[内核源码走读] 网络协议栈 socket (tcp)

2021-07-13

本文基于 tcp,走读 socket 的(Linux - 5.0.1 下载)内核源码实现。

socket 是管理网络通信的对象,适合本地或网络环境的进程间通信,它主要分两部分:与文件系统关系密切的部分,与通信关系密切的部分。


1. 应用层

应用层创建 socket 对象返回整型的文件描述符。详细参考(文档 - 可能要翻墙

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/* family:被称为协议族,或者协议域。
 * type:套接字类型。
 * protocol:某个协议的类型常值,可以设置为 0。
 * return:返回整型的文件描述符,如果返回 -1 就失败。
 */
#include <sys/socket.h>
int socket(int domain/family, int type, int protocol);

图片来源:《UNIX 网络编程_卷1》


2. 系统调用

从用户层到内核系统调用流程:user(socket()) –> glibc –> kernel。

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# System.map
ffffffff81792870 T __x64_sys_socket

# socket
__do_sys_socket() (/root/linux-5.0.1/net/socket.c:1355)
__se_sys_socket() (/root/linux-5.0.1/net/socket.c:1353)
__x64_sys_socket(const struct pt_regs * regs) (/root/linux-5.0.1/net/socket.c:1353)
do_syscall_64(unsigned long nr, struct pt_regs * regs) (/root/linux-5.0.1/arch/x86/entry/common.c:290)
entry_SYSCALL_64() (/root/linux-5.0.1/arch/x86/entry/entry_64.S:175)

参考:vscode + gdb 远程调试 linux (EPOLL) 内核源码


3. socket 结构

socket 结构主要分两部分:与文件系统关系密切的部分,与通信关系密切的部分。

图片来源:sys-socket - linux 内核 socket 结构关系

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/** ./include/linux/net.h
 *  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 socket_wq   *wq;
    struct file        *file;
    struct sock        *sk;
    const struct proto_ops  *ops;
};

/* ./include/linux/net.h */
struct proto_ops {
    ...
}

/* ./include/net/sock.h */
struct tcp_sock {
    /* inet_connection_sock has to be the first member of tcp_sock */
    struct inet_connection_sock inet_conn;
    ...
}

struct inet_connection_sock {
    struct inet_sock icsk_inet;
    ...
}

/* ./include/net/inet_sock.h */
struct inet_sock {
    /* sk and pinet6 has to be the first two members of inet_sock */
    struct sock sk;
    ...
}

struct sock {
    struct sock_common __sk_common;
    ...
};

4. 创建 socket

函数调用关系。

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#------------------- *用户态* ---------------------------
socket
#------------------- *内核态* ---------------------------
__x64_sys_socket # 内核系统调用。
__sys_socket # ./net/socket.c
    |-- sock_create # ./net/socket.c
        |-- __sock_create # ./net/socket.c
#------------------- 文件部分 ---------------------------
            |-- sock_alloc # ./net/socket.c
                |-- new_inode_pseudo # ./fs/inode.c
                    |-- alloc_inode # ./fs/inode.c
                        |-- sock_alloc_inode # ./net/socket.c
                            |-- kmem_cache_alloc
#------------------- 网络部分 ---------------------------
            |-- inet_create # pf->create -- af_inet.c
                |-- sk_alloc # ./net/core/sock.c
                    |-- sk_prot_alloc # ./net/core/sock.c
                        |-- kmem_cache_alloc
                |-- inet_sk
                |-- sock_init_data # ./net/core/sock.c
                    |-- sk_init_common # ./net/core/sock.c
                    |-- timer_setup
                |-- sk->sk_prot->init(sk) # tcp_v4_init_sock  -- ./net/ipv4/tcp_ipv4.c
                    |-- tcp_init_sock
#------------------- 文件+网络+关联进程 ------------------------
    |-- sock_map_fd # ./net/socket.c
        |-- get_unused_fd_flags # ./fs/file.c -- 进程分配空闲 fd。
        |-- sock_alloc_file # ./net/socket.c
            |-- alloc_file_pseudo # ./fs/file_table.c
        |-- fd_install # ./fs/file.c
            |-- __fd_install # ./fs/file.c
                |-- fdt = rcu_dereference_sched(files->fdt);
                |-- rcu_assign_pointer(fdt->fd[fd], file); # file 关联到进程。
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int __sys_socket(int family, int type, int protocol) {
    struct socket *sock;
    ...
    retval = sock_create(family, type, protocol, &sock);
    if (retval < 0)
        return retval;

