本文将以socket函数为例，分析它在Linux5.12.10内核中的实现，先观此图，宏观上把握它在内核中的函数调用关系：

socket函数API

socket 函数原型：

#include <sys/socket.h>

int socket(int domain, int type, int protocol)

该函数用于创建一个新的socket。

第一个参数：

domain：协议簇，常用的协议簇有：AF_INET, AF_INET6, AF_LOCAL。这个参数决定了socket的地址类型，这个应该很好理解AF_INET用于ipv4地址，AF_INET6用于ipv6地址，AF_LOCAL用于本地进程间通信。

第二个参数：

type：socket类型有好几种，主要是两种：SOCK_STREAM、SOCK_DGRAM(数据报)，通俗说就是字节流socket和数据报socket，当你在创建的使用使用哪一种由第二个参数指定。stream socket基于TCP协议，是一个有序、可靠、全双工的字节流通道。datagram socket基于UDP协议，不需要建立和维持连接，可能会丢失或错乱。

第三个参数：

protocol：指定协议，常用协议有IPPROTO_TCP、IPPROTO_UDP、IPPROTO_STCP、IPPROTO_TICP等，分别对应TCP协议，UDP协议，STCP协议，TICP协议。通常这个参数设置为0，表示自适应协议

所以这个函数通常这样用：

int socket_fd = socket(AF_INET, SOCK_STREAM, 0);

在Linux下一个进程默认打开的文件描述符是1024个，也就是说一个进程最多能创建1024个socket，超过就会报Too many open files(这个问题在工作中也会遇到)。通过ulimit命令可以查看到

# ulimit -a
core file size          (blocks, -c) unlimited
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 29414
max locked memory       (kbytes, -l) 16384
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 29414
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

如果你要修改这个上限到2021个：

# ulimit -HSn 2021

内核源码

//~/linux-5.12.10/include/linux/socket.h 头文件
extern int __sys_socket(int family, int type, int protocol);

socket函数调用结束后，用户层看到返回一个整型的句柄，但是内核在内部会创建一系列的socket相关的内核对象(不是只有一个对象)

// ~/linux-5.12.10/net/socket.c line:1481
/* Mask which covers at least up to SOCK_MASK-1.  The
 * remaining bits are used as flags. */
#define SOCK_TYPE_MASK 0xf

int __sys_socket(int family, int type, int protocol)
{
	int retval;
	struct socket *sock;
	int flags;
    
	//... 略去参数合法性校验代码

	retval = sock_create(family, type, protocol, &sock);
	if (retval < 0)
		return retval;

	return sock_map_fd(sock, flags & (O_CLOEXEC | O_NONBLOCK));
}

SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)
{
	return __sys_socket(family, type, protocol);
}

sock_create

sock_create是创建socket的主要位置，其中sock_create又调用__sock_create

// ~/linux-5.12.10/net/socket.c line:1337
/*
//net_proto_family结构体定义了每一个协议族的新建socket句柄
struct net_proto_family {
	int		family;
	int		(*create)(struct net *net, struct socket *sock,
				  int protocol, int kern);
	struct module	*owner;
};

static const struct net_proto_family __rcu *net_families[NPROTO] __read_mostly;
*/
int __sock_create(struct net *net, int family, int type, int protocol,
			 struct socket **res, int kern)
{
	int err;
	struct socket *sock;
	const struct net_proto_family *pf;

	/*
	 *      Check protocol is in range
	 */
	if (family < 0 || family >= NPROTO)
		return -EAFNOSUPPORT;
	if (type < 0 || type >= SOCK_MAX)
		return -EINVAL;

	/* Compatibility.

