Turn your Linux computer into a wireless access point using hostapd

話說手持無線裝置越來越多，習慣晚上用我的android手機上網，不過有天竟然把AP帶回老家忘記帶回來，只好拿我的HP Compaq CQ45來充當AP了，網路上已經很多Turn your Linux computer into a wireless access point using hostapd這類的文章了，我就不多加闡述，只是單純的紀錄。
我的CQ45的網卡是Broadcom的Chip，用的Linux kernel是3.0.0，預設的wireless driver是去load wl.ko，產生的interface name是eth1，不過hostap好像不支援，也沒去深究他，索性把他換成舊的b43.ko，然後簡單設定一下hostapd.conf就執行hostapd了。

hostapd.conf

ssid=test
hw_mode=g
channel=1
interface=wlan1
#bridge=br1
driver=nl80211
ignore_broadcast_ssid=0
macaddr_acl=0
wmm_enabled=0

接著還要設定wireless的IP，不過我並沒有啟動DHCP server，Client端就用靜態IP吧。

接著還要讓我的NB具有route和NAT的功能，請執行
sudo iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
sudo sysctl -w sys.net.ipv4.conf.all.forwarding=1

send signal to user-space

某天有個需求是希望當kernel發生某事件時通知user-space的process，心裡想最快就是送signal，於是google一下，果然有人有類似的需求，signals handling in the kernel，於是改了一下把他放上來，值得一提的是，其實這樣並不被鼓勵的，而且原本的kill_proc_info並沒有被export出來，所以如果是module要使用的話，就必須把他export出來，EXPORT_SYMBOL(kill_proc_info)。

#include <linux/module.h>
#include <linux/init.h>
#include <linux/moduleparam.h>

#include <linux/sched.h>
#include <linux/kernel.h> /* printk() */
// #include <linux/slab.h> /* kmalloc() */
#include <linux/errno.h>  /* error codes */
#include <linux/types.h>  /* size_t */
#include <linux/signal.h>
#include <linux/proc_fs.h>
#include <linux/uaccess.h>

#define PROC_NAME "sig2pid"

/**
 * 送signal 到pid去
 */
static int send_sig_to_pid(int sig, pid_t pid)
{
    struct siginfo info;

    info.si_signo = sig;
    info.si_errno = 0;
    info.si_code = SI_USER; // sent by kill, sigsend, raise
    info.si_pid = get_current()->pid; // sender's pid
    info.si_uid = current_uid(); // sender's uid

    return kill_proc_info(sig, &info, pid);
}

/**
 * /proc/sig2pid的write ops
 */
static int
sig2pid_proc_write(struct file *file, const char __user * buffer,
                     unsigned long count, void *data)
{
    int sig, pid, ret;
    char line[count];
    ret = copy_from_user(line, buffer, count);

    if (ret) {
        return -EFAULT;
    }
    sscanf(line, "%d %d", &pid, &sig);
    printk("%s(#%d): pid(%d), sig(%d)\n",
            __func__, __LINE__, pid, sig);
    send_sig_to_pid(sig, (pid_t) pid);
    return count;
}

/**
 * 建立/proc/sig2pid
 */
static int create_proc_file(void)
{
    struct proc_dir_entry *p;
    p = create_proc_entry(PROC_NAME, S_IFREG | S_IWUGO, NULL);
    if (!p) {
        printk("%s(#%d): create proc entry failed\n", __func__, __LINE__);
        return -EFAULT;
    }
    p->write_proc = sig2pid_proc_write;
    return 0;
}

int sig2pid_init_module(void)
{
    return create_proc_file();
}

void sig2pid_exit_module(void)
{
    remove_proc_entry(PROC_NAME, NULL);
}

module_init(sig2pid_init_module);
module_exit(sig2pid_exit_module);

參考資料：
• http://old.nabble.com/signals-handling-in-the-kernel-to12032525.html#a12032525 , signals handling in the kernel.
• http://kerneltrap.org/node/5800, how to send signal from kernel space to user space.

github： https://github.com/brook-kuo/Linux_Module/tree/master/process/send_sig_to_userspace

Allow Unix sockets to be treated like normal files.

某天忽然想要用echo/cat的方式直接對unix socket做存取，結果得到error，只好有請google大神，覓得此良方Allow Unix sockets to be treated like normal files，try了一下沒問題。

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdlib.h>

#define handle_error(msg) \
        do { perror(msg); exit(EXIT_FAILURE); } while (0)

int main(int argc, char *argv[])
{
    int srv_fd, cli_fd;
    socklen_t cli_len;
    struct sockaddr_un srv_addr, cli_addr;
    char buf[128] = "Brook: ";
    ssize_t len;

    unlink("server_socket");
    if ((srv_fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
        handle_error("socket");
    }

    srv_addr.sun_family = AF_UNIX;
    strcpy(srv_addr.sun_path, "/tmp/unix_sock");
    if (bind(srv_fd, (struct sockaddr *)&srv_addr, sizeof(srv_addr)) < 0) {
        handle_error("bind");
    }

    if (listen(srv_fd, 1) < 0) {
         handle_error("listen");
    }

    while (1) {
        cli_fd = accept(srv_fd, (struct sockaddr *)&cli_addr, &cli_len);
        len = read(cli_fd, buf + 6, sizeof(buf) - 6);
        buf[6 + len] = 0;
        write(cli_fd, buf, strlen(buf));
        close(cli_fd);
    }
    return 0;
}

參考資料：
• http://lwn.net/Articles/415651/ , net/unix: Allow Unix sockets to be treated like normal files.

github：
https://github.com/brook-kuo/Linux_Module/tree/master/socket/unix_as_normal_file