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

int sock_create(int family, int type, int protocol, struct socket **res) {
    return __sock_create(current->nsproxy->net_ns, family, type, protocol, res, 0);
}

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;
    ...
    sock = sock_alloc();
    ...
    pf = rcu_dereference(net_families[family]);
    ...
    err = pf->create(net, sock, protocol, kern);
    ...
    *res = sock;
    return 0;
    ...
}

4.1. 文件部分

Linux 系统一切皆文件,Linux 通过 vfs(虚拟文件系统)管理文件,内核为 socket 定义了一种特殊的文件类型,形成了一种特殊的文件系统:sockfs,系统初始化时,进行安装。

图片来源:《Linux 内核源代码情景分析》- 第五章 - 文件系统

创建一个 socket,要把 socket 关联到一个已打开文件,方便进程进行管理。

  • 相关结构。
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/* ./include/linux/mount.h */
struct vfsmount {
    struct dentry *mnt_root;    /* root of the mounted tree */
    struct super_block *mnt_sb;    /* pointer to superblock */
    int mnt_flags;
} __randomize_layout;

/* ./net/socket.c */
static struct vfsmount *sock_mnt __read_mostly;

/* sock 文件类型。 */
static struct file_system_type sock_fs_type = {
    .name = "sockfs",
    .mount = sockfs_mount,
    .kill_sb = kill_anon_super,
};

/* sock 文件操作。 */
static const struct super_operations sockfs_ops = {
    .alloc_inode    = sock_alloc_inode,
    .destroy_inode  = sock_destroy_inode,
    .statfs         = simple_statfs,
};

/* ./include/sock.h 
 * sock 与 inode 文件节点关联结构。*/
struct socket_alloc {
    struct socket socket;
    struct inode vfs_inode;
};

/* ./include/net/sock.h 
 * 从文件节点结构获得 socket 成员。*/
static inline struct socket *SOCKET_I(struct inode *inode) {
    return &container_of(inode, struct socket_alloc, vfs_inode)->socket;
}

/* ./include/linux/fs.h */
struct file_operations {
    struct module *owner;
    loff_t (*llseek) (struct file *, loff_t, int);
    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
    ...
} __randomize_layout;

/* ./net/socket.c
 * Socket files have a set of 'special' operations as well as the generic file ones. These don't appear
 * in the operation structures but are done directly via the socketcall() multiplexor.
 */
static const struct file_operations socket_file_ops = {
    .owner      =    THIS_MODULE,
    .llseek     =    no_llseek,
    .read_iter  =    sock_read_iter,
    .write_iter =    sock_write_iter,
    .poll       =    sock_poll,
    .unlocked_ioctl = sock_ioctl,
#ifdef CONFIG_COMPAT
    .compat_ioctl = compat_sock_ioctl,
#endif
    .mmap         = sock_mmap,
    .release      = sock_close,
    .fasync       = sock_fasync,
    .sendpage     = sock_sendpage,
    .splice_write = generic_splice_sendpage,
    .splice_read  = sock_splice_read,
};
  • 函数调用堆栈
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sockfs_mount(struct file_system_type * fs_type, int flags, const char * dev_name, void * data) (/root/linux-5.0.1/net/socket.c:357)
mount_fs(struct file_system_type * type, int flags, const char * name, void * data) (/root/linux-5.0.1/fs/super.c:1258)
vfs_kern_mount(struct file_system_type * type, int flags, const char * name, void * data) (/root/linux-5.0.1/fs/namespace.c:959)
kern_mount_data(struct file_system_type * type, void * data) (/root/linux-5.0.1/fs/namespace.c:3301)
sock_init() (/root/linux-5.0.1/net/socket.c:2737)
...
kernel_init(void * unused) (/root/linux-5.0.1/init/main.c:1054)
...
  • 系统初始化,安装 sockfs。
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/* ./net/socket.c */
static int __init sock_init(void) {
    ...
    sock_mnt = kern_mount(&sock_fs_type);
    ...
}
core_initcall(sock_init);    /* early initcall */