	   This uglymoron is moved from INET layer to here to avoid
	   deadlock in module load.
	 */
	if (family == PF_INET && type == SOCK_PACKET) {
		pr_info_once("%s uses obsolete (PF_INET,SOCK_PACKET)\n",
			     current->comm);
		family = PF_PACKET;
	}

	err = security_socket_create(family, type, protocol, kern);
	if (err)
		return err;

	/*
	 *	Allocate the socket and allow the family to set things up. if
	 *	the protocol is 0, the family is instructed to select an appropriate
	 *	default.
	 */
    // 分配socket对象，如果protocol为0 将会被设置合适的协议
	sock = sock_alloc();
	if (!sock) {
		net_warn_ratelimited("socket: no more sockets\n");
		return -ENFILE;	/* Not exactly a match, but its the
				   closest posix thing */
	}

	sock->type = type;

#ifdef CONFIG_MODULES
	/* Attempt to load a protocol module if the find failed.
	 *
	 * 12/09/1996 Marcin: But! this makes REALLY only sense, if the user
	 * requested real, full-featured networking support upon configuration.
	 * Otherwise module support will break!
	 */
	if (rcu_access_pointer(net_families[family]) == NULL)
		request_module("net-pf-%d", family);
#endif
	
    // 获取每个协议族的操作表
	rcu_read_lock();
	pf = rcu_dereference(net_families[family]);
	err = -EAFNOSUPPORT;
	if (!pf)
		goto out_release;

	/*
	 * We will call the ->create function, that possibly is in a loadable
	 * module, so we have to bump that loadable module refcnt first.
	 */
	if (!try_module_get(pf->owner))
		goto out_release;

	/* Now protected by module ref count */
	rcu_read_unlock();
	
    /// 调用指定协议族的创建函数，对于AF_INET对应的是inet_create
	err = pf->create(net, sock, protocol, kern);
	if (err < 0)
		goto out_module_put;

	/*
	 * Now to bump the refcnt of the [loadable] module that owns this
	 * socket at sock_release time we decrement its refcnt.
	 */
	if (!try_module_get(sock->ops->owner))
		goto out_module_busy;

	/*
	 * Now that we're done with the ->create function, the [loadable]
	 * module can have its refcnt decremented
	 */
	module_put(pf->owner);
	err = security_socket_post_create(sock, family, type, protocol, kern);
	if (err)
		goto out_sock_release;
	*res = sock;

	return 0;

out_module_busy:
	err = -EAFNOSUPPORT;
out_module_put:
	sock->ops = NULL;
	module_put(pf->owner);
out_sock_release:
	sock_release(sock);
	return err;

out_release:
	rcu_read_unlock();
	goto out_sock_release;
}

inet_create

在 __sock_create 里，首先调用sock_alloc来分配一个struct socket内核对象，接着获取协议族的操作函数表，并调用其create方法。对于AF_INET协议族来说，执行到的是inet_create方法

//~/linux-5.12.10/net/ipv4/af_inet.c
/*
/* This is used to register socket interfaces for IP protocols.  */
struct inet_protosw {
	struct list_head list;

        /* These two fields form the lookup key.  */
	unsigned short	 type;	   /* This is the 2nd argument to socket(2). */
	unsigned short	 protocol; /* This is the L4 protocol number.  */

	struct proto	 *prot;
	const struct proto_ops *ops;
  
	unsigned char	 flags;      /* See INET_PROTOSW_* below.  */
};
#define list_for_each_entry_rcu		list_for_each_entry

#define list_for_each_entry(pos, head, member)				\
	for (pos = list_first_entry(head, typeof(*pos), member);	\
	     &pos->member != (head);					\
	     pos = list_next_entry(pos, member))

*/
static int inet_create(struct net *net, struct socket *sock, int protocol,
		       int kern)
{
	struct sock *sk;
	struct inet_protosw *answer;
	struct inet_sock *inet;
	struct proto *answer_prot;
	unsigned char answer_flags;
	int try_loading_module = 0;
	int err;

	if (protocol < 0 || protocol >= IPPROTO_MAX)
		return -EINVAL;

	sock->state = SS_UNCONNECTED;

