- return len;
- if(addr!=NULL && (err=move_addr_to_user(address,alen, addr, addr_len))<0)//将发送发地址从内核空间COPY到用户空间
- return err;
- return len;
- }
在inet_recvfrom()函数中会调用具体的协议操作函数。UDP的协议操作函数定义如下:
- struct proto udp_prot = {
- sock_wmalloc,
- sock_rmalloc,
- sock_wfree,
- sock_rfree,
- sock_rspace,
- sock_wspace,
- udp_close,
- udp_read,
- udp_write,
- udp_sendto,
- udp_recvfrom,
- ip_build_header,
- udp_connect,
- NULL,
- ip_queue_xmit,
- NULL,
- NULL,
- NULL,
- udp_rcv,
- datagram_select,
- udp_ioctl,
- NULL,
- NULL,
- ip_setsockopt,
- ip_getsockopt,
- 128,
- 0,
- {NULL,},
- "UDP",
- 0, 0
- };
可以看到,其对应的函数对udp_recvfrom()
- /*
- * This should be easy, if there is something there we\
- * return it, otherwise we block.
- */
- int udp_recvfrom(struct sock *sk, unsigned char *to, int len,
- int noblock, unsigned flags, struct sockaddr_in *sin,
- int *addr_len)
- {
- int copied = 0;
- int truesize;
- struct sk_buff *skb;
- int er;
- /*
- * Check any passed addresses
- */
- if (addr_len)
- *addr_len=sizeof(*sin);
- /*
- * From here the generic datagram does a lot of the work. Come
- * the finished NET3, it will do _ALL_ the work!
- */
- skb=skb_recv_datagram(sk,flags,noblock,&er);
- if(skb==NULL)
- return er;
- truesize = skb->len;
- copied = min(len, truesize);
- /*
- * FIXME : should use udp header size info value
- */
- skb_copy_datagram(skb,sizeof(struct udphdr),to,copied);//从sk_buff结构中取出数据部分
- sk->stamp=skb->stamp;
- /* Copy the address. */
- if (sin)
- {
- sin->sin_family = AF_INET;
- sin->sin_port = skb->h.uh->source;
- sin->sin_addr.s_addr = skb->daddr;
- }
- skb_free_datagram(skb);
- release_sock(sk);
- return(truesize);
- }
这样数据就到达了用户空间。
普通文件操作函数接口
最主要的函数就是读写函数:sock_read和sock_write,可以通过文件操作来完成网络数据的读写。谈到文件,就得有文件描述符,文件描述符中的f_inode指针指向文件的存储结点结构。
文件操作集定义如下:
- static struct file_operations socket_file_ops = {
- sock_lseek,
- sock_read,
- sock_write,
- sock_readdir,
- sock_select,
- sock_ioctl,
- NULL, /* mmap */
- NULL, /* no special open code... */
- sock_close,
- NULL, /* no fsync */
- sock_fasync
- };
read函数和write函数与recvfrom和send类似,这里列出函数,方便查看。
- /*
- * Read data from a socket. ubuf is a user mode pointer. We make sure the user
- * area ubuf...ubuf+size-1 is writable before asking the protocol.
- */
- static int sock_read(struct inode *inode, struct file *file, char *ubuf, int size)
- {
- struct socket *sock;
- int err;
- if (!(sock = socki_lookup(inode)))
- {
- printk("NET: sock_read: can‘t find socket for inode!\n");
- return(-EBADF);
- }
- if (sock->flags & SO_ACCEPTCON)
- return(-EINVAL);
- if(size<0)
- return -EINVAL;
- if(size==0)
- return 0;
- if ((err=verify_area(VERIFY_WRITE,ubuf,size))<0)
- return err;
- return(sock->ops->read(sock, ubuf, size, (file->f_flags & O_NONBLOCK)));//和recvfrom函数类似,调用INET域相应函数
- }
这两种方式是内核网络栈对用户的接口。
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