/* 初始化的时候绑定 socket 的信息。*/
static struct dentry *sockfs_mount(struct file_system_type *fs_type,
             int flags, const char *dev_name, void *data) {
    return mount_pseudo_xattr(fs_type, "socket:", &sockfs_ops,
                  sockfs_xattr_handlers,
                  &sockfs_dentry_operations, SOCKFS_MAGIC);
}
  • 创建 socket_alloc 对象。
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struct socket *sock_alloc(void) {
    struct inode *inode;
    struct socket *sock;

    /* 创建文件节点。 */
    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;
}

struct inode *new_inode_pseudo(struct super_block *sb) {
    struct inode *inode = alloc_inode(sb);
    ...
    return inode;
}

static struct inode *alloc_inode(struct super_block *sb) {
    struct inode *inode;

    if (sb->s_op->alloc_inode)
        /* socket 调用这个。 */
        inode = sb->s_op->alloc_inode(sb);
    ...
    return inode;
}

/* 初始化 socket 结构成员。 */
static struct inode *sock_alloc_inode(struct super_block *sb) {
    struct socket_alloc *ei;
    struct socket_wq *wq;

    ei = kmem_cache_alloc(sock_inode_cachep, GFP_KERNEL);
    if (!ei)
        return NULL;
    wq = kmalloc(sizeof(*wq), GFP_KERNEL);
    if (!wq) {
        kmem_cache_free(sock_inode_cachep, ei);
        return NULL;
    }
    init_waitqueue_head(&wq->wait);
    wq->fasync_list = NULL;
    wq->flags = 0;
    ei->socket.wq = wq;

    ei->socket.state = SS_UNCONNECTED;
    ei->socket.flags = 0;
    ei->socket.ops = NULL;
    ei->socket.sk = NULL;
    ei->socket.file = NULL;

    return &ei->vfs_inode;
}
  • sock 与进程(task_struct)关联。
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static int sock_map_fd(struct socket *sock, int flags) {
    struct file *newfile;

    /* 进程分配空闲 fd。 */
    int fd = get_unused_fd_flags(flags);
    if (unlikely(fd < 0)) {
        sock_release(sock);
        return fd;
    }

    /* 进程为 sock 分配新的文件。 */
    newfile = sock_alloc_file(sock, flags, NULL);
    if (likely(!IS_ERR(newfile))) {
        /* fd 与 file 进行关联。 */
        fd_install(fd, newfile);
        return fd;
    }
    ...
}

struct file *sock_alloc_file(struct socket *sock, int flags, const char *dname) {
    ...
    file = alloc_file_pseudo(SOCK_INODE(sock), sock_mnt, dname,
                O_RDWR | (flags & O_NONBLOCK),
                &socket_file_ops);
    ...
    sock->file = file;
    file->private_data = sock;
    return file;
}

void fd_install(unsigned int fd, struct file *file) {
    __fd_install(current->files, fd, file);
}

void __fd_install(struct files_struct *files, unsigned int fd,
        struct file *file) {
    struct fdtable *fdt;
    ...
    fdt = rcu_dereference_sched(files->fdt);
    ...
    rcu_assign_pointer(fdt->fd[fd], file);
    ..
}

4.2. 网络部分

协议:socket 层 –> 传输层 –> 网络层。

  • net_proto_family:网域(PF_INET/AF_INET)数据结构。
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/* ./include/linux/net.h */
struct net_proto_family {
    int family;
    int (*create)(struct net *net, struct socket *sock, int protocol, int kern);
    struct module *owner;
};