	/* Look for the requested type/protocol pair. */
lookup_protocol:
	err = -ESOCKTNOSUPPORT;
	rcu_read_lock();
	list_for_each_entry_rcu(answer, &inetsw[sock->type], list) {

		err = 0;
		/* Check the non-wild match. */
		if (protocol == answer->protocol) {
			if (protocol != IPPROTO_IP)
				break;
		} else {
			/* Check for the two wild cases. */
			if (IPPROTO_IP == protocol) {
				protocol = answer->protocol;
				break;
			}
			if (IPPROTO_IP == answer->protocol)
				break;
		}
		err = -EPROTONOSUPPORT;
	}

	//...

	err = -EPERM;
	if (sock->type == SOCK_RAW && !kern &&
	    !ns_capable(net->user_ns, CAP_NET_RAW))
		goto out_rcu_unlock;
	
    //将 inet_stream_ops 赋值到sock->ops
	sock->ops = answer->ops;
	answer_prot = answer->prot;
	answer_flags = answer->flags;
	rcu_read_unlock();

	WARN_ON(!answer_prot->slab);

	err = -ENOBUFS;
    // 分配sock对象，并把answer_prot赋值到sock->sk_prot
	sk = sk_alloc(net, PF_INET, GFP_KERNEL, answer_prot, kern);
	if (!sk)
		goto out;

	err = 0;
	if (INET_PROTOSW_REUSE & answer_flags)
		sk->sk_reuse = SK_CAN_REUSE;

	inet = inet_sk(sk);
	inet->is_icsk = (INET_PROTOSW_ICSK & answer_flags) != 0;

	inet->nodefrag = 0;

	if (SOCK_RAW == sock->type) {
		inet->inet_num = protocol;
		if (IPPROTO_RAW == protocol)
			inet->hdrincl = 1;
	}

	if (net->ipv4.sysctl_ip_no_pmtu_disc)
		inet->pmtudisc = IP_PMTUDISC_DONT;
	else
		inet->pmtudisc = IP_PMTUDISC_WANT;

	inet->inet_id = 0;
    // 对sock对象进行初始化
	sock_init_data(sock, sk);

	sk->sk_destruct	   = inet_sock_destruct;
	sk->sk_protocol	   = protocol;
	sk->sk_backlog_rcv = sk->sk_prot->backlog_rcv;

	inet->uc_ttl	= -1;
	inet->mc_loop	= 1;
	inet->mc_ttl	= 1;
	inet->mc_all	= 1;
	inet->mc_index	= 0;
	inet->mc_list	= NULL;
	inet->rcv_tos	= 0;

	sk_refcnt_debug_inc(sk);

	if (inet->inet_num) {
		/* It assumes that any protocol which allows
		 * the user to assign a number at socket
		 * creation time automatically
		 * shares.
		 */
		inet->inet_sport = htons(inet->inet_num);
		/* Add to protocol hash chains. */
		err = sk->sk_prot->hash(sk);
		if (err) {
			sk_common_release(sk);
			goto out;
		}
	}

	if (sk->sk_prot->init) {
		err = sk->sk_prot->init(sk);
		if (err) {
			sk_common_release(sk);
			goto out;
		}
	}

	if (!kern) {
		err = BPF_CGROUP_RUN_PROG_INET_SOCK(sk);
		if (err) {
			sk_common_release(sk);
			goto out;
		}
	}
out:
	return err;
out_rcu_unlock:
	rcu_read_unlock();
	goto out;
}

当流程走到inet_create函数的时候根据type去inetsw数组中找到对应类型套接字的inet_protosw结构体，我们前面提到协议栈中已经定义了PF_INET协议族支持的inet_protosw结构体，总共有4个。

找到inet_protosw结构体以后还需要进一步判断protocol和inet_protosw中定义的protocol是否是一致的。内核中定义支持的protocol有一个特殊的值IPPROTO_IP(IPPROTO_IP为0)，可以理解为一个通配符也可以理解为一个默认值，就是说我不指定protocol，由内核自己决定使用哪一个protocol。

