用户层通过socket使用XFRM_MSG_GETSA获取xfrm_algo_auth信息

用户层可以通过socket接口调用XFRM_MSG_GETSA消息来获取xfrm_algo_auth信息。具体步骤如下：

1. 创建socket并绑定到XFRM_NETLINK协议上：

int sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_XFRM);
struct sockaddr_nl sa_nl;
memset(&sa_nl, 0, sizeof(sa_nl));
sa_nl.nl_family = AF_NETLINK;
sa_nl.nl_groups = XFRMNLGRP_NONE;
bind(sock, (struct sockaddr *)&sa_nl, sizeof(sa_nl));

2. 构造查询消息：

struct xfrm_usersa_info info;
memset(&info, 0, sizeof(info));
info.xusa_family = AF_INET;
info.xusa_sproto = IPPROTO_ESP;
info.xusa_mode = XFRM_MODE_TUNNEL;
struct nlmsghdr *nlh = nlmsg_alloc();
nlh->nlmsg_len = NLMSG_LENGTH(sizeof(info));
nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
nlh->nlmsg_type = XFRM_MSG_GETSA;
nlh->nlmsg_seq = 1;
nlh->nlmsg_pid = getpid();
nlmsg_append(nlh, &info, sizeof(info), NLMSG_ALIGNTO);

3. 发送查询消息并接收响应：

send(sock, nlh, nlh->nlmsg_len, 0);
struct msghdr msg;
struct iovec iov;
char buf[4096];
memset(&msg, 0, sizeof(msg));
memset(&iov, 0, sizeof(iov));
iov.iov_base = buf;
iov.iov_len = sizeof(buf);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
recvmsg(sock, &msg, 0);

4. 解析响应消息，获取xfrm_algo_auth信息：

struct nlmsghdr *nh;
for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, msg.msg_len); nh = NLMSG_NEXT(nh, msg.msg_len)) {
    if (nh->nlmsg_type == XFRM_MSG_NEWSA) {
        struct xfrm_usersa_info *info = NLMSG_DATA(nh);
        if (info->xusa_auth) {
            struct xfrm_algo_auth *auth = (struct xfrm_algo_auth *)(((char *)info) + info->xusa_auth);
            // auth指向xfrm_algo_auth信息
        }
    }
}

注意，以上代码仅为示例代码，实际使用时需要进行错误处理和内存管理