/* ./net/ipv4/af_inet.c */
static const struct net_proto_family inet_family_ops = {
    .family = PF_INET,
    .create = inet_create,
    .owner  = THIS_MODULE,
};
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# 保存网域数据结构指针到 net_families[] 数组。
sock_register(const struct net_proto_family * ops) (/root/linux-5.0.1/net/socket.c:2661)
inet_init() (/root/linux-5.0.1/net/ipv4/af_inet.c:1927)
...
kernel_init(void * unused) (/root/linux-5.0.1/init/main.c:1054)
...
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/**
 * sock_register - add a socket protocol handler
 * @ops: description of protocol
 *
 * This function is called by a protocol handler that wants to
 * advertise its address family, and have it linked into the
 * socket interface. The value ops->family corresponds to the
 * socket system call protocol family.
 */
int sock_register(const struct net_proto_family *ops) {
    ...
    rcu_assign_pointer(net_families[ops->family], ops);
    ...
}
EXPORT_SYMBOL(sock_register);
  • inet_protosw。
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/* ./include/net/protocol.h
 * 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.  */
};

/* af_inet.c
 * Upon startup we insert all the elements in inetsw_array[] into
 * the linked list inetsw.
 */
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,
    },
    ...
};
  • proto_ops。
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/* ./net/ipv4/af_inet.c */
const struct proto_ops inet_stream_ops = {
    .family        = PF_INET,
    .owner         = THIS_MODULE,
    .release       = inet_release,
    .bind          = inet_bind,
    .connect       = inet_stream_connect,
    .socketpair    = sock_no_socketpair,
    .accept        = inet_accept,
    ...
};
EXPORT_SYMBOL(inet_stream_ops);
  • proto。
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/* ./include/net/sock.h
 * Networking protocol blocks we attach to sockets.
 * socket layer -> transport layer interface
 */
struct proto {
    void           (*close)(struct sock *sk, long timeout);
    int            (*pre_connect)(struct sock *sk, struct sockaddr *uaddr, int addr_len);
    int            (*connect)(struct sock *sk, struct sockaddr *uaddr, int addr_len);
    int            (*disconnect)(struct sock *sk, int flags);

    struct sock *  (*accept)(struct sock *sk, int flags, int *err, bool kern);
    ...
} __randomize_layout;

/* ./net/ipv4/tcp_ipv4.c */
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,
    ...
    .obj_size         = sizeof(struct tcp_sock),
    ...
};
EXPORT_SYMBOL(tcp_prot);
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# 初始化 tcp 协议。
proto_register(struct proto * prot, int alloc_slab) (/root/linux-5.0.1/net/core/sock.c:3209)
inet_init() (/root/linux-5.0.1/net/ipv4/af_inet.c:1907)
...
kernel_init(void * unused) (/root/linux-5.0.1/init/main.c:1054)
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static int __init inet_init(void) {
    ...
    rc = proto_register(&tcp_prot, 1);
    ...
}

/* 为 tcp 协议分配空间。*/
int proto_register(struct proto *prot, int alloc_slab) {
    if (alloc_slab) {
        prot->slab = kmem_cache_create_usercopy(prot->name,
                    prot->obj_size, 0,
                    SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT |
                    prot->slab_flags,
                    prot->useroffset, prot->usersize,
                    NULL);
    ...
}

/* 分配的空间其实是 tcp_sock,struct proto --> struct proto tcp_prot.obj_size */
static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority, int family) {
    struct sock *sk;
    struct kmem_cache *slab;

    slab = prot->slab;
    if (slab != NULL) {
        sk = kmem_cache_alloc(slab, priority & ~__GFP_ZERO);
        if (!sk)
            return sk;
        if (priority & __GFP_ZERO)
            sk_prot_clear_nulls(sk, prot->obj_size);
    }
    ...
}
  • 详细源码。
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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;
    ...
    sock->state = SS_UNCONNECTED;