那么内核根据什么来选择protocol呢?就是根据内核定义的全局inetsw中对应类型的inet_protosw中的protocol。

/* Upon startup we insert all the elements in inetsw_array[] into
 * the linked list inetsw.
 */
// static struct list_head inetsw[SOCK_MAX];
// inetsw_array挂在链表上
static struct inet_protosw inetsw_array[] =
{
	{
		.type =       SOCK_STREAM,
		.protocol =   IPPROTO_TCP,
		.prot =       &tcp_prot,
		.ops =        &inet_stream_ops,
		.flags =      INET_PROTOSW_PERMANENT |
			      INET_PROTOSW_ICSK,
	},

	{
		.type =       SOCK_DGRAM,
		.protocol =   IPPROTO_UDP,
		.prot =       &udp_prot,
		.ops =        &inet_dgram_ops,
		.flags =      INET_PROTOSW_PERMANENT,
       },

       {
		.type =       SOCK_DGRAM,
		.protocol =   IPPROTO_ICMP,
		.prot =       &ping_prot,
		.ops =        &inet_sockraw_ops,
		.flags =      INET_PROTOSW_REUSE,
       },

       {
	       .type =       SOCK_RAW,
	       .protocol =   IPPROTO_IP,	/* wild card */  //0
	       .prot =       &raw_prot,
	       .ops =        &inet_sockraw_ops,
	       .flags =      INET_PROTOSW_REUSE,
       }
};

// ~/linux-5.12.10/net/ipv4/af_inet.c  inet_create函数
// int socket_fd = socket(AF_INET, SOCK_STREAM, 0);
// 初始化protocol为0， type为SOCK_STREAM
// 经过list_for_each_entry_rcu遍历，protocol修正为IPPROTO_TCP
// protocol = answer->protocol --> protocol = IPPROTO_TCP
// 如果type为SOCK_DGRAM, 则protocol被修正为IPPROTO_UDP
list_for_each_entry_rcu(answer, &inetsw[sock->type], list) {

		err = 0;
		/* Check the non-wild match. */
		if (protocol == answer->protocol) {
			if (protocol != IPPROTO_IP)
				break;
		} else {
			/* Check for the two wild cases. */
			if (IPPROTO_IP == protocol) {
				protocol = answer->protocol;
				break;
			}
			if (IPPROTO_IP == answer->protocol)
				break;
		}
		err = -EPROTONOSUPPORT;
	}

继续看sock_init_data实现

// ~/linux-5.12.10/net/core/sock.c
void sock_init_data(struct socket *sock, struct sock *sk)
{
	sk_init_common(sk);
	sk->sk_send_head	=	NULL;

	timer_setup(&sk->sk_timer, NULL, 0);

	sk->sk_allocation	=	GFP_KERNEL;
	sk->sk_rcvbuf		=	sysctl_rmem_default;
	sk->sk_sndbuf		=	sysctl_wmem_default;
	sk->sk_state		=	TCP_CLOSE;
	sk_set_socket(sk, sock);

	sock_set_flag(sk, SOCK_ZAPPED);

	if (sock) {
		sk->sk_type	=	sock->type;
		RCU_INIT_POINTER(sk->sk_wq, &sock->wq);
		sock->sk	=	sk;
		sk->sk_uid	=	SOCK_INODE(sock)->i_uid;
	} else {
		RCU_INIT_POINTER(sk->sk_wq, NULL);
		sk->sk_uid	=	make_kuid(sock_net(sk)->user_ns, 0);
	}

	rwlock_init(&sk->sk_callback_lock);
	if (sk->sk_kern_sock)
		lockdep_set_class_and_name(
			&sk->sk_callback_lock,
			af_kern_callback_keys + sk->sk_family,
			af_family_kern_clock_key_strings[sk->sk_family]);
	else
		lockdep_set_class_and_name(
			&sk->sk_callback_lock,
			af_callback_keys + sk->sk_family,
			af_family_clock_key_strings[sk->sk_family]);

	sk->sk_state_change	=	sock_def_wakeup;
	sk->sk_data_ready	=	sock_def_readable;
	sk->sk_write_space	=	sock_def_write_space;
	sk->sk_error_report	=	sock_def_error_report;
	sk->sk_destruct		=	sock_def_destruct;

