setsockopt的工作流程（从用户空间到内核）

一、用户空间

获取套接口选项：

1.  int getsockopt ( int sockfd, int level, int optname, void * optval, socklen_t *opteln ) 设置套接口选项：

2.  int setsockopt ( int sockfd, int level, int optname, const void * optval, socklen_t *opteln )

 

二、内核空间

系统调用setsockopt是通过内核sys_setsockopt函数实现的

asmlinkage long sys_setsockopt(int fd, int level, int optname, char __user *optval, int optlen)
{
……
    if ((sock = sockfd_lookup_light(fd, &err, &fput_needed)) != NULL)
    {
……
        if (level == SOL_SOCKET)//基本套接口
套接口类型（linux/socket.h）：
/* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */
#define SOL_IP           0
/* #define SOL_ICMP        1       No-no-no! Due to Linux :-) we cannot use SOL_ICMP=1 */
#define SOL_TCP                6
#define SOL_UDP               17
……
            err=sock_setsockopt(sock,level,optname,optval,optlen);
        else
            err=sock->ops->setsockopt(sock, level, optname, optval, optlen);
    ……
    }
    return err;
}

假如我们定义的套接口是IPPROTO_IP=0，所以我们将调用SOL_IP套接口，err=sock->ops->setsockopt(sock, level, optname, optval, optlen)，该钩子为proto_ops类型，所以它指向了我们注册的inet_stream_ops钩子，所以setsockopt指向了sock_common_setsockopt函数（net/core/sock.c）

在net/ipv4/af_inet.c文件中，函数inet_init注册了inetsw_array数组

for (q = inetsw_array; q < &inetsw_array[INETSW_ARRAY_LEN]; ++q)
        inet_register_protosw(q);

static struct inet_protosw inetsw_array[] =
{
        {
                .type = SOCK_STREAM,
                .protocol = IPPROTO_TCP,
                .prot = &tcp_prot,
                .ops = &inet_stream_ops,
                .capability = -1,
                .no_check = 0,
                .flags = INET_PROTOSW_PERMANENT |
             INET_PROTOSW_ICSK,
        },
        ……
};

譬如它注册了inet_stream_ops针对tcp协议

const struct proto_ops inet_stream_ops = {
    .family         = PF_INET,
    .owner         = THIS_MODULE,
    ……
    .setsockopt     = sock_common_setsockopt,
    .getsockopt     = sock_common_getsockopt,
……
};

现在我们看接口函数sock_common_setsockopt

int sock_common_setsockopt(struct socket *sock, int level, int optname,
             char __user *optval, int optlen)
{
    struct sock *sk = sock->sk;
    return sk->sk_prot->setsockopt(sk, level, optname, optval, optlen);
}

由于sk->sk_prot是指向协议的钩子，如果它指向了tcp协议钩子，setsockopt指向了tcp_setsockopt函数（net/ipv4/tcp.c）

struct proto tcp_prot = {
    .name            = "TCP",
    .owner            = THIS_MODULE,
    ……
    .setsockopt        = tcp_setsockopt,
    .getsockopt        = tcp_getsockopt,
    ……
}

 

int tcp_setsockopt(struct sock *sk, int level, int optname, char __user *optval,
         int optlen)
{
    struct inet_connection_sock *icsk = inet_csk(sk);

    if (level != SOL_TCP)//显然我们注册的是SOL_IP所以执行下面语句
        return icsk->icsk_af_ops->setsockopt(sk, level, optname,
                         optval, optlen);
    return do_tcp_setsockopt(sk, level, optname, optval, optlen);//如果注册时SOL_TCP就执行
}

如果注册时SOL_IP则由于isck->isck_af_ops是inet_connection_sock_af_ops类型的钩子，此钩子的注册为

struct inet_connection_sock_af_ops ipv4_specific = {
……
    .setsockopt     = ip_setsockopt,
    .getsockopt     = ip_getsockopt,
……
};

注册ip_setsockopt（net/ipv4/tcp_ipv4.c）接口

int ip_setsockopt(struct sock *sk, int level,
        int optname, char __user *optval, int optlen)
{
    int err;
    if (level != SOL_IP)
        return -ENOPROTOOPT;
    err = do_ip_setsockopt(sk, level, optname, optval, optlen);//注册是IPPROTO_IP执行
#ifdef CONFIG_NETFILTER
    /* we need to exclude all possible ENOPROTOOPTs except default case */
    if (err == -ENOPROTOOPT && optname != IP_HDRINCL &&
        optname != IP_IPSEC_POLICY && optname != IP_XFRM_POLICY
#ifdef CONFIG_IP_MROUTE
        && (optname < MRT_BASE || optname > (MRT_BASE + 10))
#endif
     ) {
        lock_sock(sk);
        err = nf_setsockopt(sk, PF_INET, optname, optval, optlen);//如果编译选项中设置了CONFIG_NETFILTER，就调用nf_setsockopt函数
        release_sock(sk);
    }
#endif
    return err;
}

到此我们知道了如果注册时IPPROTO_IP最终会调用do_ip_setsockopt和nf_setsockopt函数

如果注册了IPPROTO_TCP最终会调用do_tcp_setsockopt函数