    /* Look for the requested type/protocol pair. */
lookup_protocol:
    ...
    /* 查找对应的协议。 */
    list_for_each_entry_rcu(answer, &inetsw[sock->type], list) {
        ...
    }
    ...
    sock->ops = answer->ops;
    answer_prot = answer->prot;
    answer_flags = answer->flags;
    ...
    /* 为 sock 分配空间。 */
    sk = sk_alloc(net, PF_INET, GFP_KERNEL, answer_prot, kern);
    ...
    sock_init_data(sock, sk);
    ...
    if (sk->sk_prot->init) {
        /* tcp_v4_init_sock */
        err = sk->sk_prot->init(sk);
        ...
    }
    ...
out:
    return err;
    ...
}
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/*
 * sk_alloc - All socket objects are allocated here
 * @net: the applicable net namespace
 * @family: protocol family
 * @priority: for allocation (%GFP_KERNEL, %GFP_ATOMIC, etc)
 * @prot: struct proto associated with this new sock instance
 * @kern: is this to be a kernel socket?
 */
struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
              struct proto *prot, int kern)
{
    struct sock *sk;

    sk = sk_prot_alloc(prot, priority | __GFP_ZERO, family);
    if (sk) {
        sk->sk_family = family;
        /*
         * See comment in struct sock definition to understand
         * why we need sk_prot_creator -acme
         */
        sk->sk_prot = sk->sk_prot_creator = prot;
        sk->sk_kern_sock = kern;
        sock_lock_init(sk);
        ...
        sock_net_set(sk, net);
        ...
    }

    return sk;
}
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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;
        sk->sk_wq    =    sock->wq;
        sock->sk     =    sk;
        sk->sk_uid   =    SOCK_INODE(sock)->i_uid;
    } else {
        sk->sk_wq    =    NULL;
        sk->sk_uid   =    make_kuid(sock_net(sk)->user_ns, 0);
    }
    ...
    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;
    sk->sk_pacing_shift = 10;
    sk->sk_incoming_cpu = -1;
    ...
}
EXPORT_SYMBOL(sock_init_data);
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static void sk_init_common(struct sock *sk) {
    /* 传输层的接收和发送数据缓冲区。 */
    skb_queue_head_init(&sk->sk_receive_queue);
    skb_queue_head_init(&sk->sk_write_queue);
    ...
}

static int tcp_v4_init_sock(struct sock *sk) {
    struct inet_connection_sock *icsk = inet_csk(sk);
    tcp_init_sock(sk);
    ...
}

/* Address-family independent initialization for a tcp_sock.
 *
 * NOTE: A lot of things set to zero explicitly by call to
 *       sk_alloc() so need not be done here.
 */
void tcp_init_sock(struct sock *sk) {
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);

    tp->out_of_order_queue = RB_ROOT;
    sk->tcp_rtx_queue = RB_ROOT;
    tcp_init_xmit_timers(sk);
    INIT_LIST_HEAD(&tp->tsq_node);
    INIT_LIST_HEAD(&tp->tsorted_sent_queue);

    icsk->icsk_rto = TCP_TIMEOUT_INIT;
    tp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
    minmax_reset(&tp->rtt_min, tcp_jiffies32, ~0U);

    /* So many TCP implementations out there (incorrectly) count the
     * initial SYN frame in their delayed-ACK and congestion control
     * algorithms that we must have the following bandaid to talk
     * efficiently to them.  -DaveM
     */
    tp->snd_cwnd = TCP_INIT_CWND;

    /* There's a bubble in the pipe until at least the first ACK. */
    tp->app_limited = ~0U;

    /* See draft-stevens-tcpca-spec-01 for discussion of the
     * initialization of these values.
     */
    tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
    tp->snd_cwnd_clamp = ~0;
    tp->mss_cache = TCP_MSS_DEFAULT;

    tp->reordering = sock_net(sk)->ipv4.sysctl_tcp_reordering;
    tcp_assign_congestion_control(sk);

    tp->tsoffset = 0;
    tp->rack.reo_wnd_steps = 1;

    sk->sk_state = TCP_CLOSE;

    sk->sk_write_space = sk_stream_write_space;
    sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);

    icsk->icsk_sync_mss = tcp_sync_mss;

    /* 发送缓冲区大小。 */
    sk->sk_sndbuf = sock_net(sk)->ipv4.sysctl_tcp_wmem[1];
    /* 接收缓冲区大小。 */
    sk->sk_rcvbuf = sock_net(sk)->ipv4.sysctl_tcp_rmem[1];

    sk_sockets_allocated_inc(sk);
    sk->sk_route_forced_caps = NETIF_F_GSO;
}
EXPORT_SYMBOL(tcp_init_sock);

5. 参考