	sk->sk_frag.page	=	NULL;
	sk->sk_frag.offset	=	0;
	sk->sk_peek_off		=	-1;

	sk->sk_peer_pid 	=	NULL;
	sk->sk_peer_cred	=	NULL;
	sk->sk_write_pending	=	0;
	sk->sk_rcvlowat		=	1;
	sk->sk_rcvtimeo		=	MAX_SCHEDULE_TIMEOUT;
	sk->sk_sndtimeo		=	MAX_SCHEDULE_TIMEOUT;

	sk->sk_stamp = SK_DEFAULT_STAMP;
#if BITS_PER_LONG==32
	seqlock_init(&sk->sk_stamp_seq);
#endif
	atomic_set(&sk->sk_zckey, 0);

#ifdef CONFIG_NET_RX_BUSY_POLL
	sk->sk_napi_id		=	0;
	sk->sk_ll_usec		=	sysctl_net_busy_read;
#endif

	sk->sk_max_pacing_rate = ~0UL;
	sk->sk_pacing_rate = ~0UL;
	WRITE_ONCE(sk->sk_pacing_shift, 10);
	sk->sk_incoming_cpu = -1;

	sk_rx_queue_clear(sk);
	/*
	 * Before updating sk_refcnt, we must commit prior changes to memory
	 * (Documentation/RCU/rculist_nulls.rst for details)
	 */
	smp_wmb();
	refcount_set(&sk->sk_refcnt, 1);
	atomic_set(&sk->sk_drops, 0);
}

当软中断上收到数据包时会调用sk_data_ready函数指针(实际被设置成了sock_def_readable())来唤醒在sock上等待的进程。

sock_alloc

sock_alloc函数分配一个struct socket结构体，将sockfs相关属性填充在socket_alloc结构体的vfs_inode变量中，以限定后续对这个sock文件允许的操作。sock_alloc()里体现了linux一切皆文件(Everything is a file)理念，即使用文件系统来管理socket，这也是VFS所要达到的效果

struct socket *sock_alloc(void)
{
	struct inode *inode;
	struct socket *sock;

	inode = new_inode_pseudo(sock_mnt->mnt_sb);
	if (!inode)
		return NULL;

	sock = SOCKET_I(inode);

	inode->i_ino = get_next_ino();
	inode->i_mode = S_IFSOCK | S_IRWXUGO;
	inode->i_uid = current_fsuid();
	inode->i_gid = current_fsgid();
	inode->i_op = &sockfs_inode_ops;

	return sock;
}

sock_map_fd

static int sock_map_fd(struct socket *sock, int flags)
{
	struct file *newfile;
	int fd = get_unused_fd_flags(flags);
	if (unlikely(fd < 0)) {
		sock_release(sock);
		return fd;
	}

	newfile = sock_alloc_file(sock, flags, NULL);
	if (!IS_ERR(newfile)) {
		fd_install(fd, newfile);
		return fd;
	}

	put_unused_fd(fd);
	return PTR_ERR(newfile);
}

// linux-5.12.10/fs/file.c
int __get_unused_fd_flags(unsigned flags, unsigned long nofile)
{
	return alloc_fd(0, nofile, flags);
}

int get_unused_fd_flags(unsigned flags)
{
	return __get_unused_fd_flags(flags, rlimit(RLIMIT_NOFILE));
}

sock_map_fd–>get_unused_fd_flags–>__get_unused_fd_flags–>alloc_fd获取一个可用的fd

/*
 * allocate a file descriptor, mark it busy.
 */
static int alloc_fd(unsigned start, unsigned end, unsigned flags)
{
	struct files_struct *files = current->files;
	unsigned int fd;
	int error;
	struct fdtable *fdt;

	spin_lock(&files->file_lock);
repeat:
	fdt = files_fdtable(files);
	fd = start;
	if (fd < files->next_fd)
		fd = files->next_fd;

	if (fd < fdt->max_fds)
		fd = find_next_fd(fdt, fd);

	/*
	 * N.B. For clone tasks sharing a files structure, this test
	 * will limit the total number of files that can be opened.
	 */
	error = -EMFILE;
	if (fd >= end)
		goto out;

	error = expand_files(files, fd);
	if (error < 0)
		goto out;

	/*
	 * If we needed to expand the fs array we
	 * might have blocked - try again.
	 */
	if (error)
		goto repeat;

	if (start <= files->next_fd)
		files->next_fd = fd + 1;

	__set_open_fd(fd, fdt);
	if (flags & O_CLOEXEC)
		__set_close_on_exec(fd, fdt);
	else
		__clear_close_on_exec(fd, fdt);
	error = fd;
#if 1
	/* Sanity check */
	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
		rcu_assign_pointer(fdt->fd[fd], NULL);
	}
#endif

out:
	spin_unlock(&files->file_lock);
	return error;
}

sock_map_fd–>get_unused_fd_flags–>__get_unused_fd_flags–>sock_alloc_file分配struct file结构

// net/socket.c
/*
 *	Obtains the first available file descriptor and sets it up for use.
 *
 *	These functions create file structures and maps them to fd space
 *	of the current process. On success it returns file descriptor
 *	and file struct implicitly stored in sock->file.
 *	Note that another thread may close file descriptor before we return
 *	from this function. We use the fact that now we do not refer
 *	to socket after mapping. If one day we will need it, this
 *	function will increment ref. count on file by 1.
 *
 *	In any case returned fd MAY BE not valid!
 *	This race condition is unavoidable
 *	with shared fd spaces, we cannot solve it inside kernel,
 *	but we take care of internal coherence yet.
 */

/**
 *	sock_alloc_file - Bind a &socket to a &file
 *	@sock: socket
 *	@flags: file status flags
 *	@dname: protocol name
 *
 *	Returns the &file bound with @sock, implicitly storing it
 *	in sock->file. If dname is %NULL, sets to "".
 *	On failure the return is a ERR pointer (see linux/err.h).
 *	This function uses GFP_KERNEL internally.
 */

struct file *sock_alloc_file(struct socket *sock, int flags, const char *dname)
{
	struct file *file;

	if (!dname)
		dname = sock->sk ? sock->sk->sk_prot_creator->name : "";

	file = alloc_file_pseudo(SOCK_INODE(sock), sock_mnt, dname,
				O_RDWR | (flags & O_NONBLOCK),
				&socket_file_ops);
	if (IS_ERR(file)) {
		sock_release(sock);
		return file;
	}

	sock->file = file;
	file->private_data = sock;
	stream_open(SOCK_INODE(sock), file);
	return file;
}

相关数据结构

// file: include/linux/net.h
struct socket_wq {
	/* Note: wait MUST be first field of socket_wq */
	wait_queue_head_t	wait;
	struct fasync_struct	*fasync_list;
	unsigned long		flags; /* %SOCKWQ_ASYNC_NOSPACE, etc */
	struct rcu_head		rcu;
} ____cacheline_aligned_in_smp;

/**
 *  struct socket - general BSD socket
 *  @state: socket state (%SS_CONNECTED, etc)
 *  @type: socket type (%SOCK_STREAM, etc)
 *  @flags: socket flags (%SOCK_NOSPACE, etc)
 *  @ops: protocol specific socket operations
 *  @file: File back pointer for gc
 *  @sk: internal networking protocol agnostic socket representation
 *  @wq: wait queue for several uses
 */
struct socket {
	socket_state		state;

	short			type;

	unsigned long		flags;

	struct file		*file;
	struct sock		*sk;
	const struct proto_ops	*ops;

	struct socket_wq	wq;
};

至此，一个tcp对象，确切地说是AF_INET协议族下的SOCK_STREAM对象就算创建完成了。这里花费了一个socket系统调用的开销。

ref: https://www.cnblogs.com/liyuanhong/articles/